[U-Boot-Users] RFC: Booting the Linux/ppc64 kernel without Open Firmware HOWTO (#2)

Benjamin Herrenschmidt
2005-05-19T04:56:53Z

On Wed, 2005-05-18 at 17:09 +1000, Benjamin Herrenschmidt wrote:
> Hi !
>
> Here's the very first draft of my HOWTO about booting the linux/ppc64
> kernel without open firmware. It's still incomplete, the main chapter
> describing which nodes & properties are required and their format is
> still missing (though it will basically be a subset of the Open Firmware
> specification & bindings). The format of the flattened device-tree is
> documented.

And here is a second draft with more infos.

Booting the Linux/ppc64 kernel without Open Firmware
----------------------------------------------------

(c) 2005 Benjamin Herrenschmidt <benh@kernel.crashing.org>, IBM Corp.

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.

I- Introduction
===============

During the recent developpements 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 has 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 follows them properly.

The main requirement that will be defined in mmore details 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 details 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 busses that
doesn't specifically fit in an existing OF specification, like on chip
devices, this creates a great flexibility in the way the kernel can
them 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 impacting significantly
the kernel code or cluttering it with special cases.

1) Entry point
--------------

There is one and one single entry point to the kernel, at the start
of the kernel image. That entry point support 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 bits client interface
is currently supported

r3, r4 : address & lenght of an initrd if any or 0

MMU is either on or off, the kernel will run the
trampoline located in arch/ppc64/kernel/prom_init.c to
extract the device-tree and other informations 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 it's binding to the
PowerPC platform. The entry point definition then becomes:

r3 : physical pointer to the device-tree block
(defined in chapter II)

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 differenciate 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 mean, 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.

2) Board support
----------------

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 "known" 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/ppc64/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/ppc64/kernel/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

c) Add a reference to your "ppc_md" structure in the
"machines" table in arch/ppc64/kernel/setup.c

d) request and get assigned a platform number (see PLATFORM_*
constants in include/asm-ppc64/processor.h

I will describe later the boot process and various callbacks that
your platform should implement.

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/ppc64/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.

1) Header
---------

The kernel is entered with r3 pointing to an area of memory that is
roughtly described in include/asm-ppc64/prom.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 */
};

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
"offsets" values are in bytes from the start of the header, that is
from r3 value.

- 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
bits 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 initialisation. 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 bits aligned.

- version

This is the version of this structure. Version 1 stops
here. Version 2 adds an additional field boot_cpuid_phys. You
should always generate a structure of the highest version defined
at the time of your implementation. That is version 2.

- last_comp_version

Last compatible version. This indicates down to what version of
the DT block you are backward compatible with. 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 unless a new
incompatible version of the DT block is defined.

- 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)

So the typical layout of a DT block (though the various parts don't
need to be in that order) looks like (addresses go from top to bottom):

------------------------------
r3 -> | struct boot_param_header |
------------------------------
| (alignment gap) (*) |
------------------------------
| memory reserve map |
------------------------------
| (alignment gap) |
------------------------------
| |
| device-tree structure |
| |
------------------------------
| (alignment gap) |
------------------------------
| |
| device-tree strings |
| |
-----> ------------------------------
|
|
--- (r3 + 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 page
aligned.

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 contained in a property of
type "name" in the node property list whose value is a zero terminated
string and is mandatory. There is also a "unit name" that is used to
differenciate nodes with the same name at the same level, it is
usually made of the node name's, 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 these will be provided later. The
kernel ppc64 generic code does not make any formal use of the unit
address though (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. Nodes with no notion of address
and no possible sibling of the same name (like /memory or /cpus) may
ommit 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 nodes unit names
separated with "/".

The root node is defined as beeing named "device-tree" and has no unit
address (no @ symbol followed by a unit address). When manipulating
device-tree "path", the root of the tree is generally represented by a
simple slash sign "/".

Every node who actually represents an actual device (that is who isn't
only a virtual "container" for more nodes, like "/cpus" is) is also
required to have a "device_type" property indicating the type of node

Finally, every node is required to have a "linux,phandle"
property. Real open firmware implementations don't provide it as it's
generated on the fly by the prom_init.c trampoline from the Open
Firmware "phandle". Implementations providing a flattened device-tree
directly should provide this property. This propery is a 32 bits value
that uniquely identify a node. You are free to use whatever values or
system of values, internal pointers, or whatever to genrate these, the
only requirement is that every single node of the tree you are passing
to the kernel has a unique value in this property.

This can be used in some cases for nodes to reference other nodes.

Here is an example of a simple device-tree. In this example, a "o"
designates a node followed by the node unit name. Properties are
presented with their name followed by their content. "content"
represent an ASCII string (zero terminated) value, while <content>
represent a 32 bits hexadecimal value. The various nodes in this
example will be discusse in a later chapter. At this point, it is
only meant to give you a idea of what a device-tree looks like

/ 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 at 0
| |- name = "PowerPC,970"
| |- device_type = "cpu"
| |- reg = <0>
| |- clock-frequency = <5f5e1000>
| |- linux,boot-cpu
| |- linux,phandle = <2>
|
o memory at 0
| |- name = "memory"
| |- device_type = "memory"
| |- reg = <00000000 00000000 00000000 20000000>
| |- linux,phandle = <3>
|
o chosen
|- name = "chosen"
|- bootargs = "root=/dev/sda2"
|- linux,platform = <00000600>
|- linux,phandle = <4>

This tree is an example of 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, the
physical memory layout, and misc informations passed through /chosen
like in this example, the platform type (mandatory) and the kernel
command line arguments (optional).

