[PATCH v3 3/3 RFC] dt: add documentation of ARM dt boot interface
From: Grant Likely
Date: Mon Jan 31 2011 - 15:20:46 EST
v3: added details to Documentation/arm/Booting
This is RFC only. I'm not asking for this to be merged until arm-dt
support is accepted in mainline.
Signed-off-by: Grant Likely <grant.likely@xxxxxxxxxxxx>
Documentation/arm/Booting | 33 +++++++++++++++++--
Documentation/devicetree/booting-without-of.txt | 40 +++++++++++++++++++++++
2 files changed, 69 insertions(+), 4 deletions(-)
diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting
index 7685029..4e686a2 100644
@@ -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
+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)
diff --git a/Documentation/devicetree/booting-without-of.txt b/Documentation/devicetree/booting-without-of.txt
index 28b1c9d..9381a14 100644
@@ -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
@@ -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
+ 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
+ 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
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