About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / devicetree / booting-without-of.txt




Custom Search

Based on kernel version 4.1. Page generated on 2015-06-28 12:11 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	
20	  II - The DT block format
21	    1) Header
22	    2) Device tree generalities
23	    3) Device tree "structure" block
24	    4) Device tree "strings" block
25	
26	  III - Required content of the device tree
27	    1) Note about cells and address representation
28	    2) Note about "compatible" properties
29	    3) Note about "name" properties
30	    4) Note about node and property names and character set
31	    5) Required nodes and properties
32	      a) The root node
33	      b) The /cpus node
34	      c) The /cpus/* nodes
35	      d) the /memory node(s)
36	      e) The /chosen node
37	      f) the /soc<SOCname> node
38	
39	  IV - "dtc", the device tree compiler
40	
41	  V - Recommendations for a bootloader
42	
43	  VI - System-on-a-chip devices and nodes
44	    1) Defining child nodes of an SOC
45	    2) Representing devices without a current OF specification
46	
47	  VII - Specifying interrupt information for devices
48	    1) interrupts property
49	    2) interrupt-parent property
50	    3) OpenPIC Interrupt Controllers
51	    4) ISA Interrupt Controllers
52	
53	  VIII - Specifying device power management information (sleep property)
54	
55	  IX - Specifying dma bus information
56	
57	  Appendix A - Sample SOC node for MPC8540
58	
59	
60	Revision Information
61	====================
62	
63	   May 18, 2005: Rev 0.1 - Initial draft, no chapter III yet.
64	
65	   May 19, 2005: Rev 0.2 - Add chapter III and bits & pieces here or
66	                           clarifies the fact that a lot of things are
67	                           optional, the kernel only requires a very
68	                           small device tree, though it is encouraged
69	                           to provide an as complete one as possible.
70	
71	   May 24, 2005: Rev 0.3 - Precise that DT block has to be in RAM
72				 - Misc fixes
73				 - Define version 3 and new format version 16
74				   for the DT block (version 16 needs kernel
75				   patches, will be fwd separately).
76				   String block now has a size, and full path
77				   is replaced by unit name for more
78				   compactness.
79				   linux,phandle is made optional, only nodes
80				   that are referenced by other nodes need it.
81				   "name" property is now automatically
82				   deduced from the unit name
83	
84	   June 1, 2005: Rev 0.4 - Correct confusion between OF_DT_END and
85	                           OF_DT_END_NODE in structure definition.
86	                         - Change version 16 format to always align
87	                           property data to 4 bytes. Since tokens are
88	                           already aligned, that means no specific
89	                           required alignment between property size
90	                           and property data. The old style variable
91	                           alignment would make it impossible to do
92	                           "simple" insertion of properties using
93	                           memmove (thanks Milton for
94	                           noticing). Updated kernel patch as well
95				 - Correct a few more alignment constraints
96				 - Add a chapter about the device-tree
97	                           compiler and the textural representation of
98	                           the tree that can be "compiled" by dtc.
99	
100	   November 21, 2005: Rev 0.5
101				 - Additions/generalizations for 32-bit
102				 - Changed to reflect the new arch/powerpc
103				   structure
104				 - Added chapter VI
105	
106	
107	 ToDo:
108		- Add some definitions of interrupt tree (simple/complex)
109		- Add some definitions for PCI host bridges
110		- Add some common address format examples
111		- Add definitions for standard properties and "compatible"
112		  names for cells that are not already defined by the existing
113		  OF spec.
114		- Compare FSL SOC use of PCI to standard and make sure no new
115		  node definition required.
116		- Add more information about node definitions for SOC devices
117	  	  that currently have no standard, like the FSL CPM.
118	
119	
120	I - Introduction
121	================
122	
123	During the development of the Linux/ppc64 kernel, and more
124	specifically, the addition of new platform types outside of the old
125	IBM pSeries/iSeries pair, it was decided to enforce some strict rules
126	regarding the kernel entry and bootloader <-> kernel interfaces, in
127	order to avoid the degeneration that had become the ppc32 kernel entry
128	point and the way a new platform should be added to the kernel. The
129	legacy iSeries platform breaks those rules as it predates this scheme,
130	but no new board support will be accepted in the main tree that
131	doesn't follow them properly.  In addition, since the advent of the
132	arch/powerpc merged architecture for ppc32 and ppc64, new 32-bit
133	platforms and 32-bit platforms which move into arch/powerpc will be
134	required to use these rules as well.
135	
136	The main requirement that will be defined in more detail below is
137	the presence of a device-tree whose format is defined after Open
138	Firmware specification. However, in order to make life easier
139	to embedded board vendors, the kernel doesn't require the device-tree
140	to represent every device in the system and only requires some nodes
141	and properties to be present. This will be described in detail in
142	section III, but, for example, the kernel does not require you to
143	create a node for every PCI device in the system. It is a requirement
144	to have a node for PCI host bridges in order to provide interrupt
145	routing information and memory/IO ranges, among others. It is also
146	recommended to define nodes for on chip devices and other buses that
147	don't specifically fit in an existing OF specification. This creates a
148	great flexibility in the way the kernel can then probe those and match
149	drivers to device, without having to hard code all sorts of tables. It
150	also makes it more flexible for board vendors to do minor hardware
151	upgrades without significantly impacting the kernel code or cluttering
152	it with special cases.
153	
154	
155	1) Entry point for arch/arm
156	---------------------------
157	
158	   There is one single entry point to the kernel, at the start
159	   of the kernel image. That entry point supports two calling
160	   conventions.  A summary of the interface is described here.  A full
161	   description of the boot requirements is documented in
162	   Documentation/arm/Booting
163	
164	        a) ATAGS interface.  Minimal information is passed from firmware
165	        to the kernel with a tagged list of predefined parameters.
166	
167	                r0 : 0
168	
169	                r1 : Machine type number
170	
171	                r2 : Physical address of tagged list in system RAM
172	
173	        b) Entry with a flattened device-tree block.  Firmware loads the
174	        physical address of the flattened device tree block (dtb) into r2,
175	        r1 is not used, but it is considered good practice to use a valid
176	        machine number as described in Documentation/arm/Booting.
177	
178	                r0 : 0
179	
180	                r1 : Valid machine type number.  When using a device tree,
181	                a single machine type number will often be assigned to
182	                represent a class or family of SoCs.
183	
184	                r2 : physical pointer to the device-tree block
185	                (defined in chapter II) in RAM.  Device tree can be located
186	                anywhere in system RAM, but it should be aligned on a 64 bit
187	                boundary.
188	
189	   The kernel will differentiate between ATAGS and device tree booting by
190	   reading the memory pointed to by r2 and looking for either the flattened
191	   device tree block magic value (0xd00dfeed) or the ATAG_CORE value at
192	   offset 0x4 from r2 (0x54410001).
193	
194	2) Entry point for arch/powerpc
195	-------------------------------
196	
197	   There is one single entry point to the kernel, at the start
198	   of the kernel image. That entry point supports two calling
199	   conventions:
200	
201	        a) Boot from Open Firmware. If your firmware is compatible
202	        with Open Firmware (IEEE 1275) or provides an OF compatible
203	        client interface API (support for "interpret" callback of
204	        forth words isn't required), you can enter the kernel with:
205	
206	              r5 : OF callback pointer as defined by IEEE 1275
207	              bindings to powerpc. Only the 32-bit client interface
208	              is currently supported
209	
210	              r3, r4 : address & length of an initrd if any or 0
211	
212	              The MMU is either on or off; the kernel will run the
213	              trampoline located in arch/powerpc/kernel/prom_init.c to
214	              extract the device-tree and other information from open
215	              firmware and build a flattened device-tree as described
216	              in b). prom_init() will then re-enter the kernel using
217	              the second method. This trampoline code runs in the
218	              context of the firmware, which is supposed to handle all
219	              exceptions during that time.
