dt: add documentation of ARM dt boot interface

v3: added details to Documentation/arm/Booting

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

1 files changed, 40 insertions, 0 deletions

diff --git a/Documentation/devicetree/booting-without-of.txt b/Documentation/devicetree/booting-without-of.txt
index 28b1c9d3d351..9381a1481027 100644
--- a/Documentation/devicetree/booting-without-of.txt
+++ b/Documentation/devicetree/booting-without-of.txt
@@ -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
 ========================