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>
diff --git a/Documentation/devicetree/booting-without-of.txt b/Documentation/devicetree/booting-without-of.txt
index 50619a0..7c1329d 100644
--- a/Documentation/devicetree/booting-without-of.txt
+++ b/Documentation/devicetree/booting-without-of.txt
@@ -12,8 +12,9 @@
=================
I - Introduction
- 1) Entry point for arch/powerpc
- 2) Entry point for arch/x86
+ 1) Entry point for arch/arm
+ 2) Entry point for arch/powerpc
+ 3) Entry point for arch/x86
II - The DT block format
1) Header
@@ -148,7 +149,46 @@
it with special cases.
-1) Entry point for arch/powerpc
+1) 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 64 bit
+ boundary.
+
+ The kernel will differentiate between ATAGS and device tree booting by
+ reading the memory pointed to by r2 and looking for either the flattened
+ device tree block magic value (0xd00dfeed) or the ATAG_CORE value at
+ offset 0x4 from r2 (0x54410001).
+
+2) Entry point for arch/powerpc
-------------------------------
There is one single entry point to the kernel, at the start
@@ -226,7 +266,7 @@
cannot support both configurations with Book E and configurations
with classic Powerpc architectures.
-2) Entry point for arch/x86
+3) Entry point for arch/x86
-------------------------------
There is one single 32bit entry point to the kernel at code32_start,