220	
221	        b) Direct entry with a flattened device-tree block. This entry
222	        point is called by a) after the OF trampoline and can also be
223	        called directly by a bootloader that does not support the Open
224	        Firmware client interface. It is also used by "kexec" to
225	        implement "hot" booting of a new kernel from a previous
226	        running one. This method is what I will describe in more
227	        details in this document, as method a) is simply standard Open
228	        Firmware, and thus should be implemented according to the
229	        various standard documents defining it and its binding to the
230	        PowerPC platform. The entry point definition then becomes:
231	
232	                r3 : physical pointer to the device-tree block
233	                (defined in chapter II) in RAM
234	
235	                r4 : physical pointer to the kernel itself. This is
236	                used by the assembly code to properly disable the MMU
237	                in case you are entering the kernel with MMU enabled
238	                and a non-1:1 mapping.
239	
240	                r5 : NULL (as to differentiate with method a)
241	
242	        Note about SMP entry: Either your firmware puts your other
243	        CPUs in some sleep loop or spin loop in ROM where you can get
244	        them out via a soft reset or some other means, in which case
245	        you don't need to care, or you'll have to enter the kernel
246	        with all CPUs. The way to do that with method b) will be
247	        described in a later revision of this document.
248	
249	   Board supports (platforms) are not exclusive config options. An
250	   arbitrary set of board supports can be built in a single kernel
251	   image. The kernel will "know" what set of functions to use for a
252	   given platform based on the content of the device-tree. Thus, you
253	   should:
254	
255	        a) add your platform support as a _boolean_ option in
256	        arch/powerpc/Kconfig, following the example of PPC_PSERIES,
257	        PPC_PMAC and PPC_MAPLE. The later is probably a good
258	        example of a board support to start from.
259	
260	        b) create your main platform file as
261	        "arch/powerpc/platforms/myplatform/myboard_setup.c" and add it
262	        to the Makefile under the condition of your CONFIG_
263	        option. This file will define a structure of type "ppc_md"
264	        containing the various callbacks that the generic code will
265	        use to get to your platform specific code
266	
267	  A kernel image may support multiple platforms, but only if the
268	  platforms feature the same core architecture.  A single kernel build
269	  cannot support both configurations with Book E and configurations
270	  with classic Powerpc architectures.
271	
272	3) Entry point for arch/x86
273	-------------------------------
274	
275	  There is one single 32bit entry point to the kernel at code32_start,
276	  the decompressor (the real mode entry point goes to the same  32bit
277	  entry point once it switched into protected mode). That entry point
278	  supports one calling convention which is documented in
279	  Documentation/x86/boot.txt
280	  The physical pointer to the device-tree block (defined in chapter II)
281	  is passed via setup_data which requires at least boot protocol 2.09.
282	  The type filed is defined as
283	
284	  #define SETUP_DTB                      2
285	
286	  This device-tree is used as an extension to the "boot page". As such it
287	  does not parse / consider data which is already covered by the boot
288	  page. This includes memory size, reserved ranges, command line arguments
289	  or initrd address. It simply holds information which can not be retrieved
290	  otherwise like interrupt routing or a list of devices behind an I2C bus.
291	
292	4) Entry point for arch/mips/bmips
293	----------------------------------
294	
295	  Some bootloaders only support a single entry point, at the start of the
296	  kernel image.  Other bootloaders will jump to the ELF start address.
297	  Both schemes are supported; CONFIG_BOOT_RAW=y and CONFIG_NO_EXCEPT_FILL=y,
298	  so the first instruction immediately jumps to kernel_entry().
299	
300	  Similar to the arch/arm case (b), a DT-aware bootloader is expected to
301	  set up the following registers:
302	
303	         a0 : 0
304	
305	         a1 : 0xffffffff
306	
307	         a2 : Physical pointer to the device tree block (defined in chapter
308	         II) in RAM.  The device tree can be located anywhere in the first
309	         512MB of the physical address space (0x00000000 - 0x1fffffff),
310	         aligned on a 64 bit boundary.
311	
312	  Legacy bootloaders do not use this convention, and they do not pass in a
313	  DT block.  In this case, Linux will look for a builtin DTB, selected via
314	  CONFIG_DT_*.
315	
316	  This convention is defined for 32-bit systems only, as there are not
317	  currently any 64-bit BMIPS implementations.
318	
319	II - The DT block format
320	========================
321	
322	
323	This chapter defines the actual format of the flattened device-tree
324	passed to the kernel. The actual content of it and kernel requirements
325	are described later. You can find example of code manipulating that
326	format in various places, including arch/powerpc/kernel/prom_init.c
327	which will generate a flattened device-tree from the Open Firmware
328	representation, or the fs2dt utility which is part of the kexec tools
329	which will generate one from a filesystem representation. It is
330	expected that a bootloader like uboot provides a bit more support,
331	that will be discussed later as well.
332	
333	Note: The block has to be in main memory. It has to be accessible in
334	both real mode and virtual mode with no mapping other than main
335	memory. If you are writing a simple flash bootloader, it should copy
336	the block to RAM before passing it to the kernel.
337	
338	
339	1) Header
340	---------
341	
342	   The kernel is passed the physical address pointing to an area of memory
343	   that is roughly described in include/linux/of_fdt.h by the structure
344	   boot_param_header:
345	
346	struct boot_param_header {
347	        u32     magic;                  /* magic word OF_DT_HEADER */
348	        u32     totalsize;              /* total size of DT block */
349	        u32     off_dt_struct;          /* offset to structure */
350	        u32     off_dt_strings;         /* offset to strings */
351	        u32     off_mem_rsvmap;         /* offset to memory reserve map
352	                                           */
353	        u32     version;                /* format version */
354	        u32     last_comp_version;      /* last compatible version */
355	
356	        /* version 2 fields below */
357	        u32     boot_cpuid_phys;        /* Which physical CPU id we're
358	                                           booting on */
359	        /* version 3 fields below */
360	        u32     size_dt_strings;        /* size of the strings block */
361	
362	        /* version 17 fields below */
363	        u32	size_dt_struct;		/* size of the DT structure block */
364	};
365	
366	   Along with the constants:
367	
368	/* Definitions used by the flattened device tree */
369	#define OF_DT_HEADER            0xd00dfeed      /* 4: version,
370							   4: total size */
371	#define OF_DT_BEGIN_NODE        0x1             /* Start node: full name
372							   */
373	#define OF_DT_END_NODE          0x2             /* End node */
374	#define OF_DT_PROP              0x3             /* Property: name off,
375	                                                   size, content */
376	#define OF_DT_END               0x9
377	
378	   All values in this header are in big endian format, the various
379	   fields in this header are defined more precisely below. All
380	   "offset" values are in bytes from the start of the header; that is
381	   from the physical base address of the device tree block.
382	
383	   - magic
384	
385	     This is a magic value that "marks" the beginning of the
386	     device-tree block header. It contains the value 0xd00dfeed and is
387	     defined by the constant OF_DT_HEADER
388	
389	   - totalsize
390	
391	     This is the total size of the DT block including the header. The
392	     "DT" block should enclose all data structures defined in this
393	     chapter (who are pointed to by offsets in this header). That is,
394	     the device-tree structure, strings, and the memory reserve map.
