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diff --git a/Documentation/devicetree/booting-without-of.rst b/Documentation/devicetree/booting-without-of.rst new file mode 100644 index 000000000000..e9433350a20f --- /dev/null +++ b/Documentation/devicetree/booting-without-of.rst @@ -0,0 +1,1585 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================================== +Booting the Linux/ppc kernel without Open Firmware +================================================== + +Copyright (c) 2005 Benjamin Herrenschmidt <benh at kernel.crashing.org>, +IBM Corp. + +Copyright (c) 2005 Becky Bruce <becky.bruce at freescale.com>, +Freescale Semiconductor, FSL SOC and 32-bit additions + +Copyright (c) 2006 MontaVista Software, Inc. +Flash chip node definition + +.. Table of Contents + + I - Introduction + 1) Entry point for arch/arm + 2) Entry point for arch/powerpc + 3) Entry point for arch/x86 + 4) Entry point for arch/mips/bmips + 5) Entry point for arch/sh + + II - The DT block format + 1) Header + 2) Device tree generalities + 3) Device tree "structure" block + 4) Device tree "strings" block + + III - Required content of the device tree + 1) Note about cells and address representation + 2) Note about "compatible" properties + 3) Note about "name" properties + 4) Note about node and property names and character set + 5) Required nodes and properties + a) The root node + b) The /cpus node + c) The /cpus/* nodes + d) the /memory node(s) + e) The /chosen node + f) the /soc<SOCname> node + + IV - "dtc", the device tree compiler + + V - Recommendations for a bootloader + + VI - System-on-a-chip devices and nodes + 1) Defining child nodes of an SOC + 2) Representing devices without a current OF specification + + VII - Specifying interrupt information for devices + 1) interrupts property + 2) interrupt-parent property + 3) OpenPIC Interrupt Controllers + 4) ISA Interrupt Controllers + + VIII - Specifying device power management information (sleep property) + + IX - Specifying dma bus information + + Appendix A - Sample SOC node for MPC8540 + + +Revision Information +==================== + + May 18, 2005: Rev 0.1 + - Initial draft, no chapter III yet. + + May 19, 2005: Rev 0.2 + - Add chapter III and bits & pieces here or + clarifies the fact that a lot of things are + optional, the kernel only requires a very + small device tree, though it is encouraged + to provide an as complete one as possible. + + May 24, 2005: Rev 0.3 + - Precise that DT block has to be in RAM + - Misc fixes + - Define version 3 and new format version 16 + for the DT block (version 16 needs kernel + patches, will be fwd separately). + String block now has a size, and full path + is replaced by unit name for more + compactness. + linux,phandle is made optional, only nodes + that are referenced by other nodes need it. + "name" property is now automatically + deduced from the unit name + + June 1, 2005: Rev 0.4 + - Correct confusion between OF_DT_END and + OF_DT_END_NODE in structure definition. + - Change version 16 format to always align + property data to 4 bytes. Since tokens are + already aligned, that means no specific + required alignment between property size + and property data. The old style variable + alignment would make it impossible to do + "simple" insertion of properties using + memmove (thanks Milton for + noticing). Updated kernel patch as well + - Correct a few more alignment constraints + - Add a chapter about the device-tree + compiler and the textural representation of + the tree that can be "compiled" by dtc. + + November 21, 2005: Rev 0.5 + - Additions/generalizations for 32-bit + - Changed to reflect the new arch/powerpc + structure + - Added chapter VI + + + ToDo: + - Add some definitions of interrupt tree (simple/complex) + - Add some definitions for PCI host bridges + - Add some common address format examples + - Add definitions for standard properties and "compatible" + names for cells that are not already defined by the existing + OF spec. + - Compare FSL SOC use of PCI to standard and make sure no new + node definition required. + - Add more information about node definitions for SOC devices + that currently have no standard, like the FSL CPM. + + +I - Introduction +================ + +During the development of the Linux/ppc64 kernel, and more +specifically, the addition of new platform types outside of the old +IBM pSeries/iSeries pair, it was decided to enforce some strict rules +regarding the kernel entry and bootloader <-> kernel interfaces, in +order to avoid the degeneration that had become the ppc32 kernel entry +point and the way a new platform should be added to the kernel. The +legacy iSeries platform breaks those rules as it predates this scheme, +but no new board support will be accepted in the main tree that +doesn't follow them properly. In addition, since the advent of the +arch/powerpc merged architecture for ppc32 and ppc64, new 32-bit +platforms and 32-bit platforms which move into arch/powerpc will be +required to use these rules as well. + +The main requirement that will be defined in more detail below is +the presence of a device-tree whose format is defined after Open +Firmware specification. However, in