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Based on kernel version 3.19. Page generated on 2015-02-13 21:19 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	
19	  II - The DT block format
20	    1) Header
21	    2) Device tree generalities
22	    3) Device tree "structure" block
23	    4) Device tree "strings" block
24	
25	  III - Required content of the device tree
26	    1) Note about cells and address representation
27	    2) Note about "compatible" properties
28	    3) Note about "name" properties
29	    4) Note about node and property names and character set
30	    5) Required nodes and properties
31	      a) The root node
32	      b) The /cpus node
33	      c) The /cpus/* nodes
34	      d) the /memory node(s)
35	      e) The /chosen node
36	      f) the /soc<SOCname> node
37	
38	  IV - "dtc", the device tree compiler
39	
40	  V - Recommendations for a bootloader
41	
42	  VI - System-on-a-chip devices and nodes
43	    1) Defining child nodes of an SOC
44	    2) Representing devices without a current OF specification
45	
46	  VII - Specifying interrupt information for devices
47	    1) interrupts property
48	    2) interrupt-parent property
49	    3) OpenPIC Interrupt Controllers
50	    4) ISA Interrupt Controllers
51	
52	  VIII - Specifying device power management information (sleep property)
53	
54	  IX - Specifying dma bus information
55	
56	  Appendix A - Sample SOC node for MPC8540
57	
58	
59	Revision Information
60	====================
61	
62	   May 18, 2005: Rev 0.1 - Initial draft, no chapter III yet.
63	
64	   May 19, 2005: Rev 0.2 - Add chapter III and bits & pieces here or
65	                           clarifies the fact that a lot of things are
66	                           optional, the kernel only requires a very
67	                           small device tree, though it is encouraged
68	                           to provide an as complete one as possible.
69	
70	   May 24, 2005: Rev 0.3 - Precise that DT block has to be in RAM
71				 - Misc fixes
72				 - Define version 3 and new format version 16
73				   for the DT block (version 16 needs kernel
74				   patches, will be fwd separately).
75				   String block now has a size, and full path
76				   is replaced by unit name for more
77				   compactness.
78				   linux,phandle is made optional, only nodes
79				   that are referenced by other nodes need it.
80				   "name" property is now automatically
81				   deduced from the unit name
82	
83	   June 1, 2005: Rev 0.4 - Correct confusion between OF_DT_END and
84	                           OF_DT_END_NODE in structure definition.
85	                         - Change version 16 format to always align
86	                           property data to 4 bytes. Since tokens are
87	                           already aligned, that means no specific
88	                           required alignment between property size
89	                           and property data. The old style variable
90	                           alignment would make it impossible to do
91	                           "simple" insertion of properties using
92	                           memmove (thanks Milton for
93	                           noticing). Updated kernel patch as well
94				 - Correct a few more alignment constraints
95				 - Add a chapter about the device-tree
96	                           compiler and the textural representation of
97	                           the tree that can be "compiled" by dtc.
98	
99	   November 21, 2005: Rev 0.5
100				 - Additions/generalizations for 32-bit
101				 - Changed to reflect the new arch/powerpc
102				   structure
103				 - Added chapter VI
104	
105	
106	 ToDo:
107		- Add some definitions of interrupt tree (simple/complex)
108		- Add some definitions for PCI host bridges
109		- Add some common address format examples
110		- Add definitions for standard properties and "compatible"
111		  names for cells that are not already defined by the existing
112		  OF spec.
113		- Compare FSL SOC use of PCI to standard and make sure no new
114		  node definition required.
115		- Add more information about node definitions for SOC devices
116	  	  that currently have no standard, like the FSL CPM.
117	
118	
119	I - Introduction
120	================
121	
122	During the development of the Linux/ppc64 kernel, and more
123	specifically, the addition of new platform types outside of the old
124	IBM pSeries/iSeries pair, it was decided to enforce some strict rules
125	regarding the kernel entry and bootloader <-> kernel interfaces, in
126	order to avoid the degeneration that had become the ppc32 kernel entry
127	point and the way a new platform should be added to the kernel. The
128	legacy iSeries platform breaks those rules as it predates this scheme,
129	but no new board support will be accepted in the main tree that
130	doesn't follow them properly.  In addition, since the advent of the
131	arch/powerpc merged architecture for ppc32 and ppc64, new 32-bit
132	platforms and 32-bit platforms which move into arch/powerpc will be
133	required to use these rules as well.
134	
135	The main requirement that will be defined in more detail below is
136	the presence of a device-tree whose format is defined after Open
137	Firmware specification. However, in order to make life easier
138	to embedded board vendors, the kernel doesn't require the device-tree
139	to represent every device in the system and only requires some nodes
140	and properties to be present. This will be described in detail in
141	section III, but, for example, the kernel does not require you to
142	create a node for every PCI device in the system. It is a requirement
143	to have a node for PCI host bridges in order to provide interrupt
144	routing information and memory/IO ranges, among others. It is also
145	recommended to define nodes for on chip devices and other buses that
146	don't specifically fit in an existing OF specification. This creates a
147	great flexibility in the way the kernel can then probe those and match
148	drivers to device, without having to hard code all sorts of tables. It
149	also makes it more flexible for board vendors to do minor hardware
150	upgrades without significantly impacting the kernel code or cluttering
151	it with special cases.
152	
153	
154	1) Entry point for arch/arm
155	---------------------------
156	
157	   There is one single entry point to the kernel, at the start
158	   of the kernel image. That entry point supports two calling
159	   conventions.  A summary of the interface is described here.  A full
160	   description of the boot requirements is documented in
161	   Documentation/arm/Booting
162	
163	        a) ATAGS interface.  Minimal information is passed from firmware
164	        to the kernel with a tagged list of predefined parameters.
165	
166	                r0 : 0
167	
168	                r1 : Machine type number
169	
170	                r2 : Physical address of tagged list in system RAM
171	
172	        b) Entry with a flattened device-tree block.  Firmware loads the
173	        physical address of the flattened device tree block (dtb) into r2,
174	        r1 is not used, but it is considered good practice to use a valid
175	        machine number as described in Documentation/arm/Booting.
176	
177	                r0 : 0
178	
179	                r1 : Valid machine type number.  When using a device tree,
180	                a single machine type number will often be assigned to
181	                represent a class or family of SoCs.
182	
183	                r2 : physical pointer to the device-tree block
184	                (defined in chapter II) in RAM.  Device tree can be located
185	                anywhere in system RAM, but it should be aligned on a 64 bit
186	                boundary.
187	
188	   The kernel will differentiate between ATAGS and device tree booting by
189	   reading the memory pointed to by r2 and looking for either the flattened
190	   device tree block magic value (0xd00dfeed) or the ATAG_CORE value at
191	   offset 0x4 from r2 (0x54410001).
192	
193	2) Entry point for arch/powerpc
194	-------------------------------
195	
196	   There is one single entry point to the kernel, at the start
197	   of the kernel image. That entry point supports two calling
198	   conventions:
199	
200	        a) Boot from Open Firmware. If your firmware is compatible
201	        with Open Firmware (IEEE 1275) or provides an OF compatible
202	        client interface API (support for "interpret" callback of
203	        forth words isn't required), you can enter the kernel with:
204	
205	              r5 : OF callback pointer as defined by IEEE 1275
206	              bindings to powerpc. Only the 32-bit client interface
207	              is currently supported
208	
209	              r3, r4 : address & length of an initrd if any or 0
210	
211	              The MMU is either on or off; the kernel will run the
212	              trampoline located in arch/powerpc/kernel/prom_init.c to
213	              extract the device-tree and other information from open
214	              firmware and build a flattened device-tree as described
215	              in b). prom_init() will then re-enter the kernel using
216	              the second method. This trampoline code runs in the
217	              context of the firmware, which is supposed to handle all
218	              exceptions during that time.