395	
396	   - off_dt_struct
397	
398	     This is an offset from the beginning of the header to the start
399	     of the "structure" part the device tree. (see 2) device tree)
400	
401	   - off_dt_strings
402	
403	     This is an offset from the beginning of the header to the start
404	     of the "strings" part of the device-tree
405	
406	   - off_mem_rsvmap
407	
408	     This is an offset from the beginning of the header to the start
409	     of the reserved memory map. This map is a list of pairs of 64-
410	     bit integers. Each pair is a physical address and a size. The
411	     list is terminated by an entry of size 0. This map provides the
412	     kernel with a list of physical memory areas that are "reserved"
413	     and thus not to be used for memory allocations, especially during
414	     early initialization. The kernel needs to allocate memory during
415	     boot for things like un-flattening the device-tree, allocating an
416	     MMU hash table, etc... Those allocations must be done in such a
417	     way to avoid overriding critical things like, on Open Firmware
418	     capable machines, the RTAS instance, or on some pSeries, the TCE
419	     tables used for the iommu. Typically, the reserve map should
420	     contain _at least_ this DT block itself (header,total_size). If
421	     you are passing an initrd to the kernel, you should reserve it as
422	     well. You do not need to reserve the kernel image itself. The map
423	     should be 64-bit aligned.
424	
425	   - version
426	
427	     This is the version of this structure. Version 1 stops
428	     here. Version 2 adds an additional field boot_cpuid_phys.
429	     Version 3 adds the size of the strings block, allowing the kernel
430	     to reallocate it easily at boot and free up the unused flattened
431	     structure after expansion. Version 16 introduces a new more
432	     "compact" format for the tree itself that is however not backward
433	     compatible. Version 17 adds an additional field, size_dt_struct,
434	     allowing it to be reallocated or moved more easily (this is
435	     particularly useful for bootloaders which need to make
436	     adjustments to a device tree based on probed information). You
437	     should always generate a structure of the highest version defined
438	     at the time of your implementation. Currently that is version 17,
439	     unless you explicitly aim at being backward compatible.
440	
441	   - last_comp_version
442	
443	     Last compatible version. This indicates down to what version of
444	     the DT block you are backward compatible. For example, version 2
445	     is backward compatible with version 1 (that is, a kernel build
446	     for version 1 will be able to boot with a version 2 format). You
447	     should put a 1 in this field if you generate a device tree of
448	     version 1 to 3, or 16 if you generate a tree of version 16 or 17
449	     using the new unit name format.
450	
451	   - boot_cpuid_phys
452	
453	     This field only exist on version 2 headers. It indicate which
454	     physical CPU ID is calling the kernel entry point. This is used,
455	     among others, by kexec. If you are on an SMP system, this value
456	     should match the content of the "reg" property of the CPU node in
457	     the device-tree corresponding to the CPU calling the kernel entry
458	     point (see further chapters for more information on the required
459	     device-tree contents)
460	
461	   - size_dt_strings
462	
463	     This field only exists on version 3 and later headers.  It
464	     gives the size of the "strings" section of the device tree (which
465	     starts at the offset given by off_dt_strings).
466	
467	   - size_dt_struct
468	
469	     This field only exists on version 17 and later headers.  It gives
470	     the size of the "structure" section of the device tree (which
471	     starts at the offset given by off_dt_struct).
472	
473	   So the typical layout of a DT block (though the various parts don't
474	   need to be in that order) looks like this (addresses go from top to
475	   bottom):
476	
477	
478	             ------------------------------
479	     base -> |  struct boot_param_header  |
480	             ------------------------------
481	             |      (alignment gap) (*)   |
482	             ------------------------------
483	             |      memory reserve map    |
484	             ------------------------------
485	             |      (alignment gap)       |
486	             ------------------------------
487	             |                            |
488	             |    device-tree structure   |
489	             |                            |
490	             ------------------------------
491	             |      (alignment gap)       |
492	             ------------------------------
493	             |                            |
494	             |     device-tree strings    |
495	             |                            |
496	      -----> ------------------------------
497	      |
498	      |
499	      --- (base + totalsize)
500	
501	  (*) The alignment gaps are not necessarily present; their presence
502	      and size are dependent on the various alignment requirements of
503	      the individual data blocks.
504	
505	
506	2) Device tree generalities
507	---------------------------
508	
509	This device-tree itself is separated in two different blocks, a
510	structure block and a strings block. Both need to be aligned to a 4
511	byte boundary.
512	
513	First, let's quickly describe the device-tree concept before detailing
514	the storage format. This chapter does _not_ describe the detail of the
515	required types of nodes & properties for the kernel, this is done
516	later in chapter III.
517	
518	The device-tree layout is strongly inherited from the definition of
519	the Open Firmware IEEE 1275 device-tree. It's basically a tree of
520	nodes, each node having two or more named properties. A property can
521	have a value or not.
522	
523	It is a tree, so each node has one and only one parent except for the
524	root node who has no parent.
525	
526	A node has 2 names. The actual node name is generally contained in a
527	property of type "name" in the node property list whose value is a
528	zero terminated string and is mandatory for version 1 to 3 of the
529	format definition (as it is in Open Firmware). Version 16 makes it
530	optional as it can generate it from the unit name defined below.
531	
532	There is also a "unit name" that is used to differentiate nodes with
533	the same name at the same level, it is usually made of the node
534	names, the "@" sign, and a "unit address", which definition is
535	specific to the bus type the node sits on.
536	
537	The unit name doesn't exist as a property per-se but is included in
538	the device-tree structure. It is typically used to represent "path" in
539	the device-tree. More details about the actual format of these will be
540	below.
541	
542	The kernel generic code does not make any formal use of the
543	unit address (though some board support code may do) so the only real
544	requirement here for the unit address is to ensure uniqueness of
545	the node unit name at a given level of the tree. Nodes with no notion
546	of address and no possible sibling of the same name (like /memory or
547	/cpus) may omit the unit address in the context of this specification,
548	or use the "@0" default unit address. The unit name is used to define
549	a node "full path", which is the concatenation of all parent node
550	unit names separated with "/".
551	
552	The root node doesn't have a defined name, and isn't required to have
553	a name property either if you are using version 3 or earlier of the
554	format. It also has no unit address (no @ symbol followed by a unit
555	address). The root node unit name is thus an empty string. The full
556	path to the root node is "/".
557	
558	Every node which actually represents an actual device (that is, a node
559	which isn't only a virtual "container" for more nodes, like "/cpus"
560	is) is also required to have a "compatible" property indicating the
561	specific hardware and an optional list of devices it is fully
562	backwards compatible with.
563	
564	Finally, every node that can be referenced from a property in another
565	node is required to have either a "phandle" or a "linux,phandle"
566	property. Real Open Firmware implementations provide a unique
567	"phandle" value for every node that the "prom_init()" trampoline code
568	turns into "linux,phandle" properties. However, this is made optional
569	if the flattened device tree is used directly. An example of a node
570	referencing another node via "phandle" is when laying out the
571	interrupt tree which will be described in a further version of this
572	document.
573	
574	The "phandle" property is a 32-bit value that uniquely
575	identifies a node. You are free to use whatever values or system of
576	values, internal pointers, or whatever to generate these, the only
577	requirement is that every node for which you provide that property has
578	a unique value for it.
579	
580	Here is an example of a simple device-tree. In this example, an "o"
581	designates a node followed by the node unit name. Properties are
582	presented with their name followed by their content. "content"
583	represents an ASCII string (zero terminated) value, while <content>
584	represents a 32-bit value, specified in decimal or hexadecimal (the
585	latter prefixed 0x). The various nodes in this example will be
586	discussed in a later chapter. At this point, it is only meant to give
587	you a idea of what a device-tree looks like. I have purposefully kept
588	the "name" and "linux,phandle" properties which aren't necessary in
589	order to give you a better idea of what the tree looks like in
590	practice.