order to make life easier +to embedded board vendors, the kernel doesn't require the device-tree +to represent every device in the system and only requires some nodes +and properties to be present. This will be described in detail in +section III, but, for example, the kernel does not require you to +create a node for every PCI device in the system. It is a requirement +to have a node for PCI host bridges in order to provide interrupt +routing information and memory/IO ranges, among others. It is also +recommended to define nodes for on chip devices and other buses that +don't specifically fit in an existing OF specification. This creates a +great flexibility in the way the kernel can then probe those and match +drivers to device, without having to hard code all sorts of tables. It +also makes it more flexible for board vendors to do minor hardware +upgrades without significantly impacting the kernel code or cluttering +it with special cases. + + +1) Entry point for arch/arm +--------------------------- + + There is one single entry point to the kernel, at the start + of the kernel image. That entry point supports two calling + conventions. A summary of the interface is described here. A full + description of the boot requirements is documented in + Documentation/arm/booting.rst + + a) ATAGS interface. Minimal information is passed from firmware + to the kernel with a tagged list of predefined parameters. + + r0 : 0 + + r1 : Machine type number + + r2 : Physical address of tagged list in system RAM + + b) Entry with a flattened device-tree block. Firmware loads the + physical address of the flattened device tree block (dtb) into r2, + r1 is not used, but it is considered good practice to use a valid + machine number as described in Documentation/arm/booting.rst. + + r0 : 0 + + r1 : Valid machine type number. When using a device tree, + a single machine type number will often be assigned to + represent a class or family of SoCs. + + r2 : physical pointer to the device-tree block + (defined in chapter II) in RAM. Device tree can be located + anywhere in system RAM, but it should be aligned on a 64 bit + boundary. + + The kernel will differentiate between ATAGS and device tree booting by + reading the memory pointed to by r2 and looking for either the flattened + device tree block magic value (0xd00dfeed) or the ATAG_CORE value at + offset 0x4 from r2 (0x54410001). + +2) Entry point for arch/powerpc +------------------------------- + + There is one single entry point to the kernel, at the start + of the kernel image. That entry point supports two calling + conventions: + + a) Boot from Open Firmware. If your firmware is compatible + with Open Firmware (IEEE 1275) or provides an OF compatible + client interface API (support for "interpret" callback of + forth words isn't required), you can enter the kernel with: + + r5 : OF callback pointer as defined by IEEE 1275 + bindings to powerpc. Only the 32-bit client interface + is currently supported + + r3, r4 : address & length of an initrd if any or 0 + + The MMU is either on or off; the kernel will run the + trampoline located in arch/powerpc/kernel/prom_init.c to + extract the device-tree and other information from open + firmware and build a flattened device-tree as described + in b). prom_init() will then re-enter the kernel using + the second method. This trampoline code runs in the + context of the firmware, which is supposed to handle all + exceptions during that time. + + b) Direct entry with a flattened device-tree block. This entry + point is called by a) after the OF trampoline and can also be + called directly by a bootloader that does not support the Open + Firmware client interface. It is also used by "kexec" to + implement "hot" booting of a new kernel from a previous + running one. This method is what I will describe in more + details in this document, as method a) is simply standard Open + Firmware, and thus should be implemented according to the + various standard documents defining it and its binding to the + PowerPC platform. The entry point definition then becomes: + + r3 : physical pointer to the device-tree block + (defined in chapter II) in RAM + + r4 : physical pointer to the kernel itself. This is + used by the assembly code to properly disable the MMU + in case you are entering the kernel with MMU enabled + and a non-1:1 mapping. + + r5 : NULL (as to differentiate with method a) + + Note about SMP entry: Either your firmware puts your other + CPUs in some sleep loop or spin loop in ROM where you can get + them out via a soft reset or some other means, in which case + you don't need to care, or you'll have to enter the kernel + with all CPUs. The way to do that with method b) will be + described in a later revision of this document. + + Board supports (platforms) are not exclusive config options. An + arbitrary set of board supports can be built in a single kernel + image. The kernel will "know" what set of functions to use for a + given platform based on the content of the device-tree. Thus, you + should: + + a) add your platform support as a _boolean_ option in + arch/powerpc/Kconfig, following the example of PPC_PSERIES, + PPC_PMAC and PPC_MAPLE. The