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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	           Booting the Linux/ppc kernel without Open Firmware
2	           --------------------------------------------------
3	
4	(c) 2005 Benjamin Herrenschmidt <benh at kernel.crashing.org>,
5	    IBM Corp.
6	(c) 2005 Becky Bruce <becky.bruce at freescale.com>,
7	    Freescale Semiconductor, FSL SOC and 32-bit additions
8	(c) 2006 MontaVista Software, Inc.
9	    Flash chip node definition
10	
11	Table of Contents
12	=================
13	
14	  I - Introduction
15	    1) Entry point for arch/arm
16	    2) Entry point for arch/powerpc
17	    3) Entry point for arch/x86
18	    4) Entry point for arch/mips/bmips
19	    5) Entry point for arch/sh
20	
21	  II - The DT block format
22	    1) Header
23	    2) Device tree generalities
24	    3) Device tree "structure" block
25	    4) Device tree "strings" block
26	
27	  III - Required content of the device tree
28	    1) Note about cells and address representation
29	    2) Note about "compatible" properties
30	    3) Note about "name" properties
31	    4) Note about node and property names and character set
32	    5) Required nodes and properties
33	      a) The root node
34	      b) The /cpus node
35	      c) The /cpus/* nodes
36	      d) the /memory node(s)
37	      e) The /chosen node
38	      f) the /soc<SOCname> node
39	
40	  IV - "dtc", the device tree compiler
41	
42	  V - Recommendations for a bootloader
43	
44	  VI - System-on-a-chip devices and nodes
45	    1) Defining child nodes of an SOC
46	    2) Representing devices without a current OF specification
47	
48	  VII - Specifying interrupt information for devices
49	    1) interrupts property
50	    2) interrupt-parent property
51	    3) OpenPIC Interrupt Controllers
52	    4) ISA Interrupt Controllers
53	
54	  VIII - Specifying device power management information (sleep property)
55	
56	  IX - Specifying dma bus information
57	
58	  Appendix A - Sample SOC node for MPC8540
59	
60	
61	Revision Information
62	====================
63	
64	   May 18, 2005: Rev 0.1 - Initial draft, no chapter III yet.
65	
66	   May 19, 2005: Rev 0.2 - Add chapter III and bits & pieces here or
67	                           clarifies the fact that a lot of things are
68	                           optional, the kernel only requires a very
69	                           small device tree, though it is encouraged
70	                           to provide an as complete one as possible.
71	
72	   May 24, 2005: Rev 0.3 - Precise that DT block has to be in RAM
73				 - Misc fixes
74				 - Define version 3 and new format version 16
75				   for the DT block (version 16 needs kernel
76				   patches, will be fwd separately).
77				   String block now has a size, and full path
78				   is replaced by unit name for more
79				   compactness.
80				   linux,phandle is made optional, only nodes
81				   that are referenced by other nodes need it.
82				   "name" property is now automatically
83				   deduced from the unit name
84	
85	   June 1, 2005: Rev 0.4 - Correct confusion between OF_DT_END and
86	                           OF_DT_END_NODE in structure definition.
87	                         - Change version 16 format to always align
88	                           property data to 4 bytes. Since tokens are
89	                           already aligned, that means no specific
90	                           required alignment between property size
91	                           and property data. The old style variable
92	                           alignment would make it impossible to do
93	                           "simple" insertion of properties using
94	                           memmove (thanks Milton for
95	                           noticing). Updated kernel patch as well
96				 - Correct a few more alignment constraints
97				 - Add a chapter about the device-tree
98	                           compiler and the textural representation of
99	                           the tree that can be "compiled" by dtc.
100	
101	   November 21, 2005: Rev 0.5
102				 - Additions/generalizations for 32-bit
103				 - Changed to reflect the new arch/powerpc
104				   structure
105				 - Added chapter VI
106	
107	
108	 ToDo:
109		- Add some definitions of interrupt tree (simple/complex)
110		- Add some definitions for PCI host bridges
111		- Add some common address format examples
112		- Add definitions for standard properties and "compatible"
113		  names for cells that are not already defined by the existing
114		  OF spec.
115		- Compare FSL SOC use of PCI to standard and make sure no new
116		  node definition required.
117		- Add more information about node definitions for SOC devices
118	  	  that currently have no standard, like the FSL CPM.
119	
120	
121	I - Introduction
122	================
123	
124	During the development of the Linux/ppc64 kernel, and more
125	specifically, the addition of new platform types outside of the old
126	IBM pSeries/iSeries pair, it was decided to enforce some strict rules
127	regarding the kernel entry and bootloader <-> kernel interfaces, in
128	order to avoid the degeneration that had become the ppc32 kernel entry
129	point and the way a new platform should be added to the kernel. The
130	legacy iSeries platform breaks those rules as it predates this scheme,
131	but no new board support will be accepted in the main tree that
132	doesn't follow them properly.  In addition, since the advent of the
133	arch/powerpc merged architecture for ppc32 and ppc64, new 32-bit
134	platforms and 32-bit platforms which move into arch/powerpc will be
135	required to use these rules as well.
136	
137	The main requirement that will be defined in more detail below is
138	the presence of a device-tree whose format is defined after Open
139	Firmware specification. However, in order to make life easier
140	to embedded board vendors, the kernel doesn't require the device-tree
141	to represent every device in the system and only requires some nodes
142	and properties to be present. This will be described in detail in
143	section III, but, for example, the kernel does not require you to
144	create a node for every PCI device in the system. It is a requirement
145	to have a node for PCI host bridges in order to provide interrupt
146	routing information and memory/IO ranges, among others. It is also
147	recommended to define nodes for on chip devices and other buses that
148	don't specifically fit in an existing OF specification. This creates a
149	great flexibility in the way the kernel can then probe those and match
150	drivers to device, without having to hard code all sorts of tables. It
151	also makes it more flexible for board vendors to do minor hardware
152	upgrades without significantly impacting the kernel code or cluttering
153	it with special cases.
154	
155	
156	1) Entry point for arch/arm
157	---------------------------
158	
159	   There is one single entry point to the kernel, at the start
160	   of the kernel image. That entry point supports two calling
161	   conventions.  A summary of the interface is described here.  A full
162	   description of the boot requirements is documented in
163	   Documentation/arm/Booting
164	
165	        a) ATAGS interface.  Minimal information is passed from firmware
166	        to the kernel with a tagged list of predefined parameters.
167	
168	                r0 : 0
169	
170	                r1 : Machine type number
171	
172	                r2 : Physical address of tagged list in system RAM
173	
174	        b) Entry with a flattened device-tree block.  Firmware loads the
175	        physical address of the flattened device tree block (dtb) into r2,
176	        r1 is not used, but it is considered good practice to use a valid
177	        machine number as described in Documentation/arm/Booting.
178	
179	                r0 : 0
180	
181	                r1 : Valid machine type number.  When using a device tree,
182	                a single machine type number will often be assigned to
183	                represent a class or family of SoCs.
184	
185	                r2 : physical pointer to the device-tree block
186	                (defined in chapter II) in RAM.  Device tree can be located
187	                anywhere in system RAM, but it should be aligned on a 64 bit
188	                boundary.
189	
190	   The kernel will differentiate between ATAGS and device tree booting by
191	   reading the memory pointed to by r2 and looking for either the flattened
192	   device tree block magic value (0xd00dfeed) or the ATAG_CORE value at
193	   offset 0x4 from r2 (0x54410001).