591	
592	  / o device-tree
593	      |- name = "device-tree"
594	      |- model = "MyBoardName"
595	      |- compatible = "MyBoardFamilyName"
596	      |- #address-cells = <2>
597	      |- #size-cells = <2>
598	      |- linux,phandle = <0>
599	      |
600	      o cpus
601	      | | - name = "cpus"
602	      | | - linux,phandle = <1>
603	      | | - #address-cells = <1>
604	      | | - #size-cells = <0>
605	      | |
606	      | o PowerPC,970@0
607	      |   |- name = "PowerPC,970"
608	      |   |- device_type = "cpu"
609	      |   |- reg = <0>
610	      |   |- clock-frequency = <0x5f5e1000>
611	      |   |- 64-bit
612	      |   |- linux,phandle = <2>
613	      |
614	      o memory@0
615	      | |- name = "memory"
616	      | |- device_type = "memory"
617	      | |- reg = <0x00000000 0x00000000 0x00000000 0x20000000>
618	      | |- linux,phandle = <3>
619	      |
620	      o chosen
621	        |- name = "chosen"
622	        |- bootargs = "root=/dev/sda2"
623	        |- linux,phandle = <4>
624	
625	This tree is almost a minimal tree. It pretty much contains the
626	minimal set of required nodes and properties to boot a linux kernel;
627	that is, some basic model information at the root, the CPUs, and the
628	physical memory layout.  It also includes misc information passed
629	through /chosen, like in this example, the platform type (mandatory)
630	and the kernel command line arguments (optional).
631	
632	The /cpus/PowerPC,970@0/64-bit property is an example of a
633	property without a value. All other properties have a value. The
634	significance of the #address-cells and #size-cells properties will be
635	explained in chapter IV which defines precisely the required nodes and
636	properties and their content.
637	
638	
639	3) Device tree "structure" block
640	
641	The structure of the device tree is a linearized tree structure. The
642	"OF_DT_BEGIN_NODE" token starts a new node, and the "OF_DT_END_NODE"
643	ends that node definition. Child nodes are simply defined before
644	"OF_DT_END_NODE" (that is nodes within the node). A 'token' is a 32
645	bit value. The tree has to be "finished" with a OF_DT_END token
646	
647	Here's the basic structure of a single node:
648	
649	     * token OF_DT_BEGIN_NODE (that is 0x00000001)
650	     * for version 1 to 3, this is the node full path as a zero
651	       terminated string, starting with "/". For version 16 and later,
652	       this is the node unit name only (or an empty string for the
653	       root node)
654	     * [align gap to next 4 bytes boundary]
655	     * for each property:
656	        * token OF_DT_PROP (that is 0x00000003)
657	        * 32-bit value of property value size in bytes (or 0 if no
658	          value)
659	        * 32-bit value of offset in string block of property name
660	        * property value data if any
661	        * [align gap to next 4 bytes boundary]
662	     * [child nodes if any]
663	     * token OF_DT_END_NODE (that is 0x00000002)
664	
665	So the node content can be summarized as a start token, a full path,
666	a list of properties, a list of child nodes, and an end token. Every
667	child node is a full node structure itself as defined above.
668	
669	NOTE: The above definition requires that all property definitions for
670	a particular node MUST precede any subnode definitions for that node.
671	Although the structure would not be ambiguous if properties and
672	subnodes were intermingled, the kernel parser requires that the
673	properties come first (up until at least 2.6.22).  Any tools
674	manipulating a flattened tree must take care to preserve this
675	constraint.
676	
677	4) Device tree "strings" block
678	
679	In order to save space, property names, which are generally redundant,
680	are stored separately in the "strings" block. This block is simply the
681	whole bunch of zero terminated strings for all property names
682	concatenated together. The device-tree property definitions in the
683	structure block will contain offset values from the beginning of the
684	strings block.
685	
686	
687	III - Required content of the device tree
688	=========================================
689	
690	WARNING: All "linux,*" properties defined in this document apply only
691	to a flattened device-tree. If your platform uses a real
692	implementation of Open Firmware or an implementation compatible with
693	the Open Firmware client interface, those properties will be created
694	by the trampoline code in the kernel's prom_init() file. For example,
695	that's where you'll have to add code to detect your board model and
696	set the platform number. However, when using the flattened device-tree
697	entry point, there is no prom_init() pass, and thus you have to
698	provide those properties yourself.
699	
700	
701	1) Note about cells and address representation
702	----------------------------------------------
703	
704	The general rule is documented in the various Open Firmware
705	documentations. If you choose to describe a bus with the device-tree
706	and there exist an OF bus binding, then you should follow the
707	specification. However, the kernel does not require every single
708	device or bus to be described by the device tree.
709	
710	In general, the format of an address for a device is defined by the
711	parent bus type, based on the #address-cells and #size-cells
712	properties.  Note that the parent's parent definitions of #address-cells
713	and #size-cells are not inherited so every node with children must specify
714	them.  The kernel requires the root node to have those properties defining
715	addresses format for devices directly mapped on the processor bus.
716	
717	Those 2 properties define 'cells' for representing an address and a
718	size. A "cell" is a 32-bit number. For example, if both contain 2
719	like the example tree given above, then an address and a size are both
720	composed of 2 cells, and each is a 64-bit number (cells are
721	concatenated and expected to be in big endian format). Another example
722	is the way Apple firmware defines them, with 2 cells for an address
723	and one cell for a size.  Most 32-bit implementations should define
724	#address-cells and #size-cells to 1, which represents a 32-bit value.
725	Some 32-bit processors allow for physical addresses greater than 32
726	bits; these processors should define #address-cells as 2.
727	
728	"reg" properties are always a tuple of the type "address size" where
729	the number of cells of address and size is specified by the bus
730	#address-cells and #size-cells. When a bus supports various address
731	spaces and other flags relative to a given address allocation (like
732	prefetchable, etc...) those flags are usually added to the top level
733	bits of the physical address. For example, a PCI physical address is
734	made of 3 cells, the bottom two containing the actual address itself
735	while the top cell contains address space indication, flags, and pci
736	bus & device numbers.
737	
738	For buses that support dynamic allocation, it's the accepted practice
739	to then not provide the address in "reg" (keep it 0) though while
740	providing a flag indicating the address is dynamically allocated, and
741	then, to provide a separate "assigned-addresses" property that
742	contains the fully allocated addresses. See the PCI OF bindings for
743	details.
744	
745	In general, a simple bus with no address space bits and no dynamic
746	allocation is preferred if it reflects your hardware, as the existing
747	kernel address parsing functions will work out of the box. If you
748	define a bus type with a more complex address format, including things
749	like address space bits, you'll have to add a bus translator to the
750	prom_parse.c file of the recent kernels for your bus type.
751	
752	The "reg" property only defines addresses and sizes (if #size-cells is
753	non-0) within a given bus. In order to translate addresses upward
754	(that is into parent bus addresses, and possibly into CPU physical
755	addresses), all buses must contain a "ranges" property. If the
756	"ranges" property is missing at a given level, it's assumed that
757	translation isn't possible, i.e., the registers are not visible on the
758	parent bus.  The format of the "ranges" property for a bus is a list
759	of:
760	
761		bus address, parent bus address, size
762	
763	"bus address" is in the format of the bus this bus node is defining,
764	that is, for a PCI bridge, it would be a PCI address. Thus, (bus
765	address, size) defines a range of addresses for child devices. "parent
766	bus address" is in the format of the parent bus of this bus. For
767	example, for a PCI host controller, that would be a CPU address. For a
768	PCI<->ISA bridge, that would be a PCI address. It defines the base
769	address in the parent bus where the beginning of that range is mapped.