219	
220	        b) Direct entry with a flattened device-tree block. This entry
221	        point is called by a) after the OF trampoline and can also be
222	        called directly by a bootloader that does not support the Open
223	        Firmware client interface. It is also used by "kexec" to
224	        implement "hot" booting of a new kernel from a previous
225	        running one. This method is what I will describe in more
226	        details in this document, as method a) is simply standard Open
227	        Firmware, and thus should be implemented according to the
228	        various standard documents defining it and its binding to the
229	        PowerPC platform. The entry point definition then becomes:
230	
231	                r3 : physical pointer to the device-tree block
232	                (defined in chapter II) in RAM
233	
234	                r4 : physical pointer to the kernel itself. This is
235	                used by the assembly code to properly disable the MMU
236	                in case you are entering the kernel with MMU enabled
237	                and a non-1:1 mapping.
238	
239	                r5 : NULL (as to differentiate with method a)
240	
241	        Note about SMP entry: Either your firmware puts your other
242	        CPUs in some sleep loop or spin loop in ROM where you can get
243	        them out via a soft reset or some other means, in which case
244	        you don't need to care, or you'll have to enter the kernel
245	        with all CPUs. The way to do that with method b) will be
246	        described in a later revision of this document.
247	
248	   Board supports (platforms) are not exclusive config options. An
249	   arbitrary set of board supports can be built in a single kernel
250	   image. The kernel will "know" what set of functions to use for a
251	   given platform based on the content of the device-tree. Thus, you
252	   should:
253	
254	        a) add your platform support as a _boolean_ option in
255	        arch/powerpc/Kconfig, following the example of PPC_PSERIES,
256	        PPC_PMAC and PPC_MAPLE. The later is probably a good
257	        example of a board support to start from.
258	
259	        b) create your main platform file as
260	        "arch/powerpc/platforms/myplatform/myboard_setup.c" and add it
261	        to the Makefile under the condition of your CONFIG_
262	        option. This file will define a structure of type "ppc_md"
263	        containing the various callbacks that the generic code will
264	        use to get to your platform specific code
265	
266	  A kernel image may support multiple platforms, but only if the
267	  platforms feature the same core architecture.  A single kernel build
268	  cannot support both configurations with Book E and configurations
269	  with classic Powerpc architectures.
270	
271	3) Entry point for arch/x86
272	-------------------------------
273	
274	  There is one single 32bit entry point to the kernel at code32_start,
275	  the decompressor (the real mode entry point goes to the same  32bit
276	  entry point once it switched into protected mode). That entry point
277	  supports one calling convention which is documented in
278	  Documentation/x86/boot.txt
279	  The physical pointer to the device-tree block (defined in chapter II)
280	  is passed via setup_data which requires at least boot protocol 2.09.
281	  The type filed is defined as
282	
283	  #define SETUP_DTB                      2
284	
285	  This device-tree is used as an extension to the "boot page". As such it
286	  does not parse / consider data which is already covered by the boot
287	  page. This includes memory size, reserved ranges, command line arguments
288	  or initrd address. It simply holds information which can not be retrieved
289	  otherwise like interrupt routing or a list of devices behind an I2C bus.
290	
291	II - The DT block format
292	========================
293	
294	
295	This chapter defines the actual format of the flattened device-tree
296	passed to the kernel. The actual content of it and kernel requirements
297	are described later. You can find example of code manipulating that
298	format in various places, including arch/powerpc/kernel/prom_init.c
299	which will generate a flattened device-tree from the Open Firmware
300	representation, or the fs2dt utility which is part of the kexec tools
301	which will generate one from a filesystem representation. It is
302	expected that a bootloader like uboot provides a bit more support,
303	that will be discussed later as well.
304	
305	Note: The block has to be in main memory. It has to be accessible in
306	both real mode and virtual mode with no mapping other than main
307	memory. If you are writing a simple flash bootloader, it should copy
308	the block to RAM before passing it to the kernel.
309	
310	
311	1) Header
312	---------
313	
314	   The kernel is passed the physical address pointing to an area of memory
315	   that is roughly described in include/linux/of_fdt.h by the structure
316	   boot_param_header:
317	
318	struct boot_param_header {
319	        u32     magic;                  /* magic word OF_DT_HEADER */
320	        u32     totalsize;              /* total size of DT block */
321	        u32     off_dt_struct;          /* offset to structure */
322	        u32     off_dt_strings;         /* offset to strings */
323	        u32     off_mem_rsvmap;         /* offset to memory reserve map
324	                                           */
325	        u32     version;                /* format version */
326	        u32     last_comp_version;      /* last compatible version */
327	
328	        /* version 2 fields below */
329	        u32     boot_cpuid_phys;        /* Which physical CPU id we're
330	                                           booting on */
331	        /* version 3 fields below */
332	        u32     size_dt_strings;        /* size of the strings block */
333	
334	        /* version 17 fields below */
335	        u32	size_dt_struct;		/* size of the DT structure block */
336	};
337	
338	   Along with the constants:
339	
340	/* Definitions used by the flattened device tree */
341	#define OF_DT_HEADER            0xd00dfeed      /* 4: version,
342							   4: total size */
343	#define OF_DT_BEGIN_NODE        0x1             /* Start node: full name
344							   */
345	#define OF_DT_END_NODE          0x2             /* End node */
346	#define OF_DT_PROP              0x3             /* Property: name off,
347	                                                   size, content */
348	#define OF_DT_END               0x9
349	
350	   All values in this header are in big endian format, the various
351	   fields in this header are defined more precisely below. All
352	   "offset" values are in bytes from the start of the header; that is
353	   from the physical base address of the device tree block.
354	
355	   - magic
356	
357	     This is a magic value that "marks" the beginning of the
358	     device-tree block header. It contains the value 0xd00dfeed and is
359	     defined by the constant OF_DT_HEADER
360	
361	   - totalsize
362	
363	     This is the total size of the DT block including the header. The
364	     "DT" block should enclose all data structures defined in this
365	     chapter (who are pointed to by offsets in this header). That is,
366	     the device-tree structure, strings, and the memory reserve map.
367	
368	   - off_dt_struct
369	
370	     This is an offset from the beginning of the header to the start
371	     of the "structure" part the device tree. (see 2) device tree)
372	
373	   - off_dt_strings
374	
375	     This is an offset from the beginning of the header to the start
376	     of the "strings" part of the device-tree
377	
378	   - off_mem_rsvmap
379	
380	     This is an offset from the beginning of the header to the start
381	     of the reserved memory map. This map is a list of pairs of 64-
382	     bit integers. Each pair is a physical address and a size. The
383	     list is terminated by an entry of size 0. This map provides the
384	     kernel with a list of physical memory areas that are "reserved"
385	     and thus not to be used for memory allocations, especially during
386	     early initialization. The kernel needs to allocate memory during
387	     boot for things like un-flattening the device-tree, allocating an
388	     MMU hash table, etc... Those allocations must be done in such a
389	     way to avoid overriding critical things like, on Open Firmware
390	     capable machines, the RTAS instance, or on some pSeries, the TCE
391	     tables used for the iommu. Typically, the reserve map should
392	     contain _at least_ this DT block itself (header,total_size). If
393	     you are passing an initrd to the kernel, you should reserve it as
394	     well. You do not need to reserve the kernel image itself. The map
395	     should be 64-bit aligned.
