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diff --git a/Documentation/devicetree/booting-without-of.txt b/Documentation/devicetree/booting-without-of.txt new file mode 100644 index 0000000..9381a14 --- /dev/null +++ b/Documentation/devicetree/booting-without-of.txt @@ -0,0 +1,1430 @@ + Booting the Linux/ppc kernel without Open Firmware + -------------------------------------------------- + +(c) 2005 Benjamin Herrenschmidt <benh at kernel.crashing.org>, + IBM Corp. +(c) 2005 Becky Bruce <becky.bruce at freescale.com>, + Freescale Semiconductor, FSL SOC and 32-bit additions +(c) 2006 MontaVista Software, Inc. + Flash chip node definition + +Table of Contents +================= + + I - Introduction + 1) Entry point for arch/powerpc + 2) Entry point for arch/arm + + 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) + + 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 informations 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/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. + +2) 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 + + 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 practise to use a valid + machine number as described in Documentation/arm/Booting. + + 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 32 bit + boundary. + + The kernel will differentiate between ATAGS and device tree booting by + reading the memory pointed to by r1 and looking for either the flattened + device tree block magic value (0xd00dfeed) or the ATAG_CORE value at + offset 0x4 from r2 (0x54410001). + + +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 informations 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 hexadecimal value. 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 = <5f5e1000> + | |- 64-bit + | |- linux,phandle = <2> + | + o memory@0 + | |- name = "memory" + | |- device_type = "memory" + | |- reg = <00000000 00000000 00000000 20000000> + | |- 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 informations 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: + + - model : this is your board name/model + - #address-cells : address representation for "root" devices + - #size-cells: the size representation for "root" devices + - compatible : the board "family" generally finds its way here, + for example, if you have 2 board models with a similar layout, + that typically get driven by the same platform code in the + kernel, you would specify the exact board model in the + compatible property followed by an entry that represents the SoC + model. + + The root node is also generally where you add additional properties + specific to your board like the serial number if any, that sort of + thing. It is recommended that if you add any "custom" property whose + name may clash with standard defined ones, you prefix them with your + vendor name and a comma. + + b) The /cpus node + + This node is the parent of all individual CPU nodes. It doesn't + have any specific requirements, though it's generally good practice + to have at least: + + #address-cells = <00000001> + #size-cells = <00000000> + + This defines that the "address" for a CPU is a single cell, and has + no meaningful size. This is not necessary but the kernel will assume + that format when reading the "reg" properties of a CPU node, see + below + + c) The /cpus/* nodes + + So under /cpus, you are supposed to create a node for every CPU on + the machine. There is no specific restriction on the name of the + CPU, though it's common to call it <architecture>,<core>. For + example, Apple uses PowerPC,G5 while IBM uses PowerPC,970FX. + However, the Generic Names convention suggests that it would be + better to simply use 'cpu' for each cpu node and use the compatible + property to identify the specific cpu core. + + Required properties: + + - device_type : has to be "cpu" + - reg : This is the physical CPU number, it's a single 32-bit cell + and is also used as-is as the unit number for constructing the + unit name in the full path. For example, with 2 CPUs, you would + have the full path: + /cpus/PowerPC,970FX@0 + /cpus/PowerPC,970FX@1 + (unit addresses do not require leading zeroes) + - d-cache-block-size : one cell, L1 data cache block size in bytes (*) + - i-cache-block-size : one cell, L1 instruction cache block size in + bytes + - d-cache-size : one cell, size of L1 data cache in bytes + - i-cache-size : one cell, size of L1 instruction cache in bytes + +(*) The cache "block" size is the size on which the cache management +instructions operate. Historically, this document used the cache +"line" size here which is incorrect. The kernel will prefer the cache +block size and will fallback to cache line size for backward +compatibility. + + Recommended properties: + + - timebase-frequency : a cell indicating the frequency of the + timebase in Hz. This is not directly used by the generic code, + but you are welcome to copy/paste the pSeries code for setting + the kernel timebase/decrementer calibration based on this + value. + - clock-frequency : a cell indicating the CPU core clock frequency + in Hz. A new property will be defined for 64-bit values, but if + your frequency is < 4Ghz, one cell is