194	
195	2) Entry point for arch/powerpc
196	-------------------------------
197	
198	   There is one single entry point to the kernel, at the start
199	   of the kernel image. That entry point supports two calling
200	   conventions:
201	
202	        a) Boot from Open Firmware. If your firmware is compatible
203	        with Open Firmware (IEEE 1275) or provides an OF compatible
204	        client interface API (support for "interpret" callback of
205	        forth words isn't required), you can enter the kernel with:
206	
207	              r5 : OF callback pointer as defined by IEEE 1275
208	              bindings to powerpc. Only the 32-bit client interface
209	              is currently supported
210	
211	              r3, r4 : address & length of an initrd if any or 0
212	
213	              The MMU is either on or off; the kernel will run the
214	              trampoline located in arch/powerpc/kernel/prom_init.c to
215	              extract the device-tree and other information from open
216	              firmware and build a flattened device-tree as described
217	              in b). prom_init() will then re-enter the kernel using
218	              the second method. This trampoline code runs in the
219	              context of the firmware, which is supposed to handle all
220	              exceptions during that time.
221	
222	        b) Direct entry with a flattened device-tree block. This entry
223	        point is called by a) after the OF trampoline and can also be
224	        called directly by a bootloader that does not support the Open
225	        Firmware client interface. It is also used by "kexec" to
226	        implement "hot" booting of a new kernel from a previous
227	        running one. This method is what I will describe in more
228	        details in this document, as method a) is simply standard Open
229	        Firmware, and thus should be implemented according to the
230	        various standard documents defining it and its binding to the
231	        PowerPC platform. The entry point definition then becomes:
232	
233	                r3 : physical pointer to the device-tree block
234	                (defined in chapter II) in RAM
235	
236	                r4 : physical pointer to the kernel itself. This is
237	                used by the assembly code to properly disable the MMU
238	                in case you are entering the kernel with MMU enabled
239	                and a non-1:1 mapping.
240	
241	                r5 : NULL (as to differentiate with method a)
242	
243	        Note about SMP entry: Either your firmware puts your other
244	        CPUs in some sleep loop or spin loop in ROM where you can get
245	        them out via a soft reset or some other means, in which case
246	        you don't need to care, or you'll have to enter the kernel
247	        with all CPUs. The way to do that with method b) will be
248	        described in a later revision of this document.
249	
250	   Board supports (platforms) are not exclusive config options. An
251	   arbitrary set of board supports can be built in a single kernel
252	   image. The kernel will "know" what set of functions to use for a
253	   given platform based on the content of the device-tree. Thus, you
254	   should:
255	
256	        a) add your platform support as a _boolean_ option in
257	        arch/powerpc/Kconfig, following the example of PPC_PSERIES,
258	        PPC_PMAC and PPC_MAPLE. The later is probably a good
259	        example of a board support to start from.
260	
261	        b) create your main platform file as
262	        "arch/powerpc/platforms/myplatform/myboard_setup.c" and add it
263	        to the Makefile under the condition of your CONFIG_
264	        option. This file will define a structure of type "ppc_md"
265	        containing the various callbacks that the generic code will
266	        use to get to your platform specific code
267	
268	  A kernel image may support multiple platforms, but only if the
269	  platforms feature the same core architecture.  A single kernel build
270	  cannot support both configurations with Book E and configurations
271	  with classic Powerpc architectures.
272	
273	3) Entry point for arch/x86
274	-------------------------------
275	
276	  There is one single 32bit entry point to the kernel at code32_start,
277	  the decompressor (the real mode entry point goes to the same  32bit
278	  entry point once it switched into protected mode). That entry point
279	  supports one calling convention which is documented in
280	  Documentation/x86/boot.txt
281	  The physical pointer to the device-tree block (defined in chapter II)
282	  is passed via setup_data which requires at least boot protocol 2.09.
283	  The type filed is defined as
284	
285	  #define SETUP_DTB                      2
286	
287	  This device-tree is used as an extension to the "boot page". As such it
288	  does not parse / consider data which is already covered by the boot
289	  page. This includes memory size, reserved ranges, command line arguments
290	  or initrd address. It simply holds information which can not be retrieved
291	  otherwise like interrupt routing or a list of devices behind an I2C bus.
292	
293	4) Entry point for arch/mips/bmips
294	----------------------------------
295	
296	  Some bootloaders only support a single entry point, at the start of the
297	  kernel image.  Other bootloaders will jump to the ELF start address.
298	  Both schemes are supported; CONFIG_BOOT_RAW=y and CONFIG_NO_EXCEPT_FILL=y,
299	  so the first instruction immediately jumps to kernel_entry().
300	
301	  Similar to the arch/arm case (b), a DT-aware bootloader is expected to
302	  set up the following registers:
303	
304	         a0 : 0
305	
306	         a1 : 0xffffffff
307	
308	         a2 : Physical pointer to the device tree block (defined in chapter
309	         II) in RAM.  The device tree can be located anywhere in the first
310	         512MB of the physical address space (0x00000000 - 0x1fffffff),
311	         aligned on a 64 bit boundary.
312	
313	  Legacy bootloaders do not use this convention, and they do not pass in a
314	  DT block.  In this case, Linux will look for a builtin DTB, selected via
315	  CONFIG_DT_*.
316	
317	  This convention is defined for 32-bit systems only, as there are not
318	  currently any 64-bit BMIPS implementations.
319	
320	5) Entry point for arch/sh
321	--------------------------
322	
323	  Device-tree-compatible SH bootloaders are expected to provide the physical
324	  address of the device tree blob in r4. Since legacy bootloaders did not
325	  guarantee any particular initial register state, kernels built to
326	  inter-operate with old bootloaders must either use a builtin DTB or
327	  select a legacy board option (something other than CONFIG_SH_DEVICE_TREE)
328	  that does not use device tree. Support for the latter is being phased out
329	  in favor of device tree.
330	
331	
332	II - The DT block format
333	========================
334	
335	
336	This chapter defines the actual format of the flattened device-tree
337	passed to the kernel. The actual content of it and kernel requirements
338	are described later. You can find example of code manipulating that
339	format in various places, including arch/powerpc/kernel/prom_init.c
340	which will generate a flattened device-tree from the Open Firmware
341	representation, or the fs2dt utility which is part of the kexec tools
342	which will generate one from a filesystem representation. It is
343	expected that a bootloader like uboot provides a bit more support,
344	that will be discussed later as well.
345	
346	Note: The block has to be in main memory. It has to be accessible in
347	both real mode and virtual mode with no mapping other than main
348	memory. If you are writing a simple flash bootloader, it should copy
349	the block to RAM before passing it to the kernel.
350	
351	
352	1) Header
353	---------
354	
355	   The kernel is passed the physical address pointing to an area of memory
356	   that is roughly described in include/linux/of_fdt.h by the structure
357	   boot_param_header:
358	
359	struct boot_param_header {
360	        u32     magic;                  /* magic word OF_DT_HEADER */
361	        u32     totalsize;              /* total size of DT block */
362	        u32     off_dt_struct;          /* offset to structure */
363	        u32     off_dt_strings;         /* offset to strings */
364	        u32     off_mem_rsvmap;         /* offset to memory reserve map
365	                                           */
366	        u32     version;                /* format version */
367	        u32     last_comp_version;      /* last compatible version */
368	
369	        /* version 2 fields below */
370	        u32     boot_cpuid_phys;        /* Which physical CPU id we're
371	                                           booting on */
372	        /* version 3 fields below */
373	        u32     size_dt_strings;        /* size of the strings block */
374	
375	        /* version 17 fields below */
376	        u32	size_dt_struct;		/* size of the DT structure block */
377	};
378	
379	   Along with the constants:
380	
381	/* Definitions used by the flattened device tree */
382	#define OF_DT_HEADER            0xd00dfeed      /* 4: version,
383							   4: total size */
384	#define OF_DT_BEGIN_NODE        0x1             /* Start node: full name
385							   */
386	#define OF_DT_END_NODE          0x2             /* End node */
387	#define OF_DT_PROP              0x3             /* Property: name off,
388	                                                   size, content */
389	#define OF_DT_END               0x9
390	
391	   All values in this header are in big endian format, the various
392	   fields in this header are defined more precisely below. All
393	   "offset" values are in bytes from the start of the header; that is
394	   from the physical base address of the device tree block.