770	
771	For new 64-bit board support, I recommend either the 2/2 format or
772	Apple's 2/1 format which is slightly more compact since sizes usually
773	fit in a single 32-bit word.   New 32-bit board support should use a
774	1/1 format, unless the processor supports physical addresses greater
775	than 32-bits, in which case a 2/1 format is recommended.
776	
777	Alternatively, the "ranges" property may be empty, indicating that the
778	registers are visible on the parent bus using an identity mapping
779	translation.  In other words, the parent bus address space is the same
780	as the child bus address space.
781	
782	2) Note about "compatible" properties
783	-------------------------------------
784	
785	These properties are optional, but recommended in devices and the root
786	node. The format of a "compatible" property is a list of concatenated
787	zero terminated strings. They allow a device to express its
788	compatibility with a family of similar devices, in some cases,
789	allowing a single driver to match against several devices regardless
790	of their actual names.
791	
792	3) Note about "name" properties
793	-------------------------------
794	
795	While earlier users of Open Firmware like OldWorld macintoshes tended
796	to use the actual device name for the "name" property, it's nowadays
797	considered a good practice to use a name that is closer to the device
798	class (often equal to device_type). For example, nowadays, Ethernet
799	controllers are named "ethernet", an additional "model" property
800	defining precisely the chip type/model, and "compatible" property
801	defining the family in case a single driver can driver more than one
802	of these chips. However, the kernel doesn't generally put any
803	restriction on the "name" property; it is simply considered good
804	practice to follow the standard and its evolutions as closely as
805	possible.
806	
807	Note also that the new format version 16 makes the "name" property
808	optional. If it's absent for a node, then the node's unit name is then
809	used to reconstruct the name. That is, the part of the unit name
810	before the "@" sign is used (or the entire unit name if no "@" sign
811	is present).
812	
813	4) Note about node and property names and character set
814	-------------------------------------------------------
815	
816	While Open Firmware provides more flexible usage of 8859-1, this
817	specification enforces more strict rules. Nodes and properties should
818	be comprised only of ASCII characters 'a' to 'z', '0' to
819	'9', ',', '.', '_', '+', '#', '?', and '-'. Node names additionally
820	allow uppercase characters 'A' to 'Z' (property names should be
821	lowercase. The fact that vendors like Apple don't respect this rule is
822	irrelevant here). Additionally, node and property names should always
823	begin with a character in the range 'a' to 'z' (or 'A' to 'Z' for node
824	names).
825	
826	The maximum number of characters for both nodes and property names
827	is 31. In the case of node names, this is only the leftmost part of
828	a unit name (the pure "name" property), it doesn't include the unit
829	address which can extend beyond that limit.
830	
831	
832	5) Required nodes and properties
833	--------------------------------
834	  These are all that are currently required. However, it is strongly
835	  recommended that you expose PCI host bridges as documented in the
836	  PCI binding to Open Firmware, and your interrupt tree as documented
837	  in OF interrupt tree specification.
838	
839	  a) The root node
840	
841	  The root node requires some properties to be present:
842	
843	    - model : this is your board name/model
844	    - #address-cells : address representation for "root" devices
845	    - #size-cells: the size representation for "root" devices
846	    - compatible : the board "family" generally finds its way here,
847	      for example, if you have 2 board models with a similar layout,
848	      that typically get driven by the same platform code in the
849	      kernel, you would specify the exact board model in the
850	      compatible property followed by an entry that represents the SoC
851	      model.
852	
853	  The root node is also generally where you add additional properties
854	  specific to your board like the serial number if any, that sort of
855	  thing. It is recommended that if you add any "custom" property whose
856	  name may clash with standard defined ones, you prefix them with your
857	  vendor name and a comma.
858	
859	  b) The /cpus node
860	
861	  This node is the parent of all individual CPU nodes. It doesn't
862	  have any specific requirements, though it's generally good practice
863	  to have at least:
864	
865	               #address-cells = <00000001>
866	               #size-cells    = <00000000>
867	
868	  This defines that the "address" for a CPU is a single cell, and has
869	  no meaningful size. This is not necessary but the kernel will assume
870	  that format when reading the "reg" properties of a CPU node, see
871	  below
872	
873	  c) The /cpus/* nodes
874	
875	  So under /cpus, you are supposed to create a node for every CPU on
876	  the machine. There is no specific restriction on the name of the
877	  CPU, though it's common to call it <architecture>,<core>. For
878	  example, Apple uses PowerPC,G5 while IBM uses PowerPC,970FX.
879	  However, the Generic Names convention suggests that it would be
880	  better to simply use 'cpu' for each cpu node and use the compatible
881	  property to identify the specific cpu core.
882	
883	  Required properties:
884	
885	    - device_type : has to be "cpu"
886	    - reg : This is the physical CPU number, it's a single 32-bit cell
887	      and is also used as-is as the unit number for constructing the
888	      unit name in the full path. For example, with 2 CPUs, you would
889	      have the full path:
890	        /cpus/PowerPC,970FX@0
891	        /cpus/PowerPC,970FX@1
892	      (unit addresses do not require leading zeroes)
893	    - d-cache-block-size : one cell, L1 data cache block size in bytes (*)
894	    - i-cache-block-size : one cell, L1 instruction cache block size in
895	      bytes
896	    - d-cache-size : one cell, size of L1 data cache in bytes
897	    - i-cache-size : one cell, size of L1 instruction cache in bytes
898	
899	(*) The cache "block" size is the size on which the cache management
900	instructions operate. Historically, this document used the cache
901	"line" size here which is incorrect. The kernel will prefer the cache
902	block size and will fallback to cache line size for backward
903	compatibility.
904	
905	  Recommended properties:
906	
907	    - timebase-frequency : a cell indicating the frequency of the
908	      timebase in Hz. This is not directly used by the generic code,
909	      but you are welcome to copy/paste the pSeries code for setting
910	      the kernel timebase/decrementer calibration based on this
911	      value.
912	    - clock-frequency : a cell indicating the CPU core clock frequency
913	      in Hz. A new property will be defined for 64-bit values, but if
914	      your frequency is < 4Ghz, one cell is enough. Here as well as
915	      for the above, the common code doesn't use that property, but
916	      you are welcome to re-use the pSeries or Maple one. A future
917	      kernel version might provide a common function for this.
918	    - d-cache-line-size : one cell, L1 data cache line size in bytes
919	      if different from the block size
920	    - i-cache-line-size : one cell, L1 instruction cache line size in
921	      bytes if different from the block size
922	
923	  You are welcome to add any property you find relevant to your board,
924	  like some information about the mechanism used to soft-reset the
925	  CPUs. For example, Apple puts the GPIO number for CPU soft reset
926	  lines in there as a "soft-reset" property since they start secondary
927	  CPUs by soft-resetting them.
928	
929	
930	  d) the /memory node(s)
931	
932	  To define the physical memory layout of your board, you should
933	  create one or more memory node(s). You can either create a single
934	  node with all memory ranges in its reg property, or you can create
935	  several nodes, as you wish. The unit address (@ part) used for the
936	  full path is the address of the first range of memory defined by a
937	  given node. If you use a single memory node, this will typically be
938	  @0.
