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dt: add documentation of ARM dt boot interface

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v3: added details to Documentation/arm/Booting

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
This commit is contained in:
Grant Likely 2011-01-31 00:09:58 -07:00
parent cf4e5c6e8d
commit 9830fcd6f6
2 changed files with 69 additions and 4 deletions
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33
Documentation/arm/Booting
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@ -65,13 +65,19 @@ looks at the connected hardware is beyond the scope of this document.
The boot loader must ultimately be able to provide a MACH_TYPE_xxx
value to the kernel. (see linux/arch/arm/tools/mach-types).
4. Setup the kernel tagged list
-------------------------------
4. Setup boot data
------------------
Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED
New boot loaders: MANDATORY
The boot loader must provide either a tagged list or a dtb image for
passing configuration data to the kernel. The physical address of the
boot data is passed to the kernel in register r2.
4a. Setup the kernel tagged list
--------------------------------
The boot loader must create and initialise the kernel tagged list.
A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag
@ -101,6 +107,24 @@ The tagged list must be placed in a region of memory where neither
the kernel decompressor nor initrd 'bootp' program will overwrite
it. The recommended placement is in the first 16KiB of RAM.
4b. Setup the device tree
-------------------------
The boot loader must load a device tree image (dtb) into system ram
at a 64bit aligned address and initialize it with the boot data. The
dtb format is documented in Documentation/devicetree/booting-without-of.txt.
The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
physical address to determine if a dtb has been passed instead of a
tagged list.
The boot loader must pass at a minimum the size and location of the
system memory, and the root filesystem location. The dtb must be
placed in a region of memory where the kernel decompressor will not
overwrite it. The recommended placement is in the first 16KiB of RAM
with the caveat that it may not be located at physical address 0 since
the kernel interprets a value of 0 in r2 to mean neither a tagged list
nor a dtb were passed.
5. Calling the kernel image
---------------------------
@ -125,7 +149,8 @@ In either case, the following conditions must be met:
- CPU register settings
r0 = 0,
r1 = machine type number discovered in (3) above.
r2 = physical address of tagged list in system RAM.
r2 = physical address of tagged list in system RAM, or
physical address of device tree block (dtb) in system RAM
- CPU mode
All forms of interrupts must be disabled (IRQs and FIQs)

40
Documentation/devicetree/booting-without-of.txt
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@ -13,6 +13,7 @@ Table of Contents
I - Introduction
1) Entry point for arch/powerpc
2) Entry point for arch/arm
II - The DT block format
1) Header
@ -225,6 +226,45 @@ it with special cases.
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
========================