The /cpus/PowerPC,970 at 0/linux,boot-cpu property is an example of a
property without a value. All other properties have a value. The
signification 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" ends
that node definition. Child nodes are simply defined before
"OF_DT_END" (that is nodes within the node). A 'token' is a 32 bits value.

Here's the basic structure of a single node:

* token OF_DT_BEGIN_NODE (that is 0x00000001)
* node full path as a zero terminated string
* [align gap to next 4 bytes boundary]
* for each property:
* token OF_DT_PROP (that is 0x00000003)
* 32 bits value of property value size in bytes (or 0 of no value)
* 32 bits value of offset in string block of property name
* [align gap to either next 4 bytes boundary if the property value
size is less or equal to 4 bytes, or to next 8 bytes
boundary if the property value size is larger than 4 bytes]
* property value data if any
* [align gap to next 4 bytes boundary]
* [child nodes if any]
* token OF_DT_END (that is 0x00000002)

So the node content can be summmarised as a start token, a full path, a list of
properties, a list of child node and an end token. Every child node is
a full node structure itself as defined above

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 flatenned 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 chose 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 property. In
absence of such a property, the parent's parent values are used,
etc... 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 bits 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, that is a 64 bits number (cells are concatenated
and expected to be in big endian format). Another example is the way
Apple firmware define them, that is 2 cells for an address and one
cell for a size.

A device IO or MMIO areas on the bus are defined in the "reg"
property. The format of this property depends on the bus the device is
sitting on. Standard bus types define their "reg" properties format in
the various OF bindings for those bus types, you are free to define
your own "reg" format for proprietary busses or virtual busses
enclosing on-chip devices, though it is recommended that the parts of
the "reg" property containing addresses and sizes do respect the
defined #address-cells and #size-cells when those make sense.

Later, I will define more precisely some common address formats.

For a new ppc64 board, I recommend to use either the 2/2 format or
Apple's 2/1 format which is slightly more compact since sizes usually
fit in a single 32 bits word.

2) Note about "compatible" properties
-------------------------------------

Those properties are optional, but recommended in devices and the root
node. The format of a "compatible" property is a list of concatenated
zeto terminated strings. They allow a device to express it's
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. The kernel however doesn't generally put any
restriction on the "name" property, it is simply considered good
practice to folow the standard and it's evolutions as closely as possible.

4) Required nodes and properties
--------------------------------

Note that every node should have a "name" and a "linux,phandle"
property, those aren't specified explicitely below as their presence
is considered as implicit. The name property is defined in the cases
where it's content is defined or has a common practice.

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

Additionally, some recommended properties are:

- name : this is generally "device-tree"
- 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 use a different "model" property but put a
value in "compatible". The kernel doesn't directly use that
value (see /chosen/linux,platform for how the kernel choses a
platform type) but it is generally useful.

It's 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 CPUs 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 practice to call it PowerPC,<name>, for
example, Apple uses PowerPC,G5 while IBM uses PowerPC,970FX.

Required properties:

- device_type : has to be "cpu"
- reg : This is the physical cpu number, it's single 32 bits cell,
this 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 at 0
/cpus/PowerPC,970FX at 1
(unit addresses do not require to have leading zero's)
- d-cache-line-size : one cell, L1 data cache line size in bytes
- i-cache-line-size : one cell, L1 instruction cache line 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

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 bits value, 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.

You are welcome to add any property you find relevant to your board,
like some informations about mecanism 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 as 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 it's reg property, or you can create
several nodes, as you wishes. 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:

- name : has to be "chosen"
- device_type : has to be "memory"
- reg : This property contain all the physical memory ranges of
your board. It's a list of addresses/sizes concatenated
together, the number of cell of those beeing defined by the
#address-cells and #size-cells of the root node. For example,
with both of these properties beeing 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 hold between 2Gb and
4Gb. Some vendors prefer splitting those ranges into smaller
segments, the kernel doesn't care.

c) The /chosen node

This node is a bit "special". Normally, that's where open firmware
puts some variable environment informations, like the arguments, or
phandle pointers to nodes like the main interrupt controller, 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.

Required properties:

- name has to be "chosen"
- linux,platform : This is your platform number as assigned by the
architecture maintainers

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 it's own default console. If you look at the funciton
set_preferred_console() in arch/ppc64/kernel/setup.c, you'll see
that the kernel tries to find out the default console and has
knowledge of various types like 8250 serial ports. You may want
to extend this function to add your own.
- interrupt-controller : This is one cell containing a phandle
value that matches the "linux,phandle" property of your main
interrupt controller node. May be used for interrupt routing.

This is all that is 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.

IV - Recommendation for a bootloader
====================================

Here are some various ideas/recommendations that have been proposed
while all this has been defined and implemented.

- It should be possible to write a parser that turns an ASCII
representation of a device-tree (or even XML though I find that
less readable) into a device-tree block. This would allow to
basically build the device-tree structure and strings "blobs" at
bootloader build time, and have the bootloader just pass-them
as-is to the kernel. In fact, the device-tree blob could be then
separate from the bootloader itself, an be placed in a separate
portion of the flash that can be "personalized" for different
board types by flashing a different device-tree

- A very 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 it's 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 memmovin'g things
around. It contains no internal offsets or pointers for this purpose.

- An example of code for iterating nodes & retreiving properties
directly from the flattened tree format can be found in the kernel
file arch/ppc64/kernel/prom.c, look@scan_flat_dt() function,
it's 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
do discuss possible free licencing to any vendor who wishes to
integrate all or part of this code into a non-GPL bootloader.