939	
940	  Required properties:
941	
942	    - device_type : has to be "memory"
943	    - reg : This property contains all the physical memory ranges of
944	      your board. It's a list of addresses/sizes concatenated
945	      together, with the number of cells of each defined by the
946	      #address-cells and #size-cells of the root node. For example,
947	      with both of these properties being 2 like in the example given
948	      earlier, a 970 based machine with 6Gb of RAM could typically
949	      have a "reg" property here that looks like:
950	
951	      00000000 00000000 00000000 80000000
952	      00000001 00000000 00000001 00000000
953	
954	      That is a range starting at 0 of 0x80000000 bytes and a range
955	      starting at 0x100000000 and of 0x100000000 bytes. You can see
956	      that there is no memory covering the IO hole between 2Gb and
957	      4Gb. Some vendors prefer splitting those ranges into smaller
958	      segments, but the kernel doesn't care.
959	
960	  e) The /chosen node
961	
962	  This node is a bit "special". Normally, that's where Open Firmware
963	  puts some variable environment information, like the arguments, or
964	  the default input/output devices.
965	
966	  This specification makes a few of these mandatory, but also defines
967	  some linux-specific properties that would be normally constructed by
968	  the prom_init() trampoline when booting with an OF client interface,
969	  but that you have to provide yourself when using the flattened format.
970	
971	  Recommended properties:
972	
973	    - bootargs : This zero-terminated string is passed as the kernel
974	      command line
975	    - linux,stdout-path : This is the full path to your standard
976	      console device if any. Typically, if you have serial devices on
977	      your board, you may want to put the full path to the one set as
978	      the default console in the firmware here, for the kernel to pick
979	      it up as its own default console.
980	
981	  Note that u-boot creates and fills in the chosen node for platforms
982	  that use it.
983	
984	  (Note: a practice that is now obsolete was to include a property
985	  under /chosen called interrupt-controller which had a phandle value
986	  that pointed to the main interrupt controller)
987	
988	  f) the /soc<SOCname> node
989	
990	  This node is used to represent a system-on-a-chip (SoC) and must be
991	  present if the processor is a SoC. The top-level soc node contains
992	  information that is global to all devices on the SoC. The node name
993	  should contain a unit address for the SoC, which is the base address
994	  of the memory-mapped register set for the SoC. The name of an SoC
995	  node should start with "soc", and the remainder of the name should
996	  represent the part number for the soc.  For example, the MPC8540's
997	  soc node would be called "soc8540".
998	
999	  Required properties:
1000	
1001	    - ranges : Should be defined as specified in 1) to describe the
1002	      translation of SoC addresses for memory mapped SoC registers.
1003	    - bus-frequency: Contains the bus frequency for the SoC node.
1004	      Typically, the value of this field is filled in by the boot
1005	      loader.
1006	    - compatible : Exact model of the SoC
1007	
1008	
1009	  Recommended properties:
1010	
1011	    - reg : This property defines the address and size of the
1012	      memory-mapped registers that are used for the SOC node itself.
1013	      It does not include the child device registers - these will be
1014	      defined inside each child node.  The address specified in the
1015	      "reg" property should match the unit address of the SOC node.
1016	    - #address-cells : Address representation for "soc" devices.  The
1017	      format of this field may vary depending on whether or not the
1018	      device registers are memory mapped.  For memory mapped
1019	      registers, this field represents the number of cells needed to
1020	      represent the address of the registers.  For SOCs that do not
1021	      use MMIO, a special address format should be defined that
1022	      contains enough cells to represent the required information.
1023	      See 1) above for more details on defining #address-cells.
1024	    - #size-cells : Size representation for "soc" devices
1025	    - #interrupt-cells : Defines the width of cells used to represent
1026	       interrupts.  Typically this value is <2>, which includes a
1027	       32-bit number that represents the interrupt number, and a
1028	       32-bit number that represents the interrupt sense and level.
1029	       This field is only needed if the SOC contains an interrupt
1030	       controller.
1031	
1032	  The SOC node may contain child nodes for each SOC device that the
1033	  platform uses.  Nodes should not be created for devices which exist
1034	  on the SOC but are not used by a particular platform. See chapter VI
1035	  for more information on how to specify devices that are part of a SOC.
1036	
1037	  Example SOC node for the MPC8540:
1038	
1039		soc8540@e0000000 {
1040			#address-cells = <1>;
1041			#size-cells = <1>;
1042			#interrupt-cells = <2>;
1043			device_type = "soc";
1044			ranges = <0x00000000 0xe0000000 0x00100000>
1045			reg = <0xe0000000 0x00003000>;
1046			bus-frequency = <0>;
1047		}
1048	
1049	
1050	
1051	IV - "dtc", the device tree compiler
1052	====================================
1053	
1054	
1055	dtc source code can be found at
1056	<http://git.jdl.com/gitweb/?p=dtc.git>
1057	
1058	WARNING: This version is still in early development stage; the
1059	resulting device-tree "blobs" have not yet been validated with the
1060	kernel. The current generated block lacks a useful reserve map (it will
1061	be fixed to generate an empty one, it's up to the bootloader to fill
1062	it up) among others. The error handling needs work, bugs are lurking,
1063	etc...
1064	
1065	dtc basically takes a device-tree in a given format and outputs a
1066	device-tree in another format. The currently supported formats are:
1067	
1068	  Input formats:
1069	  -------------
1070	
1071	     - "dtb": "blob" format, that is a flattened device-tree block
1072	       with
1073	        header all in a binary blob.
1074	     - "dts": "source" format. This is a text file containing a
1075	       "source" for a device-tree. The format is defined later in this
1076	        chapter.
1077	     - "fs" format. This is a representation equivalent to the
1078	        output of /proc/device-tree, that is nodes are directories and
1079		properties are files
1080	
1081	 Output formats:
1082	 ---------------
1083	
1084	     - "dtb": "blob" format
1085	     - "dts": "source" format
1086	     - "asm": assembly language file. This is a file that can be
1087	       sourced by gas to generate a device-tree "blob". That file can
1088	       then simply be added to your Makefile. Additionally, the
1089	       assembly file exports some symbols that can be used.
1090	
1091	
1092	The syntax of the dtc tool is
1093	
1094	    dtc [-I <input-format>] [-O <output-format>]
1095	        [-o output-filename] [-V output_version] input_filename
1096	
1097	
1098	The "output_version" defines what version of the "blob" format will be
1099	generated. Supported versions are 1,2,3 and 16. The default is
1100	currently version 3 but that may change in the future to version 16.
1101	
1102	Additionally, dtc performs various sanity checks on the tree, like the
1103	uniqueness of linux, phandle properties, validity of strings, etc...
1104	
1105	The format of the .dts "source" file is "C" like, supports C and C++
1106	style comments.
1107	
1108	/ {
1109	}
1110	
1111	The above is the "device-tree" definition. It's the only statement
1112	supported currently at the toplevel.
1113	
1114	/ {
1115	  property1 = "string_value";	/* define a property containing a 0
1116	                                 * terminated string
1117					 */
1118	
1119	  property2 = <0x1234abcd>;	/* define a property containing a
1120	                                 * numerical 32-bit value (hexadecimal)
1121					 */
1122	
1123	  property3 = <0x12345678 0x12345678 0xdeadbeef>;
1124	                                /* define a property containing 3
1125	                                 * numerical 32-bit values (cells) in
1126	                                 * hexadecimal
1127					 */
1128	  property4 = [0x0a 0x0b 0x0c 0x0d 0xde 0xea 0xad 0xbe 0xef];
1129	                                /* define a property whose content is
1130	                                 * an arbitrary array of bytes
1131	                                 */
1132	
1133	  childnode@address {	/* define a child node named "childnode"
1134	                                 * whose unit name is "childnode at
1135					 * address"
1136	                                 */
1137	
1138	    childprop = "hello\n";      /* define a property "childprop" of
1139	                                 * childnode (in this case, a string)
1140	                                 */
1141	  };
1142	};
1143	
1144	Nodes can contain other nodes etc... thus defining the hierarchical
1145	structure of the tree.