396	
397	   - version
398	
399	     This is the version of this structure. Version 1 stops
400	     here. Version 2 adds an additional field boot_cpuid_phys.
401	     Version 3 adds the size of the strings block, allowing the kernel
402	     to reallocate it easily at boot and free up the unused flattened
403	     structure after expansion. Version 16 introduces a new more
404	     "compact" format for the tree itself that is however not backward
405	     compatible. Version 17 adds an additional field, size_dt_struct,
406	     allowing it to be reallocated or moved more easily (this is
407	     particularly useful for bootloaders which need to make
408	     adjustments to a device tree based on probed information). You
409	     should always generate a structure of the highest version defined
410	     at the time of your implementation. Currently that is version 17,
411	     unless you explicitly aim at being backward compatible.
412	
413	   - last_comp_version
414	
415	     Last compatible version. This indicates down to what version of
416	     the DT block you are backward compatible. For example, version 2
417	     is backward compatible with version 1 (that is, a kernel build
418	     for version 1 will be able to boot with a version 2 format). You
419	     should put a 1 in this field if you generate a device tree of
420	     version 1 to 3, or 16 if you generate a tree of version 16 or 17
421	     using the new unit name format.
422	
423	   - boot_cpuid_phys
424	
425	     This field only exist on version 2 headers. It indicate which
426	     physical CPU ID is calling the kernel entry point. This is used,
427	     among others, by kexec. If you are on an SMP system, this value
428	     should match the content of the "reg" property of the CPU node in
429	     the device-tree corresponding to the CPU calling the kernel entry
430	     point (see further chapters for more information on the required
431	     device-tree contents)
432	
433	   - size_dt_strings
434	
435	     This field only exists on version 3 and later headers.  It
436	     gives the size of the "strings" section of the device tree (which
437	     starts at the offset given by off_dt_strings).
438	
439	   - size_dt_struct
440	
441	     This field only exists on version 17 and later headers.  It gives
442	     the size of the "structure" section of the device tree (which
443	     starts at the offset given by off_dt_struct).
444	
445	   So the typical layout of a DT block (though the various parts don't
446	   need to be in that order) looks like this (addresses go from top to
447	   bottom):
448	
449	
450	             ------------------------------
451	     base -> |  struct boot_param_header  |
452	             ------------------------------
453	             |      (alignment gap) (*)   |
454	             ------------------------------
455	             |      memory reserve map    |
456	             ------------------------------
457	             |      (alignment gap)       |
458	             ------------------------------
459	             |                            |
460	             |    device-tree structure   |
461	             |                            |
462	             ------------------------------
463	             |      (alignment gap)       |
464	             ------------------------------
465	             |                            |
466	             |     device-tree strings    |
467	             |                            |
468	      -----> ------------------------------
469	      |
470	      |
471	      --- (base + totalsize)
472	
473	  (*) The alignment gaps are not necessarily present; their presence
474	      and size are dependent on the various alignment requirements of
475	      the individual data blocks.
476	
477	
478	2) Device tree generalities
479	---------------------------
480	
481	This device-tree itself is separated in two different blocks, a
482	structure block and a strings block. Both need to be aligned to a 4
483	byte boundary.
484	
485	First, let's quickly describe the device-tree concept before detailing
486	the storage format. This chapter does _not_ describe the detail of the
487	required types of nodes & properties for the kernel, this is done
488	later in chapter III.
489	
490	The device-tree layout is strongly inherited from the definition of
491	the Open Firmware IEEE 1275 device-tree. It's basically a tree of
492	nodes, each node having two or more named properties. A property can
493	have a value or not.
494	
495	It is a tree, so each node has one and only one parent except for the
496	root node who has no parent.
497	
498	A node has 2 names. The actual node name is generally contained in a
499	property of type "name" in the node property list whose value is a
500	zero terminated string and is mandatory for version 1 to 3 of the
501	format definition (as it is in Open Firmware). Version 16 makes it
502	optional as it can generate it from the unit name defined below.
503	
504	There is also a "unit name" that is used to differentiate nodes with
505	the same name at the same level, it is usually made of the node
506	names, the "@" sign, and a "unit address", which definition is
507	specific to the bus type the node sits on.
508	
509	The unit name doesn't exist as a property per-se but is included in
510	the device-tree structure. It is typically used to represent "path" in
511	the device-tree. More details about the actual format of these will be
512	below.
513	
514	The kernel generic code does not make any formal use of the
515	unit address (though some board support code may do) so the only real
516	requirement here for the unit address is to ensure uniqueness of
517	the node unit name at a given level of the tree. Nodes with no notion
518	of address and no possible sibling of the same name (like /memory or
519	/cpus) may omit the unit address in the context of this specification,
520	or use the "@0" default unit address. The unit name is used to define
521	a node "full path", which is the concatenation of all parent node
522	unit names separated with "/".
523	
524	The root node doesn't have a defined name, and isn't required to have
525	a name property either if you are using version 3 or earlier of the
526	format. It also has no unit address (no @ symbol followed by a unit
527	address). The root node unit name is thus an empty string. The full
528	path to the root node is "/".
529	
530	Every node which actually represents an actual device (that is, a node
531	which isn't only a virtual "container" for more nodes, like "/cpus"
532	is) is also required to have a "compatible" property indicating the
533	specific hardware and an optional list of devices it is fully
534	backwards compatible with.
535	
536	Finally, every node that can be referenced from a property in another
537	node is required to have either a "phandle" or a "linux,phandle"
538	property. Real Open Firmware implementations provide a unique
539	"phandle" value for every node that the "prom_init()" trampoline code
540	turns into "linux,phandle" properties. However, this is made optional
541	if the flattened device tree is used directly. An example of a node
542	referencing another node via "phandle" is when laying out the
543	interrupt tree which will be described in a further version of this
544	document.
545	
546	The "phandle" property is a 32-bit value that uniquely
547	identifies a node. You are free to use whatever values or system of
548	values, internal pointers, or whatever to generate these, the only
549	requirement is that every node for which you provide that property has
550	a unique value for it.
551	
552	Here is an example of a simple device-tree. In this example, an "o"
553	designates a node followed by the node unit name. Properties are
554	presented with their name followed by their content. "content"
555	represents an ASCII string (zero terminated) value, while <content>
556	represents a 32-bit value, specified in decimal or hexadecimal (the
557	latter prefixed 0x). The various nodes in this example will be
558	discussed in a later chapter. At this point, it is only meant to give
559	you a idea of what a device-tree looks like. I have purposefully kept
560	the "name" and "linux,phandle" properties which aren't necessary in
561	order to give you a better idea of what the tree looks like in
562	practice.
