[PATCH v6 07/15] dt-bindings: display: add a boot logo node under /chosen

["Màxim Pedraza Padilla" <maximpedraza@gmail.com>]

Products built on the same board often differ only in branding. On UEFI
systems the firmware hands the operating system its boot logo through
the ACPI BGRT; a device tree system has no such table, so today the logo
is built into the kernel, and each product needs its own kernel image.
With U-Boot's Falcon mode the device tree is the only thing that reaches
the kernel, so it is also the only place the logo can come from.

Add a "logo" node, compatible "boot-logo", under /chosen. It lives
there because a logo is configuration handed over by firmware rather
than a description of the hardware, as simple-framebuffer nodes already
are.

The image is a BMP with 24 bits per pixel and no compression, the
format of the BGRT image, so that one file serves the firmware splash,
a BMP loaded as firmware and this node alike. It is carried either in
a "logo-image" property, which dtc fills in from the file with /incbin/,
or in a reserved memory region the bootloader loaded it into, named by
"memory-region". Exactly one of the two must be present.

"logo-position" takes -1 on an axis to mean centre on that axis, which
is the only way to say it when the device tree does not know the image
size, and "logo-offset" is added afterwards. "rotation" is the one of
panel-common.yaml, degrees counter clockwise, but turns the image rather
than the screen, so position and offset stay in screen pixels. The
optional "background-color" goes with the image, since a logo designed
for one background looks wrong on another.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Màxim Pedraza Padilla <maximpedraza@gmail.com>
---
 .../bindings/display/boot-logo.yaml           | 156 ++++++++++++++++++
 MAINTAINERS                                   |   1 +
 2 files changed, 157 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/display/boot-logo.yaml