1146	
1147	Strings support common escape sequences from C: "\n", "\t", "\r",
1148	"\(octal value)", "\x(hex value)".
1149	
1150	It is also suggested that you pipe your source file through cpp (gcc
1151	preprocessor) so you can use #include's, #define for constants, etc...
1152	
1153	Finally, various options are planned but not yet implemented, like
1154	automatic generation of phandles, labels (exported to the asm file so
1155	you can point to a property content and change it easily from whatever
1156	you link the device-tree with), label or path instead of numeric value
1157	in some cells to "point" to a node (replaced by a phandle at compile
1158	time), export of reserve map address to the asm file, ability to
1159	specify reserve map content at compile time, etc...
1160	
1161	We may provide a .h include file with common definitions of that
1162	proves useful for some properties (like building PCI properties or
1163	interrupt maps) though it may be better to add a notion of struct
1164	definitions to the compiler...
1165	
1166	
1167	V - Recommendations for a bootloader
1168	====================================
1169	
1170	
1171	Here are some various ideas/recommendations that have been proposed
1172	while all this has been defined and implemented.
1173	
1174	  - The bootloader may want to be able to use the device-tree itself
1175	    and may want to manipulate it (to add/edit some properties,
1176	    like physical memory size or kernel arguments). At this point, 2
1177	    choices can be made. Either the bootloader works directly on the
1178	    flattened format, or the bootloader has its own internal tree
1179	    representation with pointers (similar to the kernel one) and
1180	    re-flattens the tree when booting the kernel. The former is a bit
1181	    more difficult to edit/modify, the later requires probably a bit
1182	    more code to handle the tree structure. Note that the structure
1183	    format has been designed so it's relatively easy to "insert"
1184	    properties or nodes or delete them by just memmoving things
1185	    around. It contains no internal offsets or pointers for this
1186	    purpose.
1187	
1188	  - An example of code for iterating nodes & retrieving properties
1189	    directly from the flattened tree format can be found in the kernel
1190	    file drivers/of/fdt.c.  Look at the of_scan_flat_dt() function,
1191	    its usage in early_init_devtree(), and the corresponding various
1192	    early_init_dt_scan_*() callbacks. That code can be re-used in a
1193	    GPL bootloader, and as the author of that code, I would be happy
1194	    to discuss possible free licensing to any vendor who wishes to
1195	    integrate all or part of this code into a non-GPL bootloader.
1196	    (reference needed; who is 'I' here? ---gcl Jan 31, 2011)
1197	
1198	
1199	
1200	VI - System-on-a-chip devices and nodes
1201	=======================================
1202	
1203	Many companies are now starting to develop system-on-a-chip
1204	processors, where the processor core (CPU) and many peripheral devices
1205	exist on a single piece of silicon.  For these SOCs, an SOC node
1206	should be used that defines child nodes for the devices that make
1207	up the SOC. While platforms are not required to use this model in
1208	order to boot the kernel, it is highly encouraged that all SOC
1209	implementations define as complete a flat-device-tree as possible to
1210	describe the devices on the SOC.  This will allow for the
1211	genericization of much of the kernel code.
1212	
1213	
1214	1) Defining child nodes of an SOC
1215	---------------------------------
1216	
1217	Each device that is part of an SOC may have its own node entry inside
1218	the SOC node.  For each device that is included in the SOC, the unit
1219	address property represents the address offset for this device's
1220	memory-mapped registers in the parent's address space.  The parent's
1221	address space is defined by the "ranges" property in the top-level soc
1222	node. The "reg" property for each node that exists directly under the
1223	SOC node should contain the address mapping from the child address space
1224	to the parent SOC address space and the size of the device's
1225	memory-mapped register file.
1226	
1227	For many devices that may exist inside an SOC, there are predefined
1228	specifications for the format of the device tree node.  All SOC child
1229	nodes should follow these specifications, except where noted in this
1230	document.
1231	
1232	See appendix A for an example partial SOC node definition for the
1233	MPC8540.
1234	
1235	
1236	2) Representing devices without a current OF specification
1237	----------------------------------------------------------
1238	
1239	Currently, there are many devices on SoCs that do not have a standard
1240	representation defined as part of the Open Firmware specifications,
1241	mainly because the boards that contain these SoCs are not currently
1242	booted using Open Firmware.  Binding documentation for new devices
1243	should be added to the Documentation/devicetree/bindings directory.
1244	That directory will expand as device tree support is added to more and
1245	more SoCs.
1246	
1247	
1248	VII - Specifying interrupt information for devices
1249	===================================================
1250	
1251	The device tree represents the buses and devices of a hardware
1252	system in a form similar to the physical bus topology of the
1253	hardware.
1254	
1255	In addition, a logical 'interrupt tree' exists which represents the
1256	hierarchy and routing of interrupts in the hardware.
1257	
1258	The interrupt tree model is fully described in the
1259	document "Open Firmware Recommended Practice: Interrupt
1260	Mapping Version 0.9".  The document is available at:
1261	<http://www.openfirmware.org/ofwg/practice/>
1262	
1263	1) interrupts property
1264	----------------------
1265	
1266	Devices that generate interrupts to a single interrupt controller
1267	should use the conventional OF representation described in the
1268	OF interrupt mapping documentation.
1269	
1270	Each device which generates interrupts must have an 'interrupt'
1271	property.  The interrupt property value is an arbitrary number of
1272	of 'interrupt specifier' values which describe the interrupt or
1273	interrupts for the device.
1274	
1275	The encoding of an interrupt specifier is determined by the
1276	interrupt domain in which the device is located in the
1277	interrupt tree.  The root of an interrupt domain specifies in
1278	its #interrupt-cells property the number of 32-bit cells
1279	required to encode an interrupt specifier.  See the OF interrupt
1280	mapping documentation for a detailed description of domains.
1281	
1282	For example, the binding for the OpenPIC interrupt controller
1283	specifies  an #interrupt-cells value of 2 to encode the interrupt
1284	number and level/sense information. All interrupt children in an
1285	OpenPIC interrupt domain use 2 cells per interrupt in their interrupts
1286	property.
1287	
1288	The PCI bus binding specifies a #interrupt-cell value of 1 to encode
1289	which interrupt pin (INTA,INTB,INTC,INTD) is used.
1290	
1291	2) interrupt-parent property
1292	----------------------------
1293	
1294	The interrupt-parent property is specified to define an explicit
1295	link between a device node and its interrupt parent in
1296	the interrupt tree.  The value of interrupt-parent is the
1297	phandle of the parent node.
1298	
1299	If the interrupt-parent property is not defined for a node, its
1300	interrupt parent is assumed to be an ancestor in the node's
1301	_device tree_ hierarchy.
1302	
1303	3) OpenPIC Interrupt Controllers
1304	--------------------------------
1305	
1306	OpenPIC interrupt controllers require 2 cells to encode
1307	interrupt information.  The first cell defines the interrupt
1308	number.  The second cell defines the sense and level
1309	information.
1310	
1311	Sense and level information should be encoded as follows:
1312	
1313		0 = low to high edge sensitive type enabled
1314		1 = active low level sensitive type enabled
1315		2 = active high level sensitive type enabled
1316		3 = high to low edge sensitive type enabled
1317	
1318	4) ISA Interrupt Controllers
1319	----------------------------
1320	
1321	ISA PIC interrupt controllers require 2 cells to encode
1322	interrupt information.  The first cell defines the interrupt
1323	number.  The second cell defines the sense and level
1324	information.