later is probably a good + example of a board support to start from. + + b) create your main platform file as + "arch/powerpc/platforms/myplatform/myboard_setup.c" and add it + to the Makefile under the condition of your ``CONFIG_`` + option. This file will define a structure of type "ppc_md" + containing the various callbacks that the generic code will + use to get to your platform specific code + + A kernel image may support multiple platforms, but only if the + platforms feature the same core architecture. A single kernel build + cannot support both configurations with Book E and configurations + with classic Powerpc architectures. + +3) Entry point for arch/x86 +--------------------------- + + There is one single 32bit entry point to the kernel at code32_start, + the decompressor (the real mode entry point goes to the same 32bit + entry point once it switched into protected mode). That entry point + supports one calling convention which is documented in + Documentation/x86/boot.rst + The physical pointer to the device-tree block (defined in chapter II) + is passed via setup_data which requires at least boot protocol 2.09. + The type filed is defined as:: + + #define SETUP_DTB 2 + + This device-tree is used as an extension to the "boot page". As such it + does not parse / consider data which is already covered by the boot + page. This includes memory size, reserved ranges, command line arguments + or initrd address. It simply holds information which can not be retrieved + otherwise like interrupt routing or a list of devices behind an I2C bus. + +4) Entry point for arch/mips/bmips +---------------------------------- + + Some bootloaders only support a single entry point, at the start of the + kernel image. Other bootloaders will jump to the ELF start address. + Both schemes are supported; CONFIG_BOOT_RAW=y and CONFIG_NO_EXCEPT_FILL=y, + so the first instruction immediately jumps to kernel_entry(). + + Similar to the arch/arm case (b), a DT-aware bootloader is expected to + set up the following registers: + + a0 : 0 + + a1 : 0xffffffff + + a2 : Physical pointer to the device tree block (defined in chapter + II) in RAM. The device tree can be located anywhere in the first + 512MB of the physical address space (0x00000000 - 0x1fffffff), + aligned on a 64 bit boundary. + + Legacy bootloaders do not use this convention, and they do not pass in a + DT block. In this case, Linux will look for a builtin DTB, selected via + CONFIG_DT_*. + + This convention is defined for 32-bit systems only, as there are not + currently any 64-bit BMIPS implementations. + +5) Entry point for arch/sh +-------------------------- + + Device-tree-compatible SH bootloaders are expected to provide the physical + address of the device tree blob in r4. Since legacy bootloaders did not + guarantee any particular initial register state, kernels built to + inter-operate with old bootloaders must either use a builtin DTB or + select a legacy board option (something other than CONFIG_SH_DEVICE_TREE) + that does not use device tree. Support for the latter is being phased out + in favor of device tree. + + +II - The DT block format +======================== + + +This chapter defines the actual format of the flattened device-tree +passed to the kernel. The actual content of it and kernel requirements +are described later. You can find example of code manipulating that +format in various places, including arch/powerpc/kernel/prom_init.c +which will generate a flattened device-tree from the Open Firmware +representation, or the fs2dt utility which is part of the kexec tools +which will generate one from a filesystem representation. It is +expected that a bootloader like uboot provides a bit more support, +that will be discussed later as well. + +Note: The block has to be in main memory. It has to be accessible in +both real mode and virtual mode with no mapping other than main +memory. If you are writing a simple flash bootloader, it should copy +the block to RAM before passing it to the kernel. + + +1) Header +--------- + + The kernel is passed the physical address pointing to an area of memory + that is roughly described in include/linux/of_fdt.h by the structure + boot_param_header::: + + struct boot_param_header { + u32 magic; /* magic word OF_DT_HEADER */ + u32 totalsize; /* total size of DT block */ + u32 off_dt_struct; /* offset to structure */ + u32 off_dt_strings; /* offset to strings */ + u32 off_mem_rsvmap; /* offset to memory reserve map + */ + u32 version; /* format version */ + u32 last_comp_version; /* last compatible version */ + + /* version 2 fields below */ + u32 boot_cpuid_phys; /* Which physical CPU id we're + booting on */ + /* version 3 fields below */ + u32 size_dt_strings; /* size of the strings block */ + + /* version 17 fields below */ + u32 size_dt_struct; /* size of the DT structure block */ + }; + + Along with the constants:: + + /* Definitions used by the flattened device tree */ + #define OF_DT_HEADER 0xd00dfeed /* 4: version, + 4: total size */ + #define OF_DT_BEGIN_NODE 0x1 /* Start node: full name + */ + #define OF_DT_END_NODE 0x2 /* End node */ + #define OF_DT_PROP 0x3 /* Property: name off, + size, content */ + #define OF_DT_END 0x9 + + All values in this header are in big endian format, the various + fields in this header are defined more precisely below. All + "offset" values are in bytes from the start of the header; that is + from the physical base address of the device tree block. + + - magic + + This is a magic value that "marks" the beginning of the + device-tree block header. It contains the value 0xd00dfeed and is + defined by the constant OF_DT_HEADER + + - totalsize + + This is the total size of the DT block including the header. The + "DT" block should enclose all data structures defined in this + chapter (who are pointed to by offsets in this header). That is, + the device-tree structure, strings, and the memory reserve map. + + - off_dt_struct + + This is an offset from the beginning of the header to the start + of the "structure" part the device tree. (see 2) device tree) + + - off_dt_strings + + This is an offset from the beginning of the header to the start + of the "strings" part of the device-tree + + - off_mem_rsvmap + + This is an offset from the beginning of the header to the start + of the reserved memory map. This map is a list of pairs of 64- + bit integers. Each pair is a physical address and a size. The + list is terminated by an entry of size 0. This map provides the + kernel with a list of physical memory areas that are "reserved" + and thus not to be used for memory allocations, especially during + early initialization. The kernel needs to allocate memory during + boot for things like un-flattening the device-tree, allocating an + MMU hash table, etc... Those allocations must be done in such a + way to avoid overriding critical things like, on Open Firmware + capable machines, the RTAS instance, or on some pSeries, the TCE + tables used for the iommu. Typically, the reserve map should + contain **at least** this DT block itself (header,total_size). If + you are passing an initrd to the kernel, you should reserve it as + well. You do not need to reserve the kernel image itself. The map + should be 64-bit aligned. + + - version + + This is the version of this structure. Version 1 stops + here. Version 2 adds an additional field boot_cpuid_phys. + Version 3 adds the size of the strings block, allowing the kernel + to reallocate it easily at boot and free up the unused flattened + structure after expansion. Version 16 introduces a new more + "compact" format for the tree itself that is however not backward + compatible. Version 17 adds an additional field, size_dt_struct, + allowing it to be reallocated or moved more easily (this is + particularly useful for bootloaders which need to make + adjustments to a device tree based on probed information). You + should always generate a structure of the highest version defined + at the time of your implementation. Currently that is version 17, + unless you explicitly aim at being backward compatible. + + - last_comp_version + + Last compatible version. This indicates down to what version of + the DT block you are backward compatible. For example, version 2 + is backward compatible with version 1 (that is, a kernel build + for version 1 will be able to boot with a version 2 format). You + should put a 1 in this field if you generate a device tree of + version 1 to 3, or 16 if you generate a tree of version 16 or 17 + using the new unit name format. + + - boot_cpuid_phys + + This field only exist on version 2 headers. It indicate which + physical CPU ID is calling the kernel entry point. This is used, + among others, by kexec. If you are on an SMP system, this value + should match the content of the "reg" property of the CPU node in + the device-tree corresponding to the CPU calling the kernel entry + point (see further chapters for more information on the required + device-tree contents) + + - size_dt_strings + + This field only exists on version 3 and later headers. It + gives the size of the "strings" section of the device tree (which + starts at the offset given by off_dt_strings). + + - size_dt_struct + + This field only exists on version 17 and later headers. It gives + the size of the "structure" section of the device tree (which + starts at the offset given by off_dt_struct). + + So the typical layout of a DT block (though the various parts don't + need to be in that order) looks like this (addresses go from top to + bottom):: + + + ------------------------------ + base -> | struct boot_param_header | + ------------------------------ + | (alignment gap) (*) | + ------------------------------ + | memory reserve map | + ------------------------------ + | (alignment gap) | + ------------------------------ + | | + | device-tree structure | + | | + ------------------------------ + | (alignment gap) | + ------------------------------ + | | + | device-tree strings | + | | + -----> ------------------------------ + | + | + --- (base + totalsize) + + (*) The alignment gaps are not necessarily present; their presence + and size are dependent on the various alignment requirements of + the individual data blocks. + + +2) Device tree generalities +--------------------------- + +This device-tree itself is separated in two different blocks, a +structure block and a strings block. Both need to be