enough. Here as well as + for the above, the common code doesn't use that property, but + you are welcome to re-use the pSeries or Maple one. A future + kernel version might provide a common function for this. + - d-cache-line-size : one cell, L1 data cache line size in bytes + if different from the block size + - i-cache-line-size : one cell, L1 instruction cache line size in + bytes if different from the block size + + You are welcome to add any property you find relevant to your board, + like some information about the mechanism used to soft-reset the + CPUs. For example, Apple puts the GPIO number for CPU soft reset + lines in there as a "soft-reset" property since they start secondary + CPUs by soft-resetting them. + + + d) the /memory node(s) + + To define the physical memory layout of your board, you should + create one or more memory node(s). You can either create a single + node with all memory ranges in its reg property, or you can create + several nodes, as you wish. The unit address (@ part) used for the + full path is the address of the first range of memory defined by a + given node. If you use a single memory node, this will typically be + @0. + + Required properties: + + - device_type : has to be "memory" + - reg : This property contains all the physical memory ranges of + your board. It's a list of addresses/sizes concatenated + together, with the number of cells of each defined by the + #address-cells and #size-cells of the root node. For example, + with both of these properties being 2 like in the example given + earlier, a 970 based machine with 6Gb of RAM could typically + have a "reg" property here that looks like: + + 00000000 00000000 00000000 80000000 + 00000001 00000000 00000001 00000000 + + That is a range starting at 0 of 0x80000000 bytes and a range + starting at 0x100000000 and of 0x100000000 bytes. You can see + that there is no memory covering the IO hole between 2Gb and + 4Gb. Some vendors prefer splitting those ranges into smaller + segments, but the kernel doesn't care. + + e) The /chosen node + + This node is a bit "special". Normally, that's where Open Firmware + puts some variable environment information, like the arguments, or + the default input/output devices. + + This specification makes a few of these mandatory, but also defines + some linux-specific properties that would be normally constructed by + the prom_init() trampoline when booting with an OF client interface, + but that you have to provide yourself when using the flattened format. + + Recommended properties: + + - bootargs : This zero-terminated string is passed as the kernel + command line + - linux,stdout-path : This is the full path to your standard + console device if any. Typically, if you have serial devices on + your board, you may want to put the full path to the one set as + the default console in the firmware here, for the kernel to pick + it up as its own default console. + + Note that u-boot creates and fills in the chosen node for platforms + that use it. + + (Note: a practice that is now obsolete was to include a property + under /chosen called interrupt-controller which had a phandle value + that pointed to the main interrupt controller) + + f) the /soc<SOCname> node + + This node is used to represent a system-on-a-chip (SoC) and must be + present if the processor is a SoC. The top-level soc node contains + information that is global to all devices on the SoC. The node name + should contain a unit address for the SoC, which is the base address + of the memory-mapped register set for the SoC. The name of an SoC + node should start with "soc", and the remainder of the name should + represent the part number for the soc. For example, the MPC8540's + soc node would be called "soc8540". + + Required properties: + + - ranges : Should be defined as specified in 1) to describe the + translation of SoC addresses for memory mapped SoC registers. + - bus-frequency: Contains the bus frequency for the SoC node. + Typically, the value of this field is filled in by the boot + loader. + - compatible : Exact model of the SoC + + + Recommended properties: + + - reg : This property defines the address and size of the + memory-mapped registers that are used for the SOC node itself. + It does not include the child device registers - these will be + defined inside each child node. The address specified in the + "reg" property should match the unit address of the SOC node. + - #address-cells : Address representation for "soc" devices. The + format of this field may vary depending on whether or not the + device registers are memory mapped. For memory mapped + registers, this field represents the number of cells needed to + represent the address of the registers. For SOCs that do not + use MMIO, a special address format should be defined that + contains enough cells to represent the required information. + See 1) above for more details on defining #address-cells. + - #size-cells : Size representation for "soc" devices + - #interrupt-cells : Defines the width of cells used to represent + interrupts. Typically this value is <2>, which includes a + 32-bit number that represents the interrupt number, and a + 32-bit