395	
396	   - magic
397	
398	     This is a magic value that "marks" the beginning of the
399	     device-tree block header. It contains the value 0xd00dfeed and is
400	     defined by the constant OF_DT_HEADER
401	
402	   - totalsize
403	
404	     This is the total size of the DT block including the header. The
405	     "DT" block should enclose all data structures defined in this
406	     chapter (who are pointed to by offsets in this header). That is,
407	     the device-tree structure, strings, and the memory reserve map.
408	
409	   - off_dt_struct
410	
411	     This is an offset from the beginning of the header to the start
412	     of the "structure" part the device tree. (see 2) device tree)
413	
414	   - off_dt_strings
415	
416	     This is an offset from the beginning of the header to the start
417	     of the "strings" part of the device-tree
418	
419	   - off_mem_rsvmap
420	
421	     This is an offset from the beginning of the header to the start
422	     of the reserved memory map. This map is a list of pairs of 64-
423	     bit integers. Each pair is a physical address and a size. The
424	     list is terminated by an entry of size 0. This map provides the
425	     kernel with a list of physical memory areas that are "reserved"
426	     and thus not to be used for memory allocations, especially during
427	     early initialization. The kernel needs to allocate memory during
428	     boot for things like un-flattening the device-tree, allocating an
429	     MMU hash table, etc... Those allocations must be done in such a
430	     way to avoid overriding critical things like, on Open Firmware
431	     capable machines, the RTAS instance, or on some pSeries, the TCE
432	     tables used for the iommu. Typically, the reserve map should
433	     contain _at least_ this DT block itself (header,total_size). If
434	     you are passing an initrd to the kernel, you should reserve it as
435	     well. You do not need to reserve the kernel image itself. The map
436	     should be 64-bit aligned.
437	
438	   - version
439	
440	     This is the version of this structure. Version 1 stops
441	     here. Version 2 adds an additional field boot_cpuid_phys.
442	     Version 3 adds the size of the strings block, allowing the kernel
443	     to reallocate it easily at boot and free up the unused flattened
444	     structure after expansion. Version 16 introduces a new more
445	     "compact" format for the tree itself that is however not backward
446	     compatible. Version 17 adds an additional field, size_dt_struct,
447	     allowing it to be reallocated or moved more easily (this is
448	     particularly useful for bootloaders which need to make
449	     adjustments to a device tree based on probed information). You
450	     should always generate a structure of the highest version defined
451	     at the time of your implementation. Currently that is version 17,
452	     unless you explicitly aim at being backward compatible.
453	
454	   - last_comp_version
455	
456	     Last compatible version. This indicates down to what version of
457	     the DT block you are backward compatible. For example, version 2
458	     is backward compatible with version 1 (that is, a kernel build
459	     for version 1 will be able to boot with a version 2 format). You
460	     should put a 1 in this field if you generate a device tree of
461	     version 1 to 3, or 16 if you generate a tree of version 16 or 17
462	     using the new unit name format.
463	
464	   - boot_cpuid_phys
465	
466	     This field only exist on version 2 headers. It indicate which
467	     physical CPU ID is calling the kernel entry point. This is used,
468	     among others, by kexec. If you are on an SMP system, this value
469	     should match the content of the "reg" property of the CPU node in
470	     the device-tree corresponding to the CPU calling the kernel entry
471	     point (see further chapters for more information on the required
472	     device-tree contents)
473	
474	   - size_dt_strings
475	
476	     This field only exists on version 3 and later headers.  It
477	     gives the size of the "strings" section of the device tree (which
478	     starts at the offset given by off_dt_strings).
479	
480	   - size_dt_struct
481	
482	     This field only exists on version 17 and later headers.  It gives
483	     the size of the "structure" section of the device tree (which
484	     starts at the offset given by off_dt_struct).
485	
486	   So the typical layout of a DT block (though the various parts don't
487	   need to be in that order) looks like this (addresses go from top to
488	   bottom):
489	
490	
491	             ------------------------------
492	     base -> |  struct boot_param_header  |
493	             ------------------------------
494	             |      (alignment gap) (*)   |
495	             ------------------------------
496	             |      memory reserve map    |
497	             ------------------------------
498	             |      (alignment gap)       |
499	             ------------------------------
500	             |                            |
501	             |    device-tree structure   |
502	             |                            |
503	             ------------------------------
504	             |      (alignment gap)       |
505	             ------------------------------
506	             |                            |
507	             |     device-tree strings    |
508	             |                            |
509	      -----> ------------------------------
510	      |
511	      |
512	      --- (base + totalsize)
513	
514	  (*) The alignment gaps are not necessarily present; their presence
515	      and size are dependent on the various alignment requirements of
516	      the individual data blocks.
517	
518	
519	2) Device tree generalities
520	---------------------------
521	
522	This device-tree itself is separated in two different blocks, a
523	structure block and a strings block. Both need to be aligned to a 4
524	byte boundary.
525	
526	First, let's quickly describe the device-tree concept before detailing
527	the storage format. This chapter does _not_ describe the detail of the
528	required types of nodes & properties for the kernel, this is done
529	later in chapter III.
530	
531	The device-tree layout is strongly inherited from the definition of
532	the Open Firmware IEEE 1275 device-tree. It's basically a tree of
533	nodes, each node having two or more named properties. A property can
534	have a value or not.
535	
536	It is a tree, so each node has one and only one parent except for the
537	root node who has no parent.
538	
539	A node has 2 names. The actual node name is generally contained in a
540	property of type "name" in the node property list whose value is a
541	zero terminated string and is mandatory for version 1 to 3 of the
542	format definition (as it is in Open Firmware). Version 16 makes it
543	optional as it can generate it from the unit name defined below.
544	
545	There is also a "unit name" that is used to differentiate nodes with
546	the same name at the same level, it is usually made of the node
547	names, the "@" sign, and a "unit address", which definition is
548	specific to the bus type the node sits on.
549	
550	The unit name doesn't exist as a property per-se but is included in
551	the device-tree structure. It is typically used to represent "path" in
552	the device-tree. More details about the actual format of these will be
553	below.
554	
555	The kernel generic code does not make any formal use of the
556	unit address (though some board support code may do) so the only real
557	requirement here for the unit address is to ensure uniqueness of
558	the node unit name at a given level of the tree. Nodes with no notion
559	of address and no possible sibling of the same name (like /memory or
560	/cpus) may omit the unit address in the context of this specification,
561	or use the "@0" default unit address. The unit name is used to define
562	a node "full path", which is the concatenation of all parent node
563	unit names separated with "/".
564	
565	The root node doesn't have a defined name, and isn't required to have
566	a name property either if you are using version 3 or earlier of the
567	format. It also has no unit address (no @ symbol followed by a unit
568	address). The root node unit name is thus an empty string. The full
569	path to the root node is "/".
570	
571	Every node which actually represents an actual device (that is, a node
572	which isn't only a virtual "container" for more nodes, like "/cpus"
573	is) is also required to have a "compatible" property indicating the
574	specific hardware and an optional list of devices it is fully
575	backwards compatible with.
576	
577	Finally, every node that can be referenced from a property in another
578	node is required to have either a "phandle" or a "linux,phandle"
579	property. Real Open Firmware implementations provide a unique
580	"phandle" value for every node that the "prom_init()" trampoline code
581	turns into "linux,phandle" properties. However, this is made optional
582	if the flattened device tree is used directly. An example of a node
583	referencing another node via "phandle" is when laying out the
584	interrupt tree which will be described in a further version of this
585	document.
586	
587	The "phandle" property is a 32-bit value that uniquely
588	identifies a node. You are free to use whatever values or system of
589	values, internal pointers, or whatever to generate these, the only
590	requirement is that every node for which you provide that property has
591	a unique value for it.