aligned to a 4 +byte boundary. + +First, let's quickly describe the device-tree concept before detailing +the storage format. This chapter does _not_ describe the detail of the +required types of nodes & properties for the kernel, this is done +later in chapter III. + +The device-tree layout is strongly inherited from the definition of +the Open Firmware IEEE 1275 device-tree. It's basically a tree of +nodes, each node having two or more named properties. A property can +have a value or not. + +It is a tree, so each node has one and only one parent except for the +root node who has no parent. + +A node has 2 names. The actual node name is generally contained in a +property of type "name" in the node property list whose value is a +zero terminated string and is mandatory for version 1 to 3 of the +format definition (as it is in Open Firmware). Version 16 makes it +optional as it can generate it from the unit name defined below. + +There is also a "unit name" that is used to differentiate nodes with +the same name at the same level, it is usually made of the node +names, the "@" sign, and a "unit address", which definition is +specific to the bus type the node sits on. + +The unit name doesn't exist as a property per-se but is included in +the device-tree structure. It is typically used to represent "path" in +the device-tree. More details about the actual format of these will be +below. + +The kernel generic code does not make any formal use of the +unit address (though some board support code may do) so the only real +requirement here for the unit address is to ensure uniqueness of +the node unit name at a given level of the tree. Nodes with no notion +of address and no possible sibling of the same name (like /memory or +/cpus) may omit the unit address in the context of this specification, +or use the "@0" default unit address. The unit name is used to define +a node "full path", which is the concatenation of all parent node +unit names separated with "/". + +The root node doesn't have a defined name, and isn't required to have +a name property either if you are using version 3 or earlier of the +format. It also has no unit address (no @ symbol followed by a unit +address). The root node unit name is thus an empty string. The full +path to the root node is "/". + +Every node which actually represents an actual device (that is, a node +which isn't only a virtual "container" for more nodes, like "/cpus" +is) is also required to have a "compatible" property indicating the +specific hardware and an optional list of devices it is fully +backwards compatible with. + +Finally, every node that can be referenced from a property in another +node is required to have either a "phandle" or a "linux,phandle" +property. Real Open Firmware implementations provide a unique +"phandle" value for every node that the "prom_init()" trampoline code +turns into "linux,phandle" properties. However, this is made optional +if the flattened device tree is used directly. An example of a node +referencing another node via "phandle" is when laying out the +interrupt tree which will be described in a further version of this +document. + +The "phandle" property is a 32-bit value that uniquely +identifies a node. You are free to use whatever values or system of +values, internal pointers, or whatever to generate these, the only +requirement is that every node for which you provide that property has +a unique value for it. + +Here is an example of a simple device-tree. In this example, an "o" +designates a node followed by the node unit name. Properties are +presented with their name followed by their content. "content" +represents an ASCII string (zero terminated) value, while <content> +represents a 32-bit value, specified in decimal or hexadecimal (the +latter prefixed 0x). The various nodes in this example will be +discussed in a later chapter. At this point, it is only meant to give +you a idea of what a device-tree looks like. I have purposefully kept +the "name" and "linux,phandle" properties which aren't necessary in +order to give you a better idea of what the tree looks like in +practice:: + + / o device-tree + |- name = "device-tree" + |- model = "MyBoardName" + |- compatible = "MyBoardFamilyName" + |- #address-cells = <2> + |- #size-cells = <2> + |- linux,phandle = <0> + | + o cpus + | | - name = "cpus" + | | - linux,phandle = <1> + | | - #address-cells = <1> + | | - #size-cells = <0> + | | + | o PowerPC,970@0 + | |- name = "PowerPC,970" + | |- device_type = "cpu" + | |- reg = <0> + | |- clock-frequency = <0x5f5e1000> + | |- 64-bit + | |- linux,phandle = <2> + | + o memory@0 + | |- name = "memory" + | |- device_type = "memory" + | |- reg = <0x00000000 0x00000000 0x00000000 0x20000000> + | |- linux,phandle = <3> + | + o chosen + |- name = "chosen" + |- bootargs = "root=/dev/sda2" + |- linux,phandle = <4> + +This tree is almost a minimal tree. It pretty much contains the +minimal set of required nodes and properties to boot a linux kernel; +that is, some basic model information at the root, the CPUs, and the +physical memory layout. It also includes misc information passed +through /chosen, like in this example, the platform type (mandatory) +and the