563	
564	  / o device-tree
565	      |- name = "device-tree"
566	      |- model = "MyBoardName"
567	      |- compatible = "MyBoardFamilyName"
568	      |- #address-cells = <2>
569	      |- #size-cells = <2>
570	      |- linux,phandle = <0>
571	      |
572	      o cpus
573	      | | - name = "cpus"
574	      | | - linux,phandle = <1>
575	      | | - #address-cells = <1>
576	      | | - #size-cells = <0>
577	      | |
578	      | o PowerPC,970@0
579	      |   |- name = "PowerPC,970"
580	      |   |- device_type = "cpu"
581	      |   |- reg = <0>
582	      |   |- clock-frequency = <0x5f5e1000>
583	      |   |- 64-bit
584	      |   |- linux,phandle = <2>
585	      |
586	      o memory@0
587	      | |- name = "memory"
588	      | |- device_type = "memory"
589	      | |- reg = <0x00000000 0x00000000 0x00000000 0x20000000>
590	      | |- linux,phandle = <3>
591	      |
592	      o chosen
593	        |- name = "chosen"
594	        |- bootargs = "root=/dev/sda2"
595	        |- linux,phandle = <4>
596	
597	This tree is almost a minimal tree. It pretty much contains the
598	minimal set of required nodes and properties to boot a linux kernel;
599	that is, some basic model information at the root, the CPUs, and the
600	physical memory layout.  It also includes misc information passed
601	through /chosen, like in this example, the platform type (mandatory)
602	and the kernel command line arguments (optional).
603	
604	The /cpus/PowerPC,970@0/64-bit property is an example of a
605	property without a value. All other properties have a value. The
606	significance of the #address-cells and #size-cells properties will be
607	explained in chapter IV which defines precisely the required nodes and
608	properties and their content.
609	
610	
611	3) Device tree "structure" block
612	
613	The structure of the device tree is a linearized tree structure. The
614	"OF_DT_BEGIN_NODE" token starts a new node, and the "OF_DT_END_NODE"
615	ends that node definition. Child nodes are simply defined before
616	"OF_DT_END_NODE" (that is nodes within the node). A 'token' is a 32
617	bit value. The tree has to be "finished" with a OF_DT_END token
618	
619	Here's the basic structure of a single node:
620	
621	     * token OF_DT_BEGIN_NODE (that is 0x00000001)
622	     * for version 1 to 3, this is the node full path as a zero
623	       terminated string, starting with "/". For version 16 and later,
624	       this is the node unit name only (or an empty string for the
625	       root node)
626	     * [align gap to next 4 bytes boundary]
627	     * for each property:
628	        * token OF_DT_PROP (that is 0x00000003)
629	        * 32-bit value of property value size in bytes (or 0 if no
630	          value)
631	        * 32-bit value of offset in string block of property name
632	        * property value data if any
633	        * [align gap to next 4 bytes boundary]
634	     * [child nodes if any]
635	     * token OF_DT_END_NODE (that is 0x00000002)
636	
637	So the node content can be summarized as a start token, a full path,
638	a list of properties, a list of child nodes, and an end token. Every
639	child node is a full node structure itself as defined above.
640	
641	NOTE: The above definition requires that all property definitions for
642	a particular node MUST precede any subnode definitions for that node.
643	Although the structure would not be ambiguous if properties and
644	subnodes were intermingled, the kernel parser requires that the
645	properties come first (up until at least 2.6.22).  Any tools
646	manipulating a flattened tree must take care to preserve this
647	constraint.
648	
649	4) Device tree "strings" block
650	
651	In order to save space, property names, which are generally redundant,
652	are stored separately in the "strings" block. This block is simply the
653	whole bunch of zero terminated strings for all property names
654	concatenated together. The device-tree property definitions in the
655	structure block will contain offset values from the beginning of the
656	strings block.
657	
658	
659	III - Required content of the device tree
660	=========================================
661	
662	WARNING: All "linux,*" properties defined in this document apply only
663	to a flattened device-tree. If your platform uses a real
664	implementation of Open Firmware or an implementation compatible with
665	the Open Firmware client interface, those properties will be created
666	by the trampoline code in the kernel's prom_init() file. For example,
667	that's where you'll have to add code to detect your board model and
668	set the platform number. However, when using the flattened device-tree
669	entry point, there is no prom_init() pass, and thus you have to
670	provide those properties yourself.
671	
672	
673	1) Note about cells and address representation
674	----------------------------------------------
675	
676	The general rule is documented in the various Open Firmware
677	documentations. If you choose to describe a bus with the device-tree
678	and there exist an OF bus binding, then you should follow the
679	specification. However, the kernel does not require every single
680	device or bus to be described by the device tree.
681	
682	In general, the format of an address for a device is defined by the
683	parent bus type, based on the #address-cells and #size-cells
684	properties.  Note that the parent's parent definitions of #address-cells
685	and #size-cells are not inherited so every node with children must specify
686	them.  The kernel requires the root node to have those properties defining
687	addresses format for devices directly mapped on the processor bus.
688	
689	Those 2 properties define 'cells' for representing an address and a
690	size. A "cell" is a 32-bit number. For example, if both contain 2
691	like the example tree given above, then an address and a size are both
692	composed of 2 cells, and each is a 64-bit number (cells are
693	concatenated and expected to be in big endian format). Another example
694	is the way Apple firmware defines them, with 2 cells for an address
695	and one cell for a size.  Most 32-bit implementations should define
696	#address-cells and #size-cells to 1, which represents a 32-bit value.
697	Some 32-bit processors allow for physical addresses greater than 32
698	bits; these processors should define #address-cells as 2.
699	
700	"reg" properties are always a tuple of the type "address size" where
701	the number of cells of address and size is specified by the bus
702	#address-cells and #size-cells. When a bus supports various address
703	spaces and other flags relative to a given address allocation (like
704	prefetchable, etc...) those flags are usually added to the top level
705	bits of the physical address. For example, a PCI physical address is
706	made of 3 cells, the bottom two containing the actual address itself
707	while the top cell contains address space indication, flags, and pci
708	bus & device numbers.
709	
710	For buses that support dynamic allocation, it's the accepted practice
711	to then not provide the address in "reg" (keep it 0) though while
712	providing a flag indicating the address is dynamically allocated, and
713	then, to provide a separate "assigned-addresses" property that
714	contains the fully allocated addresses. See the PCI OF bindings for
715	details.
716	
717	In general, a simple bus with no address space bits and no dynamic
718	allocation is preferred if it reflects your hardware, as the existing
719	kernel address parsing functions will work out of the box. If you
720	define a bus type with a more complex address format, including things
721	like address space bits, you'll have to add a bus translator to the
722	prom_parse.c file of the recent kernels for your bus type.
723	
724	The "reg" property only defines addresses and sizes (if #size-cells is
725	non-0) within a given bus. In order to translate addresses upward
726	(that is into parent bus addresses, and possibly into CPU physical
727	addresses), all buses must contain a "ranges" property. If the
728	"ranges" property is missing at a given level, it's assumed that
729	translation isn't possible, i.e., the registers are not visible on the
730	parent bus.  The format of the "ranges" property for a bus is a list
731	of:
732	
733		bus address, parent bus address, size
734	
735	"bus address" is in the format of the bus this bus node is defining,
736	that is, for a PCI bridge, it would be a PCI address. Thus, (bus
737	address, size) defines a range of addresses for child devices. "parent
738	bus address" is in the format of the parent bus of this bus. For
739	example, for a PCI host controller, that would be a CPU address. For a
740	PCI<->ISA bridge, that would be a PCI address. It defines the base
741	address in the parent bus where the beginning of that range is mapped.
742	
743	For new 64-bit board support, I recommend either the 2/2 format or
744	Apple's 2/1 format which is slightly more compact since sizes usually
745	fit in a single 32-bit word.   New 32-bit board support should use a
746	1/1 format, unless the processor supports physical addresses greater
747	than 32-bits, in which case a 2/1 format is recommended.
748	
749	Alternatively, the "ranges" property may be empty, indicating that the
750	registers are visible on the parent bus using an identity mapping
751	translation.  In other words, the parent bus address space is the same
752	as the child bus address space.