592	
593	Here is an example of a simple device-tree. In this example, an "o"
594	designates a node followed by the node unit name. Properties are
595	presented with their name followed by their content. "content"
596	represents an ASCII string (zero terminated) value, while <content>
597	represents a 32-bit value, specified in decimal or hexadecimal (the
598	latter prefixed 0x). The various nodes in this example will be
599	discussed in a later chapter. At this point, it is only meant to give
600	you a idea of what a device-tree looks like. I have purposefully kept
601	the "name" and "linux,phandle" properties which aren't necessary in
602	order to give you a better idea of what the tree looks like in
603	practice.
604	
605	  / o device-tree
606	      |- name = "device-tree"
607	      |- model = "MyBoardName"
608	      |- compatible = "MyBoardFamilyName"
609	      |- #address-cells = <2>
610	      |- #size-cells = <2>
611	      |- linux,phandle = <0>
612	      |
613	      o cpus
614	      | | - name = "cpus"
615	      | | - linux,phandle = <1>
616	      | | - #address-cells = <1>
617	      | | - #size-cells = <0>
618	      | |
619	      | o PowerPC,970@0
620	      |   |- name = "PowerPC,970"
621	      |   |- device_type = "cpu"
622	      |   |- reg = <0>
623	      |   |- clock-frequency = <0x5f5e1000>
624	      |   |- 64-bit
625	      |   |- linux,phandle = <2>
626	      |
627	      o memory@0
628	      | |- name = "memory"
629	      | |- device_type = "memory"
630	      | |- reg = <0x00000000 0x00000000 0x00000000 0x20000000>
631	      | |- linux,phandle = <3>
632	      |
633	      o chosen
634	        |- name = "chosen"
635	        |- bootargs = "root=/dev/sda2"
636	        |- linux,phandle = <4>
637	
638	This tree is almost a minimal tree. It pretty much contains the
639	minimal set of required nodes and properties to boot a linux kernel;
640	that is, some basic model information at the root, the CPUs, and the
641	physical memory layout.  It also includes misc information passed
642	through /chosen, like in this example, the platform type (mandatory)
643	and the kernel command line arguments (optional).
644	
645	The /cpus/PowerPC,970@0/64-bit property is an example of a
646	property without a value. All other properties have a value. The
647	significance of the #address-cells and #size-cells properties will be
648	explained in chapter IV which defines precisely the required nodes and
649	properties and their content.
650	
651	
652	3) Device tree "structure" block
653	
654	The structure of the device tree is a linearized tree structure. The
655	"OF_DT_BEGIN_NODE" token starts a new node, and the "OF_DT_END_NODE"
656	ends that node definition. Child nodes are simply defined before
657	"OF_DT_END_NODE" (that is nodes within the node). A 'token' is a 32
658	bit value. The tree has to be "finished" with a OF_DT_END token
659	
660	Here's the basic structure of a single node:
661	
662	     * token OF_DT_BEGIN_NODE (that is 0x00000001)
663	     * for version 1 to 3, this is the node full path as a zero
664	       terminated string, starting with "/". For version 16 and later,
665	       this is the node unit name only (or an empty string for the
666	       root node)
667	     * [align gap to next 4 bytes boundary]
668	     * for each property:
669	        * token OF_DT_PROP (that is 0x00000003)
670	        * 32-bit value of property value size in bytes (or 0 if no
671	          value)
672	        * 32-bit value of offset in string block of property name
673	        * property value data if any
674	        * [align gap to next 4 bytes boundary]
675	     * [child nodes if any]
676	     * token OF_DT_END_NODE (that is 0x00000002)
677	
678	So the node content can be summarized as a start token, a full path,
679	a list of properties, a list of child nodes, and an end token. Every
680	child node is a full node structure itself as defined above.
681	
682	NOTE: The above definition requires that all property definitions for
683	a particular node MUST precede any subnode definitions for that node.
684	Although the structure would not be ambiguous if properties and
685	subnodes were intermingled, the kernel parser requires that the
686	properties come first (up until at least 2.6.22).  Any tools
687	manipulating a flattened tree must take care to preserve this
688	constraint.
689	
690	4) Device tree "strings" block
691	
692	In order to save space, property names, which are generally redundant,
693	are stored separately in the "strings" block. This block is simply the
694	whole bunch of zero terminated strings for all property names
695	concatenated together. The device-tree property definitions in the
696	structure block will contain offset values from the beginning of the
697	strings block.
698	
699	
700	III - Required content of the device tree
701	=========================================
702	
703	WARNING: All "linux,*" properties defined in this document apply only
704	to a flattened device-tree. If your platform uses a real
705	implementation of Open Firmware or an implementation compatible with
706	the Open Firmware client interface, those properties will be created
707	by the trampoline code in the kernel's prom_init() file. For example,
708	that's where you'll have to add code to detect your board model and
709	set the platform number. However, when using the flattened device-tree
710	entry point, there is no prom_init() pass, and thus you have to
711	provide those properties yourself.
712	
713	
714	1) Note about cells and address representation
715	----------------------------------------------
716	
717	The general rule is documented in the various Open Firmware
718	documentations. If you choose to describe a bus with the device-tree
719	and there exist an OF bus binding, then you should follow the
720	specification. However, the kernel does not require every single
721	device or bus to be described by the device tree.
722	
723	In general, the format of an address for a device is defined by the
724	parent bus type, based on the #address-cells and #size-cells
725	properties.  Note that the parent's parent definitions of #address-cells
726	and #size-cells are not inherited so every node with children must specify
727	them.  The kernel requires the root node to have those properties defining
728	addresses format for devices directly mapped on the processor bus.
729	
730	Those 2 properties define 'cells' for representing an address and a
731	size. A "cell" is a 32-bit number. For example, if both contain 2
732	like the example tree given above, then an address and a size are both
733	composed of 2 cells, and each is a 64-bit number (cells are
734	concatenated and expected to be in big endian format). Another example
735	is the way Apple firmware defines them, with 2 cells for an address
736	and one cell for a size.  Most 32-bit implementations should define
737	#address-cells and #size-cells to 1, which represents a 32-bit value.
738	Some 32-bit processors allow for physical addresses greater than 32
739	bits; these processors should define #address-cells as 2.
740	
741	"reg" properties are always a tuple of the type "address size" where
742	the number of cells of address and size is specified by the bus
743	#address-cells and #size-cells. When a bus supports various address
744	spaces and other flags relative to a given address allocation (like
745	prefetchable, etc...) those flags are usually added to the top level
746	bits of the physical address. For example, a PCI physical address is
747	made of 3 cells, the bottom two containing the actual address itself
748	while the top cell contains address space indication, flags, and pci
749	bus & device numbers.
750	
751	For buses that support dynamic allocation, it's the accepted practice
752	to then not provide the address in "reg" (keep it 0) though while
753	providing a flag indicating the address is dynamically allocated, and
754	then, to provide a separate "assigned-addresses" property that
755	contains the fully allocated addresses. See the PCI OF bindings for
756	details.
757	
758	In general, a simple bus with no address space bits and no dynamic
759	allocation is preferred if it reflects your hardware, as the existing
760	kernel address parsing functions will work out of the box. If you
761	define a bus type with a more complex address format, including things
762	like address space bits, you'll have to add a bus translator to the
763	prom_parse.c file of the recent kernels for your bus type.