kernel command line arguments (optional). + +The /cpus/PowerPC,970@0/64-bit property is an example of a +property without a value. All other properties have a value. The +significance of the #address-cells and #size-cells properties will be +explained in chapter IV which defines precisely the required nodes and +properties and their content. + + +3) Device tree "structure" block +-------------------------------- + +The structure of the device tree is a linearized tree structure. The +"OF_DT_BEGIN_NODE" token starts a new node, and the "OF_DT_END_NODE" +ends that node definition. Child nodes are simply defined before +"OF_DT_END_NODE" (that is nodes within the node). A 'token' is a 32 +bit value. The tree has to be "finished" with a OF_DT_END token + +Here's the basic structure of a single node: + + * token OF_DT_BEGIN_NODE (that is 0x00000001) + * for version 1 to 3, this is the node full path as a zero + terminated string, starting with "/". For version 16 and later, + this is the node unit name only (or an empty string for the + root node) + * [align gap to next 4 bytes boundary] + * for each property: + + * token OF_DT_PROP (that is 0x00000003) + * 32-bit value of property value size in bytes (or 0 if no + value) + * 32-bit value of offset in string block of property name + * property value data if any + * [align gap to next 4 bytes boundary] + + * [child nodes if any] + * token OF_DT_END_NODE (that is 0x00000002) + +So the node content can be summarized as a start token, a full path, +a list of properties, a list of child nodes, and an end token. Every +child node is a full node structure itself as defined above. + +NOTE: The above definition requires that all property definitions for +a particular node MUST precede any subnode definitions for that node. +Although the structure would not be ambiguous if properties and +subnodes were intermingled, the kernel parser requires that the +properties come first (up until at least 2.6.22). Any tools +manipulating a flattened tree must take care to preserve this +constraint. + +4) Device tree "strings" block +------------------------------ + +In order to save space, property names, which are generally redundant, +are stored separately in the "strings" block. This block is simply the +whole bunch of zero terminated strings for all property names +concatenated together. The device-tree property definitions in the +structure block will contain offset values from the beginning of the +strings block. + + +III - Required content of the device tree +========================================= + +.. Warning:: + + All ``linux,*`` properties defined in this document apply only + to a flattened device-tree. If your platform uses a real + implementation of Open Firmware or an implementation compatible with + the Open Firmware client interface, those properties will be created + by the trampoline code in the kernel's prom_init() file. For example, + that's where you'll have to add code to detect your board model and + set the platform number. However, when using the flattened device-tree + entry point, there is no prom_init() pass, and thus you have to + provide those properties yourself. + + +1) Note about cells and address representation +---------------------------------------------- + +The general rule is documented in the various Open Firmware +documentations. If you choose to describe a bus with the device-tree +and there exist an OF bus binding, then you should follow the +specification. However, the kernel does not require every single +device or bus to be described by the device tree. + +In general, the format of an address for a device is defined by the +parent bus type, based on the #address-cells and #size-cells +properties. Note that the parent's parent definitions of #address-cells +and #size-cells are not inherited so every node with children must specify +them. The kernel requires the root node to have those properties defining +addresses format for devices directly mapped on the processor bus. + +Those 2 properties define 'cells' for representing an address and a +size. A "cell" is a 32-bit number. For example, if both contain 2 +like the example tree given above, then an address and a size are both +composed of 2 cells, and each is a 64-bit number (cells are +concatenated and expected to be in big endian format). Another example +is the way Apple firmware defines them, with 2 cells for an address +and one cell for a size. Most 32-bit implementations should define +#address-cells and #size-cells to 1, which represents a 32-bit value. +Some 32-bit processors allow for physical addresses greater than 32 +bits; these processors should define #address-cells as 2. + +"reg" properties are always a tuple of the type "address size" where +the number of cells of address and size is specified by the bus +#address-cells and #size-cells. When a bus supports various address +spaces and other flags relative to a given address allocation (like +prefetchable, etc...) those flags are usually added to the top level +bits of the physical address. For example, a PCI physical address is +made of 3 cells, the bottom two containing the actual address itself +while the top cell contains address space