753	
754	2) Note about "compatible" properties
755	-------------------------------------
756	
757	These properties are optional, but recommended in devices and the root
758	node. The format of a "compatible" property is a list of concatenated
759	zero terminated strings. They allow a device to express its
760	compatibility with a family of similar devices, in some cases,
761	allowing a single driver to match against several devices regardless
762	of their actual names.
763	
764	3) Note about "name" properties
765	-------------------------------
766	
767	While earlier users of Open Firmware like OldWorld macintoshes tended
768	to use the actual device name for the "name" property, it's nowadays
769	considered a good practice to use a name that is closer to the device
770	class (often equal to device_type). For example, nowadays, Ethernet
771	controllers are named "ethernet", an additional "model" property
772	defining precisely the chip type/model, and "compatible" property
773	defining the family in case a single driver can driver more than one
774	of these chips. However, the kernel doesn't generally put any
775	restriction on the "name" property; it is simply considered good
776	practice to follow the standard and its evolutions as closely as
777	possible.
778	
779	Note also that the new format version 16 makes the "name" property
780	optional. If it's absent for a node, then the node's unit name is then
781	used to reconstruct the name. That is, the part of the unit name
782	before the "@" sign is used (or the entire unit name if no "@" sign
783	is present).
784	
785	4) Note about node and property names and character set
786	-------------------------------------------------------
787	
788	While Open Firmware provides more flexible usage of 8859-1, this
789	specification enforces more strict rules. Nodes and properties should
790	be comprised only of ASCII characters 'a' to 'z', '0' to
791	'9', ',', '.', '_', '+', '#', '?', and '-'. Node names additionally
792	allow uppercase characters 'A' to 'Z' (property names should be
793	lowercase. The fact that vendors like Apple don't respect this rule is
794	irrelevant here). Additionally, node and property names should always
795	begin with a character in the range 'a' to 'z' (or 'A' to 'Z' for node
796	names).
797	
798	The maximum number of characters for both nodes and property names
799	is 31. In the case of node names, this is only the leftmost part of
800	a unit name (the pure "name" property), it doesn't include the unit
801	address which can extend beyond that limit.
802	
803	
804	5) Required nodes and properties
805	--------------------------------
806	  These are all that are currently required. However, it is strongly
807	  recommended that you expose PCI host bridges as documented in the
808	  PCI binding to Open Firmware, and your interrupt tree as documented
809	  in OF interrupt tree specification.
810	
811	  a) The root node
812	
813	  The root node requires some properties to be present:
814	
815	    - model : this is your board name/model
816	    - #address-cells : address representation for "root" devices
817	    - #size-cells: the size representation for "root" devices
818	    - compatible : the board "family" generally finds its way here,
819	      for example, if you have 2 board models with a similar layout,
820	      that typically get driven by the same platform code in the
821	      kernel, you would specify the exact board model in the
822	      compatible property followed by an entry that represents the SoC
823	      model.
824	
825	  The root node is also generally where you add additional properties
826	  specific to your board like the serial number if any, that sort of
827	  thing. It is recommended that if you add any "custom" property whose
828	  name may clash with standard defined ones, you prefix them with your
829	  vendor name and a comma.
830	
831	  b) The /cpus node
832	
833	  This node is the parent of all individual CPU nodes. It doesn't
834	  have any specific requirements, though it's generally good practice
835	  to have at least:
836	
837	               #address-cells = <00000001>
838	               #size-cells    = <00000000>
839	
840	  This defines that the "address" for a CPU is a single cell, and has
841	  no meaningful size. This is not necessary but the kernel will assume
842	  that format when reading the "reg" properties of a CPU node, see
843	  below
844	
845	  c) The /cpus/* nodes
846	
847	  So under /cpus, you are supposed to create a node for every CPU on
848	  the machine. There is no specific restriction on the name of the
849	  CPU, though it's common to call it <architecture>,<core>. For
850	  example, Apple uses PowerPC,G5 while IBM uses PowerPC,970FX.
851	  However, the Generic Names convention suggests that it would be
852	  better to simply use 'cpu' for each cpu node and use the compatible
853	  property to identify the specific cpu core.
854	
855	  Required properties:
856	
857	    - device_type : has to be "cpu"
858	    - reg : This is the physical CPU number, it's a single 32-bit cell
859	      and is also used as-is as the unit number for constructing the
860	      unit name in the full path. For example, with 2 CPUs, you would
861	      have the full path:
862	        /cpus/PowerPC,970FX@0
863	        /cpus/PowerPC,970FX@1
864	      (unit addresses do not require leading zeroes)
865	    - d-cache-block-size : one cell, L1 data cache block size in bytes (*)
866	    - i-cache-block-size : one cell, L1 instruction cache block size in
867	      bytes
868	    - d-cache-size : one cell, size of L1 data cache in bytes
869	    - i-cache-size : one cell, size of L1 instruction cache in bytes
870	
871	(*) The cache "block" size is the size on which the cache management
872	instructions operate. Historically, this document used the cache
873	"line" size here which is incorrect. The kernel will prefer the cache
874	block size and will fallback to cache line size for backward
875	compatibility.
876	
877	  Recommended properties:
878	
879	    - timebase-frequency : a cell indicating the frequency of the
880	      timebase in Hz. This is not directly used by the generic code,
881	      but you are welcome to copy/paste the pSeries code for setting
882	      the kernel timebase/decrementer calibration based on this
883	      value.
884	    - clock-frequency : a cell indicating the CPU core clock frequency
885	      in Hz. A new property will be defined for 64-bit values, but if
886	      your frequency is < 4Ghz, one cell is enough. Here as well as
887	      for the above, the common code doesn't use that property, but
888	      you are welcome to re-use the pSeries or Maple one. A future
889	      kernel version might provide a common function for this.
890	    - d-cache-line-size : one cell, L1 data cache line size in bytes
891	      if different from the block size
892	    - i-cache-line-size : one cell, L1 instruction cache line size in
893	      bytes if different from the block size
894	
895	  You are welcome to add any property you find relevant to your board,
896	  like some information about the mechanism used to soft-reset the
897	  CPUs. For example, Apple puts the GPIO number for CPU soft reset
898	  lines in there as a "soft-reset" property since they start secondary
899	  CPUs by soft-resetting them.
900	
901	
902	  d) the /memory node(s)
903	
904	  To define the physical memory layout of your board, you should
905	  create one or more memory node(s). You can either create a single
906	  node with all memory ranges in its reg property, or you can create
907	  several nodes, as you wish. The unit address (@ part) used for the
908	  full path is the address of the first range of memory defined by a
909	  given node. If you use a single memory node, this will typically be
910	  @0.
911	
912	  Required properties:
913	
914	    - device_type : has to be "memory"
915	    - reg : This property contains all the physical memory ranges of
916	      your board. It's a list of addresses/sizes concatenated
917	      together, with the number of cells of each defined by the
918	      #address-cells and #size-cells of the root node. For example,
919	      with both of these properties being 2 like in the example given
920	      earlier, a 970 based machine with 6Gb of RAM could typically
921	      have a "reg" property here that looks like:
922	
923	      00000000 00000000 00000000 80000000
924	      00000001 00000000 00000001 00000000
925	
926	      That is a range starting at 0 of 0x80000000 bytes and a range
927	      starting at 0x100000000 and of 0x100000000 bytes. You can see
928	      that there is no memory covering the IO hole between 2Gb and
929	      4Gb. Some vendors prefer splitting those ranges into smaller
930	      segments, but the kernel doesn't care.