764	
765	The "reg" property only defines addresses and sizes (if #size-cells is
766	non-0) within a given bus. In order to translate addresses upward
767	(that is into parent bus addresses, and possibly into CPU physical
768	addresses), all buses must contain a "ranges" property. If the
769	"ranges" property is missing at a given level, it's assumed that
770	translation isn't possible, i.e., the registers are not visible on the
771	parent bus.  The format of the "ranges" property for a bus is a list
772	of:
773	
774		bus address, parent bus address, size
775	
776	"bus address" is in the format of the bus this bus node is defining,
777	that is, for a PCI bridge, it would be a PCI address. Thus, (bus
778	address, size) defines a range of addresses for child devices. "parent
779	bus address" is in the format of the parent bus of this bus. For
780	example, for a PCI host controller, that would be a CPU address. For a
781	PCI<->ISA bridge, that would be a PCI address. It defines the base
782	address in the parent bus where the beginning of that range is mapped.
783	
784	For new 64-bit board support, I recommend either the 2/2 format or
785	Apple's 2/1 format which is slightly more compact since sizes usually
786	fit in a single 32-bit word.   New 32-bit board support should use a
787	1/1 format, unless the processor supports physical addresses greater
788	than 32-bits, in which case a 2/1 format is recommended.
789	
790	Alternatively, the "ranges" property may be empty, indicating that the
791	registers are visible on the parent bus using an identity mapping
792	translation.  In other words, the parent bus address space is the same
793	as the child bus address space.
794	
795	2) Note about "compatible" properties
796	-------------------------------------
797	
798	These properties are optional, but recommended in devices and the root
799	node. The format of a "compatible" property is a list of concatenated
800	zero terminated strings. They allow a device to express its
801	compatibility with a family of similar devices, in some cases,
802	allowing a single driver to match against several devices regardless
803	of their actual names.
804	
805	3) Note about "name" properties
806	-------------------------------
807	
808	While earlier users of Open Firmware like OldWorld macintoshes tended
809	to use the actual device name for the "name" property, it's nowadays
810	considered a good practice to use a name that is closer to the device
811	class (often equal to device_type). For example, nowadays, Ethernet
812	controllers are named "ethernet", an additional "model" property
813	defining precisely the chip type/model, and "compatible" property
814	defining the family in case a single driver can driver more than one
815	of these chips. However, the kernel doesn't generally put any
816	restriction on the "name" property; it is simply considered good
817	practice to follow the standard and its evolutions as closely as
818	possible.
819	
820	Note also that the new format version 16 makes the "name" property
821	optional. If it's absent for a node, then the node's unit name is then
822	used to reconstruct the name. That is, the part of the unit name
823	before the "@" sign is used (or the entire unit name if no "@" sign
824	is present).
825	
826	4) Note about node and property names and character set
827	-------------------------------------------------------
828	
829	While Open Firmware provides more flexible usage of 8859-1, this
830	specification enforces more strict rules. Nodes and properties should
831	be comprised only of ASCII characters 'a' to 'z', '0' to
832	'9', ',', '.', '_', '+', '#', '?', and '-'. Node names additionally
833	allow uppercase characters 'A' to 'Z' (property names should be
834	lowercase. The fact that vendors like Apple don't respect this rule is
835	irrelevant here). Additionally, node and property names should always
836	begin with a character in the range 'a' to 'z' (or 'A' to 'Z' for node
837	names).
838	
839	The maximum number of characters for both nodes and property names
840	is 31. In the case of node names, this is only the leftmost part of
841	a unit name (the pure "name" property), it doesn't include the unit
842	address which can extend beyond that limit.
843	
844	
845	5) Required nodes and properties
846	--------------------------------
847	  These are all that are currently required. However, it is strongly
848	  recommended that you expose PCI host bridges as documented in the
849	  PCI binding to Open Firmware, and your interrupt tree as documented
850	  in OF interrupt tree specification.
851	
852	  a) The root node
853	
854	  The root node requires some properties to be present:
855	
856	    - model : this is your board name/model
857	    - #address-cells : address representation for "root" devices
858	    - #size-cells: the size representation for "root" devices
859	    - compatible : the board "family" generally finds its way here,
860	      for example, if you have 2 board models with a similar layout,
861	      that typically get driven by the same platform code in the
862	      kernel, you would specify the exact board model in the
863	      compatible property followed by an entry that represents the SoC
864	      model.
865	
866	  The root node is also generally where you add additional properties
867	  specific to your board like the serial number if any, that sort of
868	  thing. It is recommended that if you add any "custom" property whose
869	  name may clash with standard defined ones, you prefix them with your
870	  vendor name and a comma.
871	
872	  Additional properties for the root node:
873	
874	    - serial-number : a string representing the device's serial number
875	
876	  b) The /cpus node
877	
878	  This node is the parent of all individual CPU nodes. It doesn't
879	  have any specific requirements, though it's generally good practice
880	  to have at least:
881	
882	               #address-cells = <00000001>
883	               #size-cells    = <00000000>
884	
885	  This defines that the "address" for a CPU is a single cell, and has
886	  no meaningful size. This is not necessary but the kernel will assume
887	  that format when reading the "reg" properties of a CPU node, see
888	  below
889	
890	  c) The /cpus/* nodes
891	
892	  So under /cpus, you are supposed to create a node for every CPU on
893	  the machine. There is no specific restriction on the name of the
894	  CPU, though it's common to call it <architecture>,<core>. For
895	  example, Apple uses PowerPC,G5 while IBM uses PowerPC,970FX.
896	  However, the Generic Names convention suggests that it would be
897	  better to simply use 'cpu' for each cpu node and use the compatible
898	  property to identify the specific cpu core.
899	
900	  Required properties:
901	
902	    - device_type : has to be "cpu"
903	    - reg : This is the physical CPU number, it's a single 32-bit cell
904	      and is also used as-is as the unit number for constructing the
905	      unit name in the full path. For example, with 2 CPUs, you would
906	      have the full path:
907	        /cpus/PowerPC,970FX@0
908	        /cpus/PowerPC,970FX@1
909	      (unit addresses do not require leading zeroes)
910	    - d-cache-block-size : one cell, L1 data cache block size in bytes (*)
911	    - i-cache-block-size : one cell, L1 instruction cache block size in
912	      bytes
913	    - d-cache-size : one cell, size of L1 data cache in bytes
914	    - i-cache-size : one cell, size of L1 instruction cache in bytes
915	
916	(*) The cache "block" size is the size on which the cache management
917	instructions operate. Historically, this document used the cache
918	"line" size here which is incorrect. The kernel will prefer the cache
919	block size and will fallback to cache line size for backward
920	compatibility.
921	
922	  Recommended properties:
923	
924	    - timebase-frequency : a cell indicating the frequency of the
925	      timebase in Hz. This is not directly used by the generic code,
926	      but you are welcome to copy/paste the pSeries code for setting
927	      the kernel timebase/decrementer calibration based on this
928	      value.
929	    - clock-frequency : a cell indicating the CPU core clock frequency
930	      in Hz. A new property will be defined for 64-bit values, but if
931	      your frequency is < 4Ghz, one cell is enough. Here as well as
932	      for the above, the common code doesn't use that property, but
933	      you are welcome to re-use the pSeries or Maple one. A future
934	      kernel version might provide a common function for this.
935	    - d-cache-line-size : one cell, L1 data cache line size in bytes
936	      if different from the block size
937	    - i-cache-line-size : one cell, L1 instruction cache line size in
938	      bytes if different from the block size
939	
940	  You are welcome to add any property you find relevant to your board,
941	  like some information about the mechanism used to soft-reset the
942	  CPUs. For example, Apple puts the GPIO number for CPU soft reset
943	  lines in there as a "soft-reset" property since they start secondary
944	  CPUs by soft-resetting them.
945	
946	
947	  d) the /memory node(s)
948	
949	  To define the physical memory layout of your board, you should
950	  create one or more memory node(s). You can either create a single
951	  node with all memory ranges in its reg property, or you can create
952	  several nodes, as you wish. The unit address (@ part) used for the
953	  full path is the address of the first range of memory defined by a
954	  given node. If you use a single memory node, this will typically be
955	  @0.
