drm/fourcc: Add DRM_FORMAT_Y8

Add greyscale Y8 format.

The 8-bit greyscale format has been discussed before, and the earlier
guidance was to use DRM_FORMAT_R8, as a single-channel 8-bit pixel.

However, adding DRM_FORMAT_Y8 makes sense, we can mark it as 'is_yuv' in
the drm_format_info, and this can help the drivers handle e.g.
full/limited range. This will distinguish two single-channel formats:
R8, which is a RGB format with the same value for all components, and
Y8, which is a Y-only YCbCr format, with Cb and Cr being neutral.

Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Vishal Sagar <vishal.sagar@amd.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Ser <contact@emersion.fr>
Link: https://patch.msgid.link/20260423-xilinx-formats-v10-3-c690c2b8ea89@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

Author: tomba

drivers/gpu/drm/drm_fourcc.c

@@ -275,6 +275,7 @@ const struct drm_format_info *__drm_format_info(u32 format)
 		{ .format = DRM_FORMAT_YVU422,		.depth = 0,  .num_planes = 3, .cpp = { 1, 1, 1 }, .hsub = 2, .vsub = 1, .is_yuv = true },
 		{ .format = DRM_FORMAT_YUV444,		.depth = 0,  .num_planes = 3, .cpp = { 1, 1, 1 }, .hsub = 1, .vsub = 1, .is_yuv = true },
 		{ .format = DRM