931	
932	  e) The /chosen node
933	
934	  This node is a bit "special". Normally, that's where Open Firmware
935	  puts some variable environment information, like the arguments, or
936	  the default input/output devices.
937	
938	  This specification makes a few of these mandatory, but also defines
939	  some linux-specific properties that would be normally constructed by
940	  the prom_init() trampoline when booting with an OF client interface,
941	  but that you have to provide yourself when using the flattened format.
942	
943	  Recommended properties:
944	
945	    - bootargs : This zero-terminated string is passed as the kernel
946	      command line
947	    - linux,stdout-path : This is the full path to your standard
948	      console device if any. Typically, if you have serial devices on
949	      your board, you may want to put the full path to the one set as
950	      the default console in the firmware here, for the kernel to pick
951	      it up as its own default console.
952	
953	  Note that u-boot creates and fills in the chosen node for platforms
954	  that use it.
955	
956	  (Note: a practice that is now obsolete was to include a property
957	  under /chosen called interrupt-controller which had a phandle value
958	  that pointed to the main interrupt controller)
959	
960	  f) the /soc<SOCname> node
961	
962	  This node is used to represent a system-on-a-chip (SoC) and must be
963	  present if the processor is a SoC. The top-level soc node contains
964	  information that is global to all devices on the SoC. The node name
965	  should contain a unit address for the SoC, which is the base address
966	  of the memory-mapped register set for the SoC. The name of an SoC
967	  node should start with "soc", and the remainder of the name should
968	  represent the part number for the soc.  For example, the MPC8540's
969	  soc node would be called "soc8540".
970	
971	  Required properties:
972	
973	    - ranges : Should be defined as specified in 1) to describe the
974	      translation of SoC addresses for memory mapped SoC registers.
975	    - bus-frequency: Contains the bus frequency for the SoC node.
976	      Typically, the value of this field is filled in by the boot
977	      loader.
978	    - compatible : Exact model of the SoC
979	
980	
981	  Recommended properties:
982	
983	    - reg : This property defines the address and size of the
984	      memory-mapped registers that are used for the SOC node itself.
985	      It does not include the child device registers - these will be
986	      defined inside each child node.  The address specified in the
987	      "reg" property should match the unit address of the SOC node.
988	    - #address-cells : Address representation for "soc" devices.  The
989	      format of this field may vary depending on whether or not the
990	      device registers are memory mapped.  For memory mapped
991	      registers, this field represents the number of cells needed to
992	      represent the address of the registers.  For SOCs that do not
993	      use MMIO, a special address format should be defined that
994	      contains enough cells to represent the required information.
995	      See 1) above for more details on defining #address-cells.
996	    - #size-cells : Size representation for "soc" devices
997	    - #interrupt-cells : Defines the width of cells used to represent
998	       interrupts.  Typically this value is <2>, which includes a
999	       32-bit number that represents the interrupt number, and a
1000	       32-bit number that represents the interrupt sense and level.
1001	       This field is only needed if the SOC contains an interrupt
1002	       controller.
1003	
1004	  The SOC node may contain child nodes for each SOC device that the
1005	  platform uses.  Nodes should not be created for devices which exist
1006	  on the SOC but are not used by a particular platform. See chapter VI
1007	  for more information on how to specify devices that are part of a SOC.
1008	
1009	  Example SOC node for the MPC8540:
1010	
1011		soc8540@e0000000 {
1012			#address-cells = <1>;
1013			#size-cells = <1>;
1014			#interrupt-cells = <2>;
1015			device_type = "soc";
1016			ranges = <0x00000000 0xe0000000 0x00100000>
1017			reg = <0xe0000000 0x00003000>;
1018			bus-frequency = <0>;
1019		}
1020	
1021	
1022	
1023	IV - "dtc", the device tree compiler
1024	====================================
1025	
1026	
1027	dtc source code can be found at
1028	<http://git.jdl.com/gitweb/?p=dtc.git>
1029	
1030	WARNING: This version is still in early development stage; the
1031	resulting device-tree "blobs" have not yet been validated with the
1032	kernel. The current generated block lacks a useful reserve map (it will
1033	be fixed to generate an empty one, it's up to the bootloader to fill
1034	it up) among others. The error handling needs work, bugs are lurking,
1035	etc...
1036	
1037	dtc basically takes a device-tree in a given format and outputs a
1038	device-tree in another format. The currently supported formats are:
1039	
1040	  Input formats:
1041	  -------------
1042	
1043	     - "dtb": "blob" format, that is a flattened device-tree block
1044	       with
1045	        header all in a binary blob.
1046	     - "dts": "source" format. This is a text file containing a
1047	       "source" for a device-tree. The format is defined later in this
1048	        chapter.
1049	     - "fs" format. This is a representation equivalent to the
1050	        output of /proc/device-tree, that is nodes are directories and
1051		properties are files
1052	
1053	 Output formats:
1054	 ---------------
1055	
1056	     - "dtb": "blob" format
1057	     - "dts": "source" format
1058	     - "asm": assembly language file. This is a file that can be
1059	       sourced by gas to generate a device-tree "blob". That file can
1060	       then simply be added to your Makefile. Additionally, the
1061	       assembly file exports some symbols that can be used.
1062	
1063	
1064	The syntax of the dtc tool is
1065	
1066	    dtc [-I <input-format>] [-O <output-format>]
1067	        [-o output-filename] [-V output_version] input_filename
1068	
1069	
1070	The "output_version" defines what version of the "blob" format will be
1071	generated. Supported versions are 1,2,3 and 16. The default is
1072	currently version 3 but that may change in the future to version 16.
1073	
1074	Additionally, dtc performs various sanity checks on the tree, like the
1075	uniqueness of linux, phandle properties, validity of strings, etc...
1076	
1077	The format of the .dts "source" file is "C" like, supports C and C++
1078	style comments.
1079	
1080	/ {
1081	}
1082	
1083	The above is the "device-tree" definition. It's the only statement
1084	supported currently at the toplevel.
1085	
1086	/ {
1087	  property1 = "string_value";	/* define a property containing a 0
1088	                                 * terminated string
1089					 */
1090	
1091	  property2 = <0x1234abcd>;	/* define a property containing a
1092	                                 * numerical 32-bit value (hexadecimal)
1093					 */
1094	
1095	  property3 = <0x12345678 0x12345678 0xdeadbeef>;
1096	                                /* define a property containing 3
1097	                                 * numerical 32-bit values (cells) in
1098	                                 * hexadecimal
1099					 */
1100	  property4 = [0x0a 0x0b 0x0c 0x0d 0xde 0xea 0xad 0xbe 0xef];
1101	                                /* define a property whose content is
1102	                                 * an arbitrary array of bytes
1103	                                 */
1104	
1105	  childnode@address {	/* define a child node named "childnode"
1106	                                 * whose unit name is "childnode at
1107					 * address"
1108	                                 */
1109	
1110	    childprop = "hello\n";      /* define a property "childprop" of
1111	                                 * childnode (in this case, a string)
1112	                                 */
1113	  };
1114	};
1115	
1116	Nodes can contain other nodes etc... thus defining the hierarchical
1117	structure of the tree.
1118	
1119	Strings support common escape sequences from C: "\n", "\t", "\r",
1120	"\(octal value)", "\x(hex value)".
1121	
1122	It is also suggested that you pipe your source file through cpp (gcc
1123	preprocessor) so you can use #include's, #define for constants, etc...
