dt: add documentation of ARM dt boot interface

v6: typo fixes
v5: clarified that dtb should be aligned on a 64 bit boundary in RAM.
v3: added details to Documentation/arm/Booting

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

1 files changed, 29 insertions, 4 deletions

diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting
index 76850295af8f..4e686a2ed91e 100644
--- a/Documentation/arm/Booting
+++ b/Documentation/arm/Booting
@@ -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)