indication, flags, and pci +bus & device numbers. + +For buses that support dynamic allocation, it's the accepted practice +to then not provide the address in "reg" (keep it 0) though while +providing a flag indicating the address is dynamically allocated, and +then, to provide a separate "assigned-addresses" property that +contains the fully allocated addresses. See the PCI OF bindings for +details. + +In general, a simple bus with no address space bits and no dynamic +allocation is preferred if it reflects your hardware, as the existing +kernel address parsing functions will work out of the box. If you +define a bus type with a more complex address format, including things +like address space bits, you'll have to add a bus translator to the +prom_parse.c file of the recent kernels for your bus type. + +The "reg" property only defines addresses and sizes (if #size-cells is +non-0) within a given bus. In order to translate addresses upward +(that is into parent bus addresses, and possibly into CPU physical +addresses), all buses must contain a "ranges" property. If the +"ranges" property is missing at a given level, it's assumed that +translation isn't possible, i.e., the registers are not visible on the +parent bus. The format of the "ranges" property for a bus is a list +of:: + + bus address, parent bus address, size + +"bus address" is in the format of the bus this bus node is defining, +that is, for a PCI bridge, it would be a PCI address. Thus, (bus +address, size) defines a range of addresses for child devices. "parent +bus address" is in the format of the parent bus of this bus. For +example, for a PCI host controller, that would be a CPU address. For a +PCI<->ISA bridge, that would be a PCI address. It defines the base +address in the parent bus where the beginning of that range is mapped. + +For new 64-bit board support, I recommend either the 2/2 format or +Apple's 2/1 format which is slightly more compact since sizes usually +fit in a single 32-bit word. New 32-bit board support should use a +1/1 format, unless the processor supports physical addresses greater +than 32-bits, in which case a 2/1 format is recommended. + +Alternatively, the "ranges" property may be empty, indicating that the +registers are visible on the parent bus using an identity mapping +translation. In other words, the parent bus address space is the same +as the child bus address space. + +2) Note about "compatible" properties +------------------------------------- + +These properties are optional, but recommended in devices and the root +node. The format of a "compatible" property is a list of concatenated +zero terminated strings. They allow a device to express its +compatibility with a family of similar devices, in some cases, +allowing a single driver to match against several devices regardless +of their actual names. + +3) Note about "name" properties +------------------------------- + +While earlier users of Open Firmware like OldWorld macintoshes tended +to use the actual device name for the "name" property, it's nowadays +considered a good practice to use a name that is closer to the device +class (often equal to device_type). For example, nowadays, Ethernet +controllers are named "ethernet", an additional "model" property +defining precisely the chip type/model, and "compatible" property +defining the family in case a single driver can driver more than one +of these chips. However, the kernel doesn't generally put any +restriction on the "name" property; it is simply considered good +practice to follow the standard and its evolutions as closely as +possible. + +Note also that the new format version 16 makes the "name" property +optional. If it's absent for a node, then the node's unit name is then +used to reconstruct the name. That is, the part of the unit name +before the "@" sign is used (or the entire unit name if no "@" sign +is present). + +4) Note about node and property names and character set +------------------------------------------------------- + +While Open Firmware provides more flexible usage of 8859-1, this +specification enforces more strict rules. Nodes and properties should +be comprised only of ASCII characters 'a' to 'z', '0' to +'9', ',', '.', '_', '+', '#', '?', and '-'. Node names additionally +allow uppercase characters 'A' to 'Z' (property names should be +lowercase. The fact that vendors like Apple don't respect this rule is +irrelevant here). Additionally, node and property names should always +begin with a character in the range 'a' to 'z' (or 'A' to 'Z' for node +names). + +The maximum number of characters for both nodes and property names +is 31. In the case of node names, this is only the leftmost part of +a unit name (the pure "name" property), it doesn't include the unit +address which can extend beyond that limit. + + +5) Required nodes and properties +-------------------------------- + These are all that are currently required. However, it is strongly + recommended that you expose PCI host bridges as documented in the + PCI binding to Open Firmware, and your interrupt tree as documented + in OF interrupt tree specification. + + a) The root node + + The root node requires some properties to be present: |