number that represents the interrupt sense and level. + This field is only needed if the SOC contains an interrupt + controller. + + The SOC node may contain child nodes for each SOC device that the + platform uses. Nodes should not be created for devices which exist + on the SOC but are not used by a particular platform. See chapter VI + for more information on how to specify devices that are part of a SOC. + + Example SOC node for the MPC8540: + + soc8540@e0000000 { + #address-cells = <1>; + #size-cells = <1>; + #interrupt-cells = <2>; + device_type = "soc"; + ranges = <00000000 e0000000 00100000> + reg = <e0000000 00003000>; + bus-frequency = <0>; + } + + + +IV - "dtc", the device tree compiler +==================================== + + +dtc source code can be found at +<http://git.jdl.com/gitweb/?p=dtc.git> + +WARNING: This version is still in early development stage; the +resulting device-tree "blobs" have not yet been validated with the +kernel. The current generated block lacks a useful reserve map (it will +be fixed to generate an empty one, it's up to the bootloader to fill +it up) among others. The error handling needs work, bugs are lurking, +etc... + +dtc basically takes a device-tree in a given format and outputs a +device-tree in another format. The currently supported formats are: + + Input formats: + ------------- + + - "dtb": "blob" format, that is a flattened device-tree block + with + header all in a binary blob. + - "dts": "source" format. This is a text file containing a + "source" for a device-tree. The format is defined later in this + chapter. + - "fs" format. This is a representation equivalent to the + output of /proc/device-tree, that is nodes are directories and + properties are files + + Output formats: + --------------- + + - "dtb": "blob" format + - "dts": "source" format + - "asm": assembly language file. This is a file that can be + sourced by gas to generate a device-tree "blob". That file can + then simply be added to your Makefile. Additionally, the + assembly file exports some symbols that can be used. + + +The syntax of the dtc tool is + + dtc [-I <input-format>] [-O <output-format>] + [-o output-filename] [-V output_version] input_filename + + +The "output_version" defines what version of the "blob" format will be +generated. Supported versions are 1,2,3 and 16. The default is +currently version 3 but that may change in the future to version 16. + +Additionally, dtc performs various sanity checks on the tree, like the +uniqueness of linux, phandle properties, validity of strings, etc... + +The format of the .dts "source" file is "C" like, supports C and C++ +style comments. + +/ { +} + +The above is the "device-tree" definition. It's the only statement +supported currently at the toplevel. + +/ { + property1 = "string_value"; /* define a property containing a 0 + * terminated string + */ + + property2 = <1234abcd>; /* define a property containing a + * numerical 32-bit value (hexadecimal) + */ + + property3 = <12345678 12345678 deadbeef>; + /* define a property containing 3 + * numerical 32-bit values (cells) in + * hexadecimal + */ + property4 = [0a 0b 0c 0d de ea ad be ef]; + /* define a property whose content is + * an arbitrary array of bytes + */ + + childnode@address { /* define a child node named "childnode" + * whose unit name is "childnode at + * address" + */ + + childprop = "hello\n"; /* define a property "childprop" of + * childnode (in this case, a string) + */ + }; +}; + +Nodes can contain other nodes etc... thus defining the hierarchical +structure of the tree. + +Strings support common escape sequences from C: "\n", "\t", "\r", +"\(octal value)", "\x(hex value)". + +It is also suggested that you pipe your source file through cpp (gcc +preprocessor) so you can use #include's, #define for constants, etc... + +Finally, various options are planned but not yet implemented, like +automatic generation of phandles, labels (exported to the asm file so +you can point to a property content and change it easily from whatever +you link the device-tree with), label or path instead of numeric value +in some cells to "point" to a node (replaced by a phandle at compile +time), export of reserve map address to the asm file, ability to +specify reserve map content at compile time, etc... + +We may provide a .h include file with common definitions of that +proves useful for some properties (like building PCI properties or +interrupt maps) though it may be better to add a notion of struct +definitions to the compiler... + + +V - Recommendations for a bootloader +==================================== + + +Here are some various ideas/recommendations that have been proposed +while all this has been defined and implemented. + + - The bootloader may want to be able to use the device-tree itself + and may want to manipulate it (to add/edit some properties, + like physical memory size or kernel arguments). At this point, 2 + choices can be made. Either the bootloader works directly on the + flattened format, or the bootloader has its own internal tree + representation with pointers (similar to the kernel one) and + re-flattens the tree when booting the kernel. The former is a