956	
957	  Required properties:
958	
959	    - device_type : has to be "memory"
960	    - reg : This property contains all the physical memory ranges of
961	      your board. It's a list of addresses/sizes concatenated
962	      together, with the number of cells of each defined by the
963	      #address-cells and #size-cells of the root node. For example,
964	      with both of these properties being 2 like in the example given
965	      earlier, a 970 based machine with 6Gb of RAM could typically
966	      have a "reg" property here that looks like:
967	
968	      00000000 00000000 00000000 80000000
969	      00000001 00000000 00000001 00000000
970	
971	      That is a range starting at 0 of 0x80000000 bytes and a range
972	      starting at 0x100000000 and of 0x100000000 bytes. You can see
973	      that there is no memory covering the IO hole between 2Gb and
974	      4Gb. Some vendors prefer splitting those ranges into smaller
975	      segments, but the kernel doesn't care.
976	
977	  Additional properties:
978	
979	    - hotpluggable : The presence of this property provides an explicit
980	      hint to the operating system that this memory may potentially be
981	      removed later. The kernel can take this into consideration when
982	      doing nonmovable allocations and when laying out memory zones.
983	
984	  e) The /chosen node
985	
986	  This node is a bit "special". Normally, that's where Open Firmware
987	  puts some variable environment information, like the arguments, or
988	  the default input/output devices.
989	
990	  This specification makes a few of these mandatory, but also defines
991	  some linux-specific properties that would be normally constructed by
992	  the prom_init() trampoline when booting with an OF client interface,
993	  but that you have to provide yourself when using the flattened format.
994	
995	  Recommended properties:
996	
997	    - bootargs : This zero-terminated string is passed as the kernel
998	      command line
999	    - linux,stdout-path : This is the full path to your standard
1000	      console device if any. Typically, if you have serial devices on
1001	      your board, you may want to put the full path to the one set as
1002	      the default console in the firmware here, for the kernel to pick
1003	      it up as its own default console.
1004	
1005	  Note that u-boot creates and fills in the chosen node for platforms
1006	  that use it.
1007	
1008	  (Note: a practice that is now obsolete was to include a property
1009	  under /chosen called interrupt-controller which had a phandle value
1010	  that pointed to the main interrupt controller)
1011	
1012	  f) the /soc<SOCname> node
1013	
1014	  This node is used to represent a system-on-a-chip (SoC) and must be
1015	  present if the processor is a SoC. The top-level soc node contains
1016	  information that is global to all devices on the SoC. The node name
1017	  should contain a unit address for the SoC, which is the base address
1018	  of the memory-mapped register set for the SoC. The name of an SoC
1019	  node should start with "soc", and the remainder of the name should
1020	  represent the part number for the soc.  For example, the MPC8540's
1021	  soc node would be called "soc8540".
1022	
1023	  Required properties:
1024	
1025	    - ranges : Should be defined as specified in 1) to describe the
1026	      translation of SoC addresses for memory mapped SoC registers.
1027	    - bus-frequency: Contains the bus frequency for the SoC node.
1028	      Typically, the value of this field is filled in by the boot
1029	      loader.
1030	    - compatible : Exact model of the SoC
1031	
1032	
1033	  Recommended properties:
1034	
1035	    - reg : This property defines the address and size of the
1036	      memory-mapped registers that are used for the SOC node itself.
1037	      It does not include the child device registers - these will be
1038	      defined inside each child node.  The address specified in the
1039	      "reg" property should match the unit address of the SOC node.
1040	    - #address-cells : Address representation for "soc" devices.  The
1041	      format of this field may vary depending on whether or not the
1042	      device registers are memory mapped.  For memory mapped
1043	      registers, this field represents the number of cells needed to
1044	      represent the address of the registers.  For SOCs that do not
1045	      use MMIO, a special address format should be defined that
1046	      contains enough cells to represent the required information.
1047	      See 1) above for more details on defining #address-cells.
1048	    - #size-cells : Size representation for "soc" devices
1049	    - #interrupt-cells : Defines the width of cells used to represent
1050	       interrupts.  Typically this value is <2>, which includes a
1051	       32-bit number that represents the interrupt number, and a
1052	       32-bit number that represents the interrupt sense and level.
1053	       This field is only needed if the SOC contains an interrupt
1054	       controller.
1055	
1056	  The SOC node may contain child nodes for each SOC device that the
1057	  platform uses.  Nodes should not be created for devices which exist
1058	  on the SOC but are not used by a particular platform. See chapter VI
1059	  for more information on how to specify devices that are part of a SOC.
1060	
1061	  Example SOC node for the MPC8540:
1062	
1063		soc8540@e0000000 {
1064			#address-cells = <1>;
1065			#size-cells = <1>;
1066			#interrupt-cells = <2>;
1067			device_type = "soc";
1068			ranges = <0x00000000 0xe0000000 0x00100000>
1069			reg = <0xe0000000 0x00003000>;
1070			bus-frequency = <0>;
1071		}
1072	
1073	
1074	
1075	IV - "dtc", the device tree compiler
1076	====================================
1077	
1078	
1079	dtc source code can be found at
1080	<http://git.jdl.com/gitweb/?p=dtc.git>
1081	
1082	WARNING: This version is still in early development stage; the
1083	resulting device-tree "blobs" have not yet been validated with the
1084	kernel. The current generated block lacks a useful reserve map (it will
1085	be fixed to generate an empty one, it's up to the bootloader to fill
1086	it up) among others. The error handling needs work, bugs are lurking,
1087	etc...
1088	
1089	dtc basically takes a device-tree in a given format and outputs a
1090	device-tree in another format. The currently supported formats are:
1091	
1092	  Input formats:
1093	  -------------
1094	
1095	     - "dtb": "blob" format, that is a flattened device-tree block
1096	       with
1097	        header all in a binary blob.
1098	     - "dts": "source" format. This is a text file containing a
1099	       "source" for a device-tree. The format is defined later in this
1100	        chapter.
1101	     - "fs" format. This is a representation equivalent to the
1102	        output of /proc/device-tree, that is nodes are directories and
1103		properties are files
1104	
1105	 Output formats:
1106	 ---------------
1107	
1108	     - "dtb": "blob" format
1109	     - "dts": "source" format
1110	     - "asm": assembly language file. This is a file that can be
1111	       sourced by gas to generate a device-tree "blob". That file can
1112	       then simply be added to your Makefile. Additionally, the
1113	       assembly file exports some symbols that can be used.
1114	
1115	
1116	The syntax of the dtc tool is
1117	
1118	    dtc [-I <input-format>] [-O <output-format>]
1119	        [-o output-filename] [-V output_version] input_filename
1120	
1121	
1122	The "output_version" defines what version of the "blob" format will be
1123	generated. Supported versions are 1,2,3 and 16. The default is
1124	currently version 3 but that may change in the future to version 16.
1125	
1126	Additionally, dtc performs various sanity checks on the tree, like the
1127	uniqueness of linux, phandle properties, validity of strings, etc...
1128	
1129	The format of the .dts "source" file is "C" like, supports C and C++
1130	style comments.
1131	
1132	/ {
1133	}
1134	
1135	The above is the "device-tree" definition. It's the only statement
1136	supported currently at the toplevel.
1137	
1138	/ {
1139	  property1 = "string_value";	/* define a property containing a 0
1140	                                 * terminated string
1141					 */
1142	
1143	  property2 = <0x1234abcd>;	/* define a property containing a
1144	                                 * numerical 32-bit value (hexadecimal)
1145					 */
1146	
1147	  property3 = <0x12345678 0x12345678 0xdeadbeef>;
1148	                                /* define a property containing 3
1149	                                 * numerical 32-bit values (cells) in
1150	                                 * hexadecimal
1151					 */
1152	  property4 = [0x0a 0x0b 0x0c 0x0d 0xde 0xea 0xad 0xbe 0xef];
1153	                                /* define a property whose content is
1154	                                 * an arbitrary array of bytes
1155	                                 */
1156	
1157	  childnode@address {	/* define a child node named "childnode"
1158	                                 * whose unit name is "childnode at
1159					 * address"
1160	                                 */
1161	
1162	    childprop = "hello\n";      /* define a property "childprop" of
1163	                                 * childnode (in this case, a string)
1164	                                 */
1165	  };
1166	};
1167	
1168	Nodes can contain other nodes etc... thus defining the hierarchical
1169	structure of the tree.