1325	
1326	ISA PIC interrupt controllers should adhere to the ISA PIC
1327	encodings listed below:
1328	
1329		0 =  active low level sensitive type enabled
1330		1 =  active high level sensitive type enabled
1331		2 =  high to low edge sensitive type enabled
1332		3 =  low to high edge sensitive type enabled
1333	
1334	VIII - Specifying Device Power Management Information (sleep property)
1335	===================================================================
1336	
1337	Devices on SOCs often have mechanisms for placing devices into low-power
1338	states that are decoupled from the devices' own register blocks.  Sometimes,
1339	this information is more complicated than a cell-index property can
1340	reasonably describe.  Thus, each device controlled in such a manner
1341	may contain a "sleep" property which describes these connections.
1342	
1343	The sleep property consists of one or more sleep resources, each of
1344	which consists of a phandle to a sleep controller, followed by a
1345	controller-specific sleep specifier of zero or more cells.
1346	
1347	The semantics of what type of low power modes are possible are defined
1348	by the sleep controller.  Some examples of the types of low power modes
1349	that may be supported are:
1350	
1351	 - Dynamic: The device may be disabled or enabled at any time.
1352	 - System Suspend: The device may request to be disabled or remain
1353	   awake during system suspend, but will not be disabled until then.
1354	 - Permanent: The device is disabled permanently (until the next hard
1355	   reset).
1356	
1357	Some devices may share a clock domain with each other, such that they should
1358	only be suspended when none of the devices are in use.  Where reasonable,
1359	such nodes should be placed on a virtual bus, where the bus has the sleep
1360	property.  If the clock domain is shared among devices that cannot be
1361	reasonably grouped in this manner, then create a virtual sleep controller
1362	(similar to an interrupt nexus, except that defining a standardized
1363	sleep-map should wait until its necessity is demonstrated).
1364	
1365	IX - Specifying dma bus information
1366	
1367	Some devices may have DMA memory range shifted relatively to the beginning of
1368	RAM, or even placed outside of kernel RAM. For example, the Keystone 2 SoC
1369	worked in LPAE mode with 4G memory has:
1370	- RAM range: [0x8 0000 0000, 0x8 FFFF FFFF]
1371	- DMA range: [  0x8000 0000,   0xFFFF FFFF]
1372	and DMA range is aliased into first 2G of RAM in HW.
1373	
1374	In such cases, DMA addresses translation should be performed between CPU phys
1375	and DMA addresses. The "dma-ranges" property is intended to be used
1376	for describing the configuration of such system in DT.
1377	
1378	In addition, each DMA master device on the DMA bus may or may not support
1379	coherent DMA operations. The "dma-coherent" property is intended to be used
1380	for identifying devices supported coherent DMA operations in DT.
1381	
1382	* DMA Bus master
1383	Optional property:
1384	- dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
1385		(child-bus-address, parent-bus-address, length). Each triplet specified
1386		describes a contiguous DMA address range.
1387		The dma-ranges property is used to describe the direct memory access (DMA)
1388		structure of a memory-mapped bus whose device tree parent can be accessed
1389		from DMA operations originating from the bus. It provides a means of
1390		defining a mapping or translation between the physical address space of
1391		the bus and the physical address space of the parent of the bus.
1392		(for more information see ePAPR specification)
1393	
1394	* DMA Bus child
1395	Optional property:
1396	- dma-ranges: <empty> value. if present - It means that DMA addresses
1397		translation has to be enabled for this device.
1398	- dma-coherent: Present if dma operations are coherent
1399	
1400	Example:
1401	soc {
1402			compatible = "ti,keystone","simple-bus";
1403			ranges = <0x0 0x0 0x0 0xc0000000>;
1404			dma-ranges = <0x80000000 0x8 0x00000000 0x80000000>;
1405	
1406			[...]
1407	
1408			usb: usb@2680000 {
1409				compatible = "ti,keystone-dwc3";
1410	
1411				[...]
1412				dma-coherent;
1413			};
1414	};
1415	
1416	Appendix A - Sample SOC node for MPC8540
1417	========================================
1418	
1419		soc@e0000000 {
1420			#address-cells = <1>;
1421			#size-cells = <1>;
1422			compatible = "fsl,mpc8540-ccsr", "simple-bus";
1423			device_type = "soc";
1424			ranges = <0x00000000 0xe0000000 0x00100000>
1425			bus-frequency = <0>;
1426			interrupt-parent = <&pic>;
1427	
1428			ethernet@24000 {
1429				#address-cells = <1>;
1430				#size-cells = <1>;
1431				device_type = "network";
1432				model = "TSEC";
1433				compatible = "gianfar", "simple-bus";
1434				reg = <0x24000 0x1000>;
1435				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x00 ];
1436				interrupts = <0x29 2 0x30 2 0x34 2>;
1437				phy-handle = <&phy0>;
1438				sleep = <&pmc 0x00000080>;
1439				ranges;
1440	
1441				mdio@24520 {
1442					reg = <0x24520 0x20>;
1443					compatible = "fsl,gianfar-mdio";
1444	
1445					phy0: ethernet-phy@0 {
1446						interrupts = <5 1>;
1447						reg = <0>;
1448					};
1449	
1450					phy1: ethernet-phy@1 {
1451						interrupts = <5 1>;
1452						reg = <1>;
1453					};
1454	
1455					phy3: ethernet-phy@3 {
1456						interrupts = <7 1>;
1457						reg = <3>;
1458					};
1459				};
1460			};
1461	
1462			ethernet@25000 {
1463				device_type = "network";
1464				model = "TSEC";
1465				compatible = "gianfar";
1466				reg = <0x25000 0x1000>;
1467				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x01 ];
1468				interrupts = <0x13 2 0x14 2 0x18 2>;
1469				phy-handle = <&phy1>;
1470				sleep = <&pmc 0x00000040>;
1471			};
1472	
1473			ethernet@26000 {
1474				device_type = "network";
1475				model = "FEC";
1476				compatible = "gianfar";
1477				reg = <0x26000 0x1000>;
1478				local-mac-address = [ 0x00 0xE0 0x0C 0x00 0x73 0x02 ];
1479				interrupts = <0x41 2>;
1480				phy-handle = <&phy3>;
1481				sleep = <&pmc 0x00000020>;
1482			};
1483	
1484			serial@4500 {
1485				#address-cells = <1>;
1486				#size-cells = <1>;
1487				compatible = "fsl,mpc8540-duart", "simple-bus";
1488				sleep = <&pmc 0x00000002>;
1489				ranges;
1490	
1491				serial@4500 {
1492					device_type = "serial";
1493					compatible = "ns16550";
1494					reg = <0x4500 0x100>;
1495					clock-frequency = <0>;
1496					interrupts = <0x42 2>;
1497				};
1498	
1499				serial@4600 {
1500					device_type = "serial";
1501					compatible = "ns16550";
1502					reg = <0x4600 0x100>;
1503					clock-frequency = <0>;
1504					interrupts = <0x42 2>;
1505				};
1506			};
1507	
1508			pic: pic@40000 {
1509				interrupt-controller;
1510				#address-cells = <0>;
1511				#interrupt-cells = <2>;
1512				reg = <0x40000 0x40000>;
1513				compatible = "chrp,open-pic";
1514				device_type = "open-pic";
1515			};
1516	
1517			i2c@3000 {
1518				interrupts = <0x43 2>;
1519				reg = <0x3000 0x100>;
1520				compatible  = "fsl-i2c";
1521				dfsrr;
1522				sleep = <&pmc 0x00000004>;
1523			};
1524	
1525			pmc: power@e0070 {
1526				compatible = "fsl,mpc8540-pmc", "fsl,mpc8548-pmc";
1527				reg = <0xe0070 0x20>;
1528			};
1529		};
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.