[RFC PATCH 4/6] dt-bindings: display: allow the boot logo in a reserved memory region

[Max Pedraza <maximpedraza@gmail.com>]

Carrying the image in the device tree ties it to the device tree, but the
image and where it goes on screen are independent axes of variation. One
board sold to several customers wants several device trees that differ in
the logo; one customer with several products built on that board wants the
same logo placed differently on each panel. The second case would otherwise
mean duplicating the same image into every device tree.

Let the node point at a reserved memory region filled in by the bootloader
instead, so one image can be shared by device trees that differ only in
placement. The region starts with a small header carrying a magic number
and the geometry, so the kernel can tell a logo from an empty or stale
region and bounds check everything against the reservation.

A phandle to a declared region is used rather than a bare address: the
reservation is what makes the memory safe to read and what gives the kernel
a size to validate against. The two ways of supplying the image are
mutually exclusive.

Signed-off-by: Max Pedraza <maximpedraza@gmail.com>
---
 .../display/linux,boot-logo-clut224.yaml      | 52 +++++++++++++++++--
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/display/linux,boot-logo-clut224.yaml b/Documentation/devicetree/bindings/display/linux,boot-logo-clut224.yaml
index 10845378d..c40683f3a 100644
--- a/Documentation/devicetree/bindings/display/linux,boot-logo-clut224.yaml
+++ b/Documentation/devicetree/bindings/display/linux,boot-logo-clut224.yaml
@@ -54,6 +54,23 @@ properties:
       index into the colour lookup table. The property length must be equal to
       width multiplied by height.
 
+  memory-region:
+    maxItems: 1
+    description: |
+      Reserved memory region holding the logo, as an alternative to carrying
+      it in the width, height, clut and data properties. The bootloader is
+      expected to have placed the image there before starting the kernel.
+
+      This lets one image be shared by several device trees that differ only
+      in where the logo goes, which is what a family of products built on the
+      same board but with different panels needs.
+
+      The region starts with a header of four little endian 32 bit words:
+      the magic number 0x4f474f4c ("LOGO"), the width, the height and the
+      number of palette entries. The palette follows, as consecutive red,
+      green and blue bytes per entry, and then one byte per pixel, each an
+      index into that palette.
+
   logo-position:
     $ref: /schemas/types.yaml#/definitions/uint32-array
     description:
@@ -85,10 +102,17 @@ properties:
 
 required:
   - compatible
-  - width
-  - height
-  - clut
-  - data
+
+# The image either lives in the device tree or in a reserved memory region,
+# never both.
+oneOf:
+  - required:
+      - width
+      - height
+      - clut
+      - data
+  - required:
+      - memory-region
 
 additionalProperties: false
 
@@ -106,3 +130,23 @@ examples:
                          0x02 0x00 0x00 0x02>;
         logo-centered;
     };
+  - |
+    // The same logo taken from a region the bootloader filled in, centred on
+    // a panel whose usable area is not the centre of the mode.
+    reserved-memory {
+        #address-cells = <1>;
+        #size-cells = <1>;
+        ranges;
+
+        logo_mem: logo@9c000000 {
+            reg = <0x9c000000 0x100000>;
+            no-map;
+        };
+    };
+
+    logo {
+        compatible = "linux,boot-logo-clut224";
+        memory-region = <&logo_mem>;
+        logo-centered;
+        logo-offset = <0 120>;
+    };
-- 
2.39.5