diff --git a/Documentation/devicetree/bindings/display/boot-logo.yaml b/Documentation/devicetree/bindings/display/boot-logo.yaml
new file mode 100644
index 000000000000..9cd4b2565112
--- /dev/null
+++ b/Documentation/devicetree/bindings/display/boot-logo.yaml
@@ -0,0 +1,156 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/boot-logo.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Boot logo supplied by the device tree
+
+maintainers:
+  - Francesco Valla <francesco@valla.it>
+  - Màxim Pedraza Padilla <maximpedraza@gmail.com>
+
+description: |
+  An image for the operating system to show on the display while it boots,
+  handed over by the firmware together with where and how to show it.
+
+  Products built on the same board often differ only in branding. Carrying
+  the logo in the device tree lets a single kernel image serve all of them,
+  each booting with its own device tree, and it works where the device tree
+  is the only thing that reaches the kernel, as with U-Boot's Falcon mode.
+  It plays the part the ACPI BGRT plays on UEFI systems.
+
+  Since a logo is configuration rather than a description of the hardware,
+  the node lives under /chosen, next to the other things firmware hands to
+  the operating system.
+
+  The image is a BMP file with a 40 byte BITMAPINFOHEADER, 24 bits per pixel
+  and no compression, the format of the BGRT image. It is carried either in
+  the node itself or in a reserved memory region the bootloader loaded it
+  into, never both.
+
+allOf:
+  - $ref: panel/panel-common.yaml#
+
+properties:
+  $nodename:
+    const: logo
+
+  compatible:
+    const: boot-logo
+
+  logo-image:
+    $ref: /schemas/types.yaml#/definitions/uint8-array
+    description:
+      The BMP file itself, byte for byte, which dtc can fill in from the file
+      with /incbin/. The device tree stays in memory for as long as the
+      system runs, and some architectures limit its size (arm64 to 2 MB), so
+      this is meant for small images; at three bytes per pixel, larger ones
+      belong in a reserved memory region.
+
+  memory-region:
+    maxItems: 1
+    description: |
+      Reserved memory region the bootloader loaded the BMP file into, starting
+      at the beginning of the region. The BMP header says how much of the
+      region is image.
+
+      This keeps the image out of the device tree, so that it can be changed
+      without rebuilding the device tree, for instance by loading it from a
+      partition of its own that userspace can update.
+
+      Memory keeps its contents across a reset, and may even across a short
+      power cycle, and the operating system cannot tell a stale image from a
+      fresh one. A bootloader that loads no image into the region therefore
+      disables the node (status = "disabled") or removes it.
+
+  logo-position:
+    $ref: /schemas/types.yaml#/definitions/int32-array
+    description:
+      X and Y coordinates, in screen pixels, of the top left corner of the
+      image once it has been rotated. A value of -1 on an axis centres the
+      image on that axis instead, which is the only way to say it when the
+      device tree does not know the image size, as with a memory region.
+      Defaults to centring on both axes.
+    default: [-1, -1]
+    items:
+      - description: X coordinate, or -1 to centre horizontally
+        minimum: -1
+        maximum: 65535
+      - description: Y coordinate, or -1 to centre vertically
+        minimum: -1
+        maximum: 65535
+
+  logo-offset:
+    $ref: /schemas/types.yaml#/definitions/int32-array
+    description:
+      X and Y displacement, in screen pixels, applied after the image has been
+      placed. Mostly useful together with a centred axis, to land the image
+      somewhere other than the middle of a panel whose visible area is not
+      the middle of the mode. Defaults to no displacement.
+    default: [0, 0]
+    items:
+      - description: X displacement
+        minimum: -65535
+        maximum: 65535
+      - description: Y displacement
+        minimum: -65535
+        maximum: 65535
+
+  rotation:
+    description:
+      Rotation applied to the image as the screen is seen, in degrees counter
+      clockwise, as for the rotation property of panels. It turns the image
+      and not the screen; a panel mounted turned is described by its own
+      rotation property, which is made up for first, so this means the same
+      whichever way the panel is mounted. A quarter turn swaps how much room
+      the image takes up, but logo-position and logo-offset stay in pixels of
+      the screen as seen either way. Defaults to 0.
+
+  background-color:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Colour of the rest of the screen, as 0xRRGGBB. When absent, the
+      operating system uses its own default.
+    maximum: 0xffffff
+
+required:
+  - compatible
+
+# The image lives either in the node or in a reserved memory region,
+# never both and never neither.
+oneOf:
+  - required:
+      - logo-image
+  - required:
+      - memory-region
+
+additionalProperties: false
+
+examples:
+  - |
+    // A 2x1 image, red and blue, centred and 120 pixels below the middle
+    / {
+        compatible = "foo";
+        model = "foo";
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        chosen {
+            logo {
+                compatible = "boot-logo";
+                /* Normally: logo-image = /incbin/("logo.bmp"); */
+                logo-image = /bits/ 8 <0x42 0x4d 0x3e 0x00 0x00 0x00 0x00 0x00
+                                  0x00 0x00 0x36 0x00 0x00 0x00 0x28 0x00
+                                  0x00 0x00 0x02 0x00 0x00 0x00 0x01 0x00
+                                  0x00 0x00 0x01 0x00 0x18 0x00 0x00 0x00
+                                  0x00 0x00 0x08 0x00 0x00 0x00 0x13 0x0b
+                                  0x00 0x00 0x13 0x0b 0x00 0x00 0x00 0x00
+                                  0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
+                                  0xff 0xff 0x00 0x00 0x00 0x00>;
+                logo-position = <(-1) (-1)>;
+                logo-offset = <0 120>;
+                background-color = <0x1e1e28>;
+            };
+        };
+    };
diff --git a/MAINTAINERS b/MAINTAINERS
index 321ecb9f5b5b..0d1decaedde0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9174,6 +9174,7 @@ M:	Màxim Pedraza Padilla <maximpedraza@gmail.com>
 L:	dri-devel@lists.freedesktop.org
 S:	Maintained
 T:	git https://gitlab.freedesktop.org/drm/misc/kernel.git
+F:	Documentation/devicetree/bindings/display/boot-logo.yaml
 F:	drivers/gpu/drm/clients/drm_splash.c
 
 DRM TTM SUBSYSTEM
-- 
2.39.5