bit + more difficult to edit/modify, the later requires probably a bit + more code to handle the tree structure. Note that the structure + format has been designed so it's relatively easy to "insert" + properties or nodes or delete them by just memmoving things + around. It contains no internal offsets or pointers for this + purpose. + + - An example of code for iterating nodes & retrieving properties + directly from the flattened tree format can be found in the kernel + file drivers/of/fdt.c. Look at the of_scan_flat_dt() function, + its usage in early_init_devtree(), and the corresponding various + early_init_dt_scan_*() callbacks. That code can be re-used in a + GPL bootloader, and as the author of that code, I would be happy + to discuss possible free licensing to any vendor who wishes to + integrate all or part of this code into a non-GPL bootloader. + (reference needed; who is 'I' here? ---gcl Jan 31, 2011) + + + +VI - System-on-a-chip devices and nodes +======================================= + +Many companies are now starting to develop system-on-a-chip +processors, where the processor core (CPU) and many peripheral devices +exist on a single piece of silicon. For these SOCs, an SOC node +should be used that defines child nodes for the devices that make +up the SOC. While platforms are not required to use this model in +order to boot the kernel, it is highly encouraged that all SOC +implementations define as complete a flat-device-tree as possible to +describe the devices on the SOC. This will allow for the +genericization of much of the kernel code. + + +1) Defining child nodes of an SOC +--------------------------------- + +Each device that is part of an SOC may have its own node entry inside +the SOC node. For each device that is included in the SOC, the unit +address property represents the address offset for this device's +memory-mapped registers in the parent's address space. The parent's +address space is defined by the "ranges" property in the top-level soc +node. The "reg" property for each node that exists directly under the +SOC node should contain the address mapping from the child address space +to the parent SOC address space and the size of the device's +memory-mapped register file. + +For many devices that may exist inside an SOC, there are predefined +specifications for the format of the device tree node. All SOC child +nodes should follow these specifications, except where noted in this +document. + +See appendix A for an example partial SOC node definition for the +MPC8540. + + +2) Representing devices without a current OF specification +---------------------------------------------------------- + +Currently, there are many devices on SoCs that do not have a standard +representation defined as part of the Open Firmware specifications, +mainly because the boards that contain these SoCs are not currently +booted using Open Firmware. Binding documentation for new devices +should be added to the Documentation/devicetree/bindings directory. +That directory will expand as device tree support is added to more and +more SoCs. + + +VII - Specifying interrupt information for devices +=================================================== + +The device tree represents the buses and devices of a hardware +system in a form similar to the physical bus topology of the +hardware. + +In addition, a logical 'interrupt tree' exists which represents the +hierarchy and routing of interrupts in the hardware. + +The interrupt tree model is fully described in the +document "Open Firmware Recommended Practice: Interrupt +Mapping Version 0.9". The document is available at: +<http://playground.sun.com/1275/practice>. + +1) interrupts property +---------------------- + +Devices that generate interrupts to a single interrupt controller +should use the conventional OF representation described in the +OF interrupt mapping documentation. + +Each device which generates interrupts must have an 'interrupt' +property. The interrupt property value is an arbitrary number of +of 'interrupt specifier' values which describe the interrupt or +interrupts for the device. + +The encoding of an interrupt specifier is determined by the +interrupt domain in which the device is located in the +interrupt tree. The root of an interrupt domain specifies in +its #interrupt-cells property the number of 32-bit cells +required to encode an interrupt specifier. See the OF interrupt +mapping documentation for a detailed description of domains. + +For example, the binding for the OpenPIC interrupt controller +specifies an #interrupt-cells value of 2 to encode the interrupt +number and level/sense information. All interrupt children in an +OpenPIC interrupt domain use 2 cells per interrupt in their interrupts +property. + +The PCI bus binding specifies a #interrupt-cell value of 1 to encode +which interrupt pin (INTA,INTB,INTC,INTD) is used. + +2) interrupt-parent property +---------------------------- + +The interrupt-parent property is specified to define an explicit +link between a device node and its interrupt parent in +the interrupt tree. The value of interrupt-parent is the +phandle of the parent node. + +If the interrupt-parent