1170	
1171	Strings support common escape sequences from C: "\n", "\t", "\r",
1172	"\(octal value)", "\x(hex value)".
1173	
1174	It is also suggested that you pipe your source file through cpp (gcc
1175	preprocessor) so you can use #include's, #define for constants, etc...
1176	
1177	Finally, various options are planned but not yet implemented, like
1178	automatic generation of phandles, labels (exported to the asm file so
1179	you can point to a property content and change it easily from whatever
1180	you link the device-tree with), label or path instead of numeric value
1181	in some cells to "point" to a node (replaced by a phandle at compile
1182	time), export of reserve map address to the asm file, ability to
1183	specify reserve map content at compile time, etc...
1184	
1185	We may provide a .h include file with common definitions of that
1186	proves useful for some properties (like building PCI properties or
1187	interrupt maps) though it may be better to add a notion of struct
1188	definitions to the compiler...
1189	
1190	
1191	V - Recommendations for a bootloader
1192	====================================
1193	
1194	
1195	Here are some various ideas/recommendations that have been proposed
1196	while all this has been defined and implemented.
1197	
1198	  - The bootloader may want to be able to use the device-tree itself
1199	    and may want to manipulate it (to add/edit some properties,
1200	    like physical memory size or kernel arguments). At this point, 2
1201	    choices can be made. Either the bootloader works directly on the
1202	    flattened format, or the bootloader has its own internal tree
1203	    representation with pointers (similar to the kernel one) and
1204	    re-flattens the tree when booting the kernel. The former is a bit
1205	    more difficult to edit/modify, the later requires probably a bit
1206	    more code to handle the tree structure. Note that the structure
1207	    format has been designed so it's relatively easy to "insert"
1208	    properties or nodes or delete them by just memmoving things
1209	    around. It contains no internal offsets or pointers for this
1210	    purpose.
1211	
1212	  - An example of code for iterating nodes & retrieving properties
1213	    directly from the flattened tree format can be found in the kernel
1214	    file drivers/of/fdt.c.  Look at the of_scan_flat_dt() function,
1215	    its usage in early_init_devtree(), and the corresponding various
1216	    early_init_dt_scan_*() callbacks. That code can be re-used in a
1217	    GPL bootloader, and as the author of that code, I would be happy
1218	    to discuss possible free licensing to any vendor who wishes to
1219	    integrate all or part of this code into a non-GPL bootloader.
1220	    (reference needed; who is 'I' here? ---gcl Jan 31, 2011)
1221	
1222	
1223	
1224	VI - System-on-a-chip devices and nodes
1225	=======================================
1226	
1227	Many companies are now starting to develop system-on-a-chip
1228	processors, where the processor core (CPU) and many peripheral devices
1229	exist on a single piece of silicon.  For these SOCs, an SOC node
1230	should be used that defines child nodes for the devices that make
1231	up the SOC. While platforms are not required to use this model in
1232	order to boot the kernel, it is highly encouraged that all SOC
1233	implementations define as complete a flat-device-tree as possible to
1234	describe the devices on the SOC.  This will allow for the
1235	genericization of much of the kernel code.
1236	
1237	
1238	1) Defining child nodes of an SOC
1239	---------------------------------
1240	
1241	Each device that is part of an SOC may have its own node entry inside
1242	the SOC node.  For each device that is included in the SOC, the unit
1243	address property represents the address offset for this device's
1244	memory-mapped registers in the parent's address space.  The parent's
1245	address space is defined by the "ranges" property in the top-level soc
1246	node. The "reg" property for each node that exists directly under the
1247	SOC node should contain the address mapping from the child address space
1248	to the parent SOC address space and the size of the device's
1249	memory-mapped register file.
1250	
1251	For many devices that may exist inside an SOC, there are predefined
1252	specifications for the format of the device tree node.  All SOC child
1253	nodes should follow these specifications, except where noted in this
1254	document.
1255	
1256	See appendix A for an example partial SOC node definition for the
1257	MPC8540.
1258	
1259	
1260	2) Representing devices without a current OF specification
1261	----------------------------------------------------------
1262	
1263	Currently, there are many devices on SoCs that do not have a standard
1264	representation defined as part of the Open Firmware specifications,
1265	mainly because the boards that contain these SoCs are not currently
1266	booted using Open Firmware.  Binding documentation for new devices
1267	should be added to the Documentation/devicetree/bindings directory.
1268	That directory will expand as device tree support is added to more and
1269	more SoCs.
1270	
1271	
1272	VII - Specifying interrupt information for devices
1273	===================================================
1274	
1275	The device tree represents the buses and devices of a hardware
1276	system in a form similar to the physical bus topology of the
1277	hardware.
1278	
1279	In addition, a logical 'interrupt tree' exists which represents the
1280	hierarchy and routing of interrupts in the hardware.
1281	
1282	The interrupt tree model is fully described in the
1283	document "Open Firmware Recommended Practice: Interrupt
1284	Mapping Version 0.9".  The document is available at:
1285	<http://www.openfirmware.org/ofwg/practice/>
1286	
1287	1) interrupts property
1288	----------------------
1289	
1290	Devices that generate interrupts to a single interrupt controller
1291	should use the conventional OF representation described in the
1292	OF interrupt mapping documentation.
1293	
1294	Each device which generates interrupts must have an 'interrupt'
1295	property.  The interrupt property value is an arbitrary number of
1296	of 'interrupt specifier' values which describe the interrupt or
1297	interrupts for the device.
1298	
1299	The encoding of an interrupt specifier is determined by the
1300	interrupt domain in which the device is located in the
1301	interrupt tree.  The root of an interrupt domain specifies in
1302	its #interrupt-cells property the number of 32-bit cells
1303	required to encode an interrupt specifier.  See the OF interrupt
1304	mapping documentation for a detailed description of domains.
1305	
1306	For example, the binding for the OpenPIC interrupt controller
1307	specifies  an #interrupt-cells value of 2 to encode the interrupt
1308	number and level/sense information. All interrupt children in an
1309	OpenPIC interrupt domain use 2 cells per interrupt in their interrupts
1310	property.
1311	
1312	The PCI bus binding specifies a #interrupt-cell value of 1 to encode
1313	which interrupt pin (INTA,INTB,INTC,INTD) is used.
1314	
1315	2) interrupt-parent property
1316	----------------------------
1317	
1318	The interrupt-parent property is specified to define an explicit
1319	link between a device node and its interrupt parent in
1320	the interrupt tree.  The value of interrupt-parent is the
1321	phandle of the parent node.
1322	
1323	If the interrupt-parent property is not defined for a node, its
1324	interrupt parent is assumed to be an ancestor in the node's
1325	_device tree_ hierarchy.
1326	
1327	3) OpenPIC Interrupt Controllers
1328	--------------------------------
1329	
1330	OpenPIC interrupt controllers require 2 cells to encode
1331	interrupt information.  The first cell defines the interrupt
1332	number.  The second cell defines the sense and level
1333	information.
1334	
1335	Sense and level information should be encoded as follows:
1336	
1337		0 = low to high edge sensitive type enabled
1338		1 = active low level sensitive type enabled
1339		2 = active high level sensitive type enabled
1340		3 = high to low edge sensitive type enabled
1341	
1342	4) ISA Interrupt Controllers
1343	----------------------------
1344	
1345	ISA PIC interrupt controllers require 2 cells to encode
1346	interrupt information.  The first cell defines the interrupt
1347	number.  The second cell defines the sense and level
1348	information.