1124	
1125	Finally, various options are planned but not yet implemented, like
1126	automatic generation of phandles, labels (exported to the asm file so
1127	you can point to a property content and change it easily from whatever
1128	you link the device-tree with), label or path instead of numeric value
1129	in some cells to "point" to a node (replaced by a phandle at compile
1130	time), export of reserve map address to the asm file, ability to
1131	specify reserve map content at compile time, etc...
1132	
1133	We may provide a .h include file with common definitions of that
1134	proves useful for some properties (like building PCI properties or
1135	interrupt maps) though it may be better to add a notion of struct
1136	definitions to the compiler...
1137	
1138	
1139	V - Recommendations for a bootloader
1140	====================================
1141	
1142	
1143	Here are some various ideas/recommendations that have been proposed
1144	while all this has been defined and implemented.
1145	
1146	  - The bootloader may want to be able to use the device-tree itself
1147	    and may want to manipulate it (to add/edit some properties,
1148	    like physical memory size or kernel arguments). At this point, 2
1149	    choices can be made. Either the bootloader works directly on the
1150	    flattened format, or the bootloader has its own internal tree
1151	    representation with pointers (similar to the kernel one) and
1152	    re-flattens the tree when booting the kernel. The former is a bit
1153	    more difficult to edit/modify, the later requires probably a bit
1154	    more code to handle the tree structure. Note that the structure
1155	    format has been designed so it's relatively easy to "insert"
1156	    properties or nodes or delete them by just memmoving things
1157	    around. It contains no internal offsets or pointers for this
1158	    purpose.
1159	
1160	  - An example of code for iterating nodes & retrieving properties
1161	    directly from the flattened tree format can be found in the kernel
1162	    file drivers/of/fdt.c.  Look at the of_scan_flat_dt() function,
1163	    its usage in early_init_devtree(), and the corresponding various
1164	    early_init_dt_scan_*() callbacks. That code can be re-used in a
1165	    GPL bootloader, and as the author of that code, I would be happy
1166	    to discuss possible free licensing to any vendor who wishes to
1167	    integrate all or part of this code into a non-GPL bootloader.
1168	    (reference needed; who is 'I' here? ---gcl Jan 31, 2011)
1169	
1170	
1171	
1172	VI - System-on-a-chip devices and nodes
1173	=======================================
1174	
1175	Many companies are now starting to develop system-on-a-chip
1176	processors, where the processor core (CPU) and many peripheral devices
1177	exist on a single piece of silicon.  For these SOCs, an SOC node
1178	should be used that defines child nodes for the devices that make
1179	up the SOC. While platforms are not required to use this model in
1180	order to boot the kernel, it is highly encouraged that all SOC
1181	implementations define as complete a flat-device-tree as possible to
1182	describe the devices on the SOC.  This will allow for the
1183	genericization of much of the kernel code.
1184	
1185	
1186	1) Defining child nodes of an SOC
1187	---------------------------------
1188	
1189	Each device that is part of an SOC may have its own node entry inside
1190	the SOC node.  For each device that is included in the SOC, the unit
1191	address property represents the address offset for this device's
1192	memory-mapped registers in the parent's address space.  The parent's
1193	address space is defined by the "ranges" property in the top-level soc
1194	node. The "reg" property for each node that exists directly under the
1195	SOC node should contain the address mapping from the child address space
1196	to the parent SOC address space and the size of the device's
1197	memory-mapped register file.
1198	
1199	For many devices that may exist inside an SOC, there are predefined
1200	specifications for the format of the device tree node.  All SOC child
1201	nodes should follow these specifications, except where noted in this
1202	document.
1203	
1204	See appendix A for an example partial SOC node definition for the
1205	MPC8540.
1206	
1207	
1208	2) Representing devices without a current OF specification
1209	----------------------------------------------------------
1210	
1211	Currently, there are many devices on SoCs that do not have a standard
1212	representation defined as part of the Open Firmware specifications,
1213	mainly because the boards that contain these SoCs are not currently
1214	booted using Open Firmware.  Binding documentation for new devices
1215	should be added to the Documentation/devicetree/bindings directory.
1216	That directory will expand as device tree support is added to more and
1217	more SoCs.
1218	
1219	
1220	VII - Specifying interrupt information for devices
1221	===================================================
1222	
1223	The device tree represents the buses and devices of a hardware
1224	system in a form similar to the physical bus topology of the
1225	hardware.
1226	
1227	In addition, a logical 'interrupt tree' exists which represents the
1228	hierarchy and routing of interrupts in the hardware.
1229	
1230	The interrupt tree model is fully described in the
1231	document "Open Firmware Recommended Practice: Interrupt
1232	Mapping Version 0.9".  The document is available at:
1233	<http://www.openfirmware.org/ofwg/practice/>
1234	
1235	1) interrupts property
1236	----------------------
1237	
1238	Devices that generate interrupts to a single interrupt controller
1239	should use the conventional OF representation described in the
1240	OF interrupt mapping documentation.
1241	
1242	Each device which generates interrupts must have an 'interrupt'
1243	property.  The interrupt property value is an arbitrary number of
1244	of 'interrupt specifier' values which describe the interrupt or
1245	interrupts for the device.
1246	
1247	The encoding of an interrupt specifier is determined by the
1248	interrupt domain in which the device is located in the
1249	interrupt tree.  The root of an interrupt domain specifies in
1250	its #interrupt-cells property the number of 32-bit cells
1251	required to encode an interrupt specifier.  See the OF interrupt
1252	mapping documentation for a detailed description of domains.
1253	
1254	For example, the binding for the OpenPIC interrupt controller
1255	specifies  an #interrupt-cells value of 2 to encode the interrupt
1256	number and level/sense information. All interrupt children in an
1257	OpenPIC interrupt domain use 2 cells per interrupt in their interrupts
1258	property.
1259	
1260	The PCI bus binding specifies a #interrupt-cell value of 1 to encode
1261	which interrupt pin (INTA,INTB,INTC,INTD) is used.
1262	
1263	2) interrupt-parent property
1264	----------------------------
1265	
1266	The interrupt-parent property is specified to define an explicit
1267	link between a device node and its interrupt parent in
1268	the interrupt tree.  The value of interrupt-parent is the
1269	phandle of the parent node.
1270	
1271	If the interrupt-parent property is not defined for a node, its
1272	interrupt parent is assumed to be an ancestor in the node's
1273	_device tree_ hierarchy.
1274	
1275	3) OpenPIC Interrupt Controllers
1276	--------------------------------
1277	
1278	OpenPIC interrupt controllers require 2 cells to encode
1279	interrupt information.  The first cell defines the interrupt
1280	number.  The second cell defines the sense and level
1281	information.
1282	
1283	Sense and level information should be encoded as follows:
1284	
1285		0 = low to high edge sensitive type enabled
1286		1 = active low level sensitive type enabled
1287		2 = active high level sensitive type enabled
1288		3 = high to low edge sensitive type enabled
1289	
1290	4) ISA Interrupt Controllers
1291	----------------------------
1292	
1293	ISA PIC interrupt controllers require 2 cells to encode
1294	interrupt information.  The first cell defines the interrupt
1295	number.  The second cell defines the sense and level
1296	information.