property is not defined for a node, its +interrupt parent is assumed to be an ancestor in the node's +_device tree_ hierarchy. + +3) OpenPIC Interrupt Controllers +-------------------------------- + +OpenPIC interrupt controllers require 2 cells to encode +interrupt information. The first cell defines the interrupt +number. The second cell defines the sense and level +information. + +Sense and level information should be encoded as follows: + + 0 = low to high edge sensitive type enabled + 1 = active low level sensitive type enabled + 2 = active high level sensitive type enabled + 3 = high to low edge sensitive type enabled + +4) ISA Interrupt Controllers +---------------------------- + +ISA PIC interrupt controllers require 2 cells to encode +interrupt information. The first cell defines the interrupt +number. The second cell defines the sense and level +information. + +ISA PIC interrupt controllers should adhere to the ISA PIC +encodings listed below: + + 0 = active low level sensitive type enabled + 1 = active high level sensitive type enabled + 2 = high to low edge sensitive type enabled + 3 = low to high edge sensitive type enabled + +VIII - Specifying Device Power Management Information (sleep property) +=================================================================== + +Devices on SOCs often have mechanisms for placing devices into low-power +states that are decoupled from the devices' own register blocks. Sometimes, +this information is more complicated than a cell-index property can +reasonably describe. Thus, each device controlled in such a manner +may contain a "sleep" property which describes these connections. + +The sleep property consists of one or more sleep resources, each of +which consists of a phandle to a sleep controller, followed by a +controller-specific sleep specifier of zero or more cells. + +The semantics of what type of low power modes are possible are defined +by the sleep controller. Some examples of the types of low power modes +that may be supported are: + + - Dynamic: The device may be disabled or enabled at any time. + - System Suspend: The device may request to be disabled or remain + awake during system suspend, but will not be disabled until then. + - Permanent: The device is disabled permanently (until the next hard + reset). + +Some devices may share a clock domain with each other, such that they should +only be suspended when none of the devices are in use. Where reasonable, +such nodes should be placed on a virtual bus, where the bus has the sleep +property. If the clock domain is shared among devices that cannot be +reasonably grouped in this manner, then create a virtual sleep controller +(similar to an interrupt nexus, except that defining a standardized +sleep-map should wait until its necessity is demonstrated). + +Appendix A - Sample SOC node for MPC8540 +======================================== + + soc@e0000000 { + #address-cells = <1>; + #size-cells = <1>; + compatible = "fsl,mpc8540-ccsr", "simple-bus"; + device_type = "soc"; + ranges = <0x00000000 0xe0000000 0x00100000> + bus-frequency = <0>; + interrupt-parent = <&pic>; + + ethernet@24000 { + #address-cells = <1>; + #size-cells = <1>; + device_type = "network"; + model = "TSEC"; + compatible = "gianfar", "simple-bus"; + reg = <0x24000 0x1000>; + local-mac-address = [ 00 E0 0C 00 73 00 ]; + interrupts = <29 2 30 2 34 2>; + phy-handle = <&phy0>; + sleep = <&pmc 00000080>; + ranges; + + mdio@24520 { + reg = <0x24520 0x20>; + compatible = "fsl,gianfar-mdio"; + + phy0: ethernet-phy@0 { + interrupts = <5 1>; + reg = <0>; + device_type = "ethernet-phy"; + }; + + phy1: ethernet-phy@1 { + interrupts = <5 1>; + reg = <1>; + device_type = "ethernet-phy"; + }; + + phy3: ethernet-phy@3 { + interrupts = <7 1>; + reg = <3>; + device_type = "ethernet-phy"; + }; + }; + }; + + ethernet@25000 { + device_type = "network"; + model = "TSEC"; + compatible = "gianfar"; + reg = <0x25000 0x1000>; + local-mac-address = [ 00 E0 0C 00 73 01 ]; + interrupts = <13 2 14 2 18 2>; + phy-handle = <&phy1>; + sleep = <&pmc 00000040>; + }; + + ethernet@26000 { + device_type = "network"; + model = "FEC"; + compatible = "gianfar"; + reg = <0x26000 0x1000>; + local-mac-address = [ 00 E0 0C 00 73 02 ]; + interrupts = <41 2>; + phy-handle = <&phy3>; + sleep = <&pmc 00000020>; + }; + + serial@4500 { + #address-cells = <1>; + #size-cells = <1>; + compatible = "fsl,mpc8540-duart", "simple-bus"; + sleep = <&pmc 00000002>; + ranges; + + serial@4500 { + device_type = "serial"; + compatible = "ns16550"; + reg = <0x4500 0x100>; + clock-frequency = <0>; + interrupts = <42 2>; + }; + + serial@4600 { + device_type = "serial"; + compatible = "ns16550"; + reg = <0x4600 0x100>; + clock-frequency = <0>; + interrupts = <42 2>; + }; + }; + + pic: pic@40000 { + interrupt-controller; + #address-cells = <0>; + #interrupt-cells = <2>; + reg = <0x40000 0x40000>; + compatible = "chrp,open-pic"; + device_type = "open-pic"; + }; + + i2c@3000 { + interrupts = <43 2>; + reg = <0x3000 0x100>; + compatible = "fsl-i2c"; + dfsrr; + sleep = <&pmc 00000004>; + }; + + pmc: power@e0070 { + compatible = "fsl,mpc8540-pmc", "fsl,mpc8548-pmc"; + reg = <0xe0070 0x20>; + }; + }; |