1349	
1350	ISA PIC interrupt controllers should adhere to the ISA PIC
1351	encodings listed below:
1352	
1353		0 =  active low level sensitive type enabled
1354		1 =  active high level sensitive type enabled
1355		2 =  high to low edge sensitive type enabled
1356		3 =  low to high edge sensitive type enabled
1357	
1358	VIII - Specifying Device Power Management Information (sleep property)
1359	===================================================================
1360	
1361	Devices on SOCs often have mechanisms for placing devices into low-power
1362	states that are decoupled from the devices' own register blocks.  Sometimes,
1363	this information is more complicated than a cell-index property can
1364	reasonably describe.  Thus, each device controlled in such a manner
1365	may contain a "sleep" property which describes these connections.
1366	
1367	The sleep property consists of one or more sleep resources, each of
1368	which consists of a phandle to a sleep controller, followed by a
1369	controller-specific sleep specifier of zero or more cells.
1370	
1371	The semantics of what type of low power modes are possible are defined
1372	by the sleep controller.  Some examples of the types of low power modes
1373	that may be supported are:
1374	
1375	 - Dynamic: The device may be disabled or enabled at any time.
1376	 - System Suspend: The device may request to be disabled or remain
1377	   awake during system suspend, but will not be disabled until then.
1378	 - Permanent: The device is disabled permanently (until the next hard
1379	   reset).
1380	
1381	Some devices may share a clock domain with each other, such that they should
1382	only be suspended when none of the devices are in use.  Where reasonable,
1383	such nodes should be placed on a virtual bus, where the bus has the sleep
1384	property.  If the clock domain is shared among devices that cannot be
1385	reasonably grouped in this manner, then create a virtual sleep controller
1386	(similar to an interrupt nexus, except that defining a standardized
1387	sleep-map should wait until its necessity is demonstrated).
1388	
1389	IX - Specifying dma bus information
1390	
1391	Some devices may have DMA memory range shifted relatively to the beginning of
1392	RAM, or even placed outside of kernel RAM. For example, the Keystone 2 SoC
1393	worked in LPAE mode with 4G memory has:
1394	- RAM range: [0x8 0000 0000, 0x8 FFFF FFFF]
1395	- DMA range: [  0x8000 0000,   0xFFFF FFFF]
1396	and DMA range is aliased into first 2G of RAM in HW.
1397	
1398	In such cases, DMA addresses translation should be performed between CPU phys
1399	and DMA addresses. The "dma-ranges" property is intended to be used
1400	for describing the configuration of such system in DT.
1401	
1402	In addition, each DMA master device on the DMA bus may or may not support
1403	coherent DMA operations. The "dma-coherent" property is intended to be used
1404	for identifying devices supported coherent DMA operations in DT.
1405	
1406	* DMA Bus master
1407	Optional property:
1408	- dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
1409		(child-bus-address, parent-bus-address, length). Each triplet specified
1410		describes a contiguous DMA address range.
1411		The dma-ranges property is used to describe the direct memory access (DMA)
1412		structure of a memory-mapped bus whose device tree parent can be accessed
1413		from DMA operations originating from the bus. It provides a means of
1414		defining a mapping or translation between the physical address space of
1415		the bus and the physical address space of the parent of the bus.
1416		(for more information see the Devicetree Specification)
1417	
1418	* DMA Bus child
1419	Optional property:
1420	- dma-ranges: <empty> value. if present - It means that DMA addresses
1421		translation has to be enabled for this device.
1422	- dma-coherent: Present if dma operations are coherent
1423	
1424	Example:
1425	soc {
1426			compatible = "ti,keystone","simple-bus";
1427			ranges = <0x0 0x0 0x0 0xc0000000>;
1428			dma-ranges = <0x80000000 0x8 0x00000000 0x80000000>;
1429	
1430			[...]
1431	
1432			usb: usb@2680000 {
1433				compatible = "ti,keystone-dwc3";
1434	
1435				[...]
1436				dma-coherent;
1437			};
1438	};
1439	
1440	Appendix A - Sample SOC node for MPC8540
1441	========================================
1442	
1443		soc@e0000000 {
1444			#address-cells = <1>;
1445			#size-cells = <1>;
1446			compatible = "fsl,mpc8540-ccsr", "simple-bus";
1447			device_type = "soc";
1448			ranges = <0x00000000 0xe0000000 0x00100000>
1449			bus-frequency = <0>;
1450			interrupt-parent = <&pic>;
1451	
1452			ethernet@24000 {
1453				#address-cells = <1>;
1454				#size-cells = <1>;
1455				device_type = "network";
1456				model = "TSEC";
1457				compatible = "gianfar", "simple-bus";
1458				reg = <0x24000 0x1000>;
1459				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x00 ];
1460				interrupts = <0x29 2 0x30 2 0x34 2>;
1461				phy-handle = <&phy0>;
1462				sleep = <&pmc 0x00000080>;
1463				ranges;
1464	
1465				mdio@24520 {
1466					reg = <0x24520 0x20>;
1467					compatible = "fsl,gianfar-mdio";
1468	
1469					phy0: ethernet-phy@0 {
1470						interrupts = <5 1>;
1471						reg = <0>;
1472					};
1473	
1474					phy1: ethernet-phy@1 {
1475						interrupts = <5 1>;
1476						reg = <1>;
1477					};
1478	
1479					phy3: ethernet-phy@3 {
1480						interrupts = <7 1>;
1481						reg = <3>;
1482					};
1483				};
1484			};
1485	
1486			ethernet@25000 {
1487				device_type = "network";
1488				model = "TSEC";
1489				compatible = "gianfar";
1490				reg = <0x25000 0x1000>;
1491				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x01 ];
1492				interrupts = <0x13 2 0x14 2 0x18 2>;
1493				phy-handle = <&phy1>;
1494				sleep = <&pmc 0x00000040>;
1495			};
1496	
1497			ethernet@26000 {
1498				device_type = "network";
1499				model = "FEC";
1500				compatible = "gianfar";
1501				reg = <0x26000 0x1000>;
1502				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x02 ];
1503				interrupts = <0x41 2>;
1504				phy-handle = <&phy3>;
1505				sleep = <&pmc 0x00000020>;
1506			};
1507	
1508			serial@4500 {
1509				#address-cells = <1>;
1510				#size-cells = <1>;
1511				compatible = "fsl,mpc8540-duart", "simple-bus";
1512				sleep = <&pmc 0x00000002>;
1513				ranges;
1514	
1515				serial@4500 {
1516					device_type = "serial";
1517					compatible = "ns16550";
1518					reg = <0x4500 0x100>;
1519					clock-frequency = <0>;
1520					interrupts = <0x42 2>;
1521				};
1522	
1523				serial@4600 {
1524					device_type = "serial";
1525					compatible = "ns16550";
1526					reg = <0x4600 0x100>;
1527					clock-frequency = <0>;
1528					interrupts = <0x42 2>;
1529				};
1530			};
1531	
1532			pic: pic@40000 {
1533				interrupt-controller;
1534				#address-cells = <0>;
1535				#interrupt-cells = <2>;
1536				reg = <0x40000 0x40000>;
1537				compatible = "chrp,open-pic";
1538				device_type = "open-pic";
1539			};
1540	
1541			i2c@3000 {
1542				interrupts = <0x43 2>;
1543				reg = <0x3000 0x100>;
1544				compatible  = "fsl-i2c";
1545				dfsrr;
1546				sleep = <&pmc 0x00000004>;
1547			};
1548	
1549			pmc: power@e0070 {
1550				compatible = "fsl,mpc8540-pmc", "fsl,mpc8548-pmc";
1551				reg = <0xe0070 0x20>;
1552			};
1553		};
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