1297	
1298	ISA PIC interrupt controllers should adhere to the ISA PIC
1299	encodings listed below:
1300	
1301		0 =  active low level sensitive type enabled
1302		1 =  active high level sensitive type enabled
1303		2 =  high to low edge sensitive type enabled
1304		3 =  low to high edge sensitive type enabled
1305	
1306	VIII - Specifying Device Power Management Information (sleep property)
1307	===================================================================
1308	
1309	Devices on SOCs often have mechanisms for placing devices into low-power
1310	states that are decoupled from the devices' own register blocks.  Sometimes,
1311	this information is more complicated than a cell-index property can
1312	reasonably describe.  Thus, each device controlled in such a manner
1313	may contain a "sleep" property which describes these connections.
1314	
1315	The sleep property consists of one or more sleep resources, each of
1316	which consists of a phandle to a sleep controller, followed by a
1317	controller-specific sleep specifier of zero or more cells.
1318	
1319	The semantics of what type of low power modes are possible are defined
1320	by the sleep controller.  Some examples of the types of low power modes
1321	that may be supported are:
1322	
1323	 - Dynamic: The device may be disabled or enabled at any time.
1324	 - System Suspend: The device may request to be disabled or remain
1325	   awake during system suspend, but will not be disabled until then.
1326	 - Permanent: The device is disabled permanently (until the next hard
1327	   reset).
1328	
1329	Some devices may share a clock domain with each other, such that they should
1330	only be suspended when none of the devices are in use.  Where reasonable,
1331	such nodes should be placed on a virtual bus, where the bus has the sleep
1332	property.  If the clock domain is shared among devices that cannot be
1333	reasonably grouped in this manner, then create a virtual sleep controller
1334	(similar to an interrupt nexus, except that defining a standardized
1335	sleep-map should wait until its necessity is demonstrated).
1336	
1337	IX - Specifying dma bus information
1338	
1339	Some devices may have DMA memory range shifted relatively to the beginning of
1340	RAM, or even placed outside of kernel RAM. For example, the Keystone 2 SoC
1341	worked in LPAE mode with 4G memory has:
1342	- RAM range: [0x8 0000 0000, 0x8 FFFF FFFF]
1343	- DMA range: [  0x8000 0000,   0xFFFF FFFF]
1344	and DMA range is aliased into first 2G of RAM in HW.
1345	
1346	In such cases, DMA addresses translation should be performed between CPU phys
1347	and DMA addresses. The "dma-ranges" property is intended to be used
1348	for describing the configuration of such system in DT.
1349	
1350	In addition, each DMA master device on the DMA bus may or may not support
1351	coherent DMA operations. The "dma-coherent" property is intended to be used
1352	for identifying devices supported coherent DMA operations in DT.
1353	
1354	* DMA Bus master
1355	Optional property:
1356	- dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
1357		(child-bus-address, parent-bus-address, length). Each triplet specified
1358		describes a contiguous DMA address range.
1359		The dma-ranges property is used to describe the direct memory access (DMA)
1360		structure of a memory-mapped bus whose device tree parent can be accessed
1361		from DMA operations originating from the bus. It provides a means of
1362		defining a mapping or translation between the physical address space of
1363		the bus and the physical address space of the parent of the bus.
1364		(for more information see ePAPR specification)
1365	
1366	* DMA Bus child
1367	Optional property:
1368	- dma-ranges: <empty> value. if present - It means that DMA addresses
1369		translation has to be enabled for this device.
1370	- dma-coherent: Present if dma operations are coherent
1371	
1372	Example:
1373	soc {
1374			compatible = "ti,keystone","simple-bus";
1375			ranges = <0x0 0x0 0x0 0xc0000000>;
1376			dma-ranges = <0x80000000 0x8 0x00000000 0x80000000>;
1377	
1378			[...]
1379	
1380			usb: usb@2680000 {
1381				compatible = "ti,keystone-dwc3";
1382	
1383				[...]
1384				dma-coherent;
1385			};
1386	};
1387	
1388	Appendix A - Sample SOC node for MPC8540
1389	========================================
1390	
1391		soc@e0000000 {
1392			#address-cells = <1>;
1393			#size-cells = <1>;
1394			compatible = "fsl,mpc8540-ccsr", "simple-bus";
1395			device_type = "soc";
1396			ranges = <0x00000000 0xe0000000 0x00100000>
1397			bus-frequency = <0>;
1398			interrupt-parent = <&pic>;
1399	
1400			ethernet@24000 {
1401				#address-cells = <1>;
1402				#size-cells = <1>;
1403				device_type = "network";
1404				model = "TSEC";
1405				compatible = "gianfar", "simple-bus";
1406				reg = <0x24000 0x1000>;
1407				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x00 ];
1408				interrupts = <0x29 2 0x30 2 0x34 2>;
1409				phy-handle = <&phy0>;
1410				sleep = <&pmc 0x00000080>;
1411				ranges;
1412	
1413				mdio@24520 {
1414					reg = <0x24520 0x20>;
1415					compatible = "fsl,gianfar-mdio";
1416	
1417					phy0: ethernet-phy@0 {
1418						interrupts = <5 1>;
1419						reg = <0>;
1420					};
1421	
1422					phy1: ethernet-phy@1 {
1423						interrupts = <5 1>;
1424						reg = <1>;
1425					};
1426	
1427					phy3: ethernet-phy@3 {
1428						interrupts = <7 1>;
1429						reg = <3>;
1430					};
1431				};
1432			};
1433	
1434			ethernet@25000 {
1435				device_type = "network";
1436				model = "TSEC";
1437				compatible = "gianfar";
1438				reg = <0x25000 0x1000>;
1439				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x01 ];
1440				interrupts = <0x13 2 0x14 2 0x18 2>;
1441				phy-handle = <&phy1>;
1442				sleep = <&pmc 0x00000040>;
1443			};
1444	
1445			ethernet@26000 {
1446				device_type = "network";
1447				model = "FEC";
1448				compatible = "gianfar";
1449				reg = <0x26000 0x1000>;
1450				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x02 ];
1451				interrupts = <0x41 2>;
1452				phy-handle = <&phy3>;
1453				sleep = <&pmc 0x00000020>;
1454			};
1455	
1456			serial@4500 {
1457				#address-cells = <1>;
1458				#size-cells = <1>;
1459				compatible = "fsl,mpc8540-duart", "simple-bus";
1460				sleep = <&pmc 0x00000002>;
1461				ranges;
1462	
1463				serial@4500 {
1464					device_type = "serial";
1465					compatible = "ns16550";
1466					reg = <0x4500 0x100>;
1467					clock-frequency = <0>;
1468					interrupts = <0x42 2>;
1469				};
1470	
1471				serial@4600 {
1472					device_type = "serial";
1473					compatible = "ns16550";
1474					reg = <0x4600 0x100>;
1475					clock-frequency = <0>;
1476					interrupts = <0x42 2>;
1477				};
1478			};
1479	
1480			pic: pic@40000 {
1481				interrupt-controller;
1482				#address-cells = <0>;
1483				#interrupt-cells = <2>;
1484				reg = <0x40000 0x40000>;
1485				compatible = "chrp,open-pic";
1486				device_type = "open-pic";
1487			};
1488	
1489			i2c@3000 {
1490				interrupts = <0x43 2>;
1491				reg = <0x3000 0x100>;
1492				compatible  = "fsl-i2c";
1493				dfsrr;
1494				sleep = <&pmc 0x00000004>;
1495			};
1496	
1497			pmc: power@e0070 {
1498				compatible = "fsl,mpc8540-pmc", "fsl,mpc8548-pmc";
1499				reg = <0xe0070 0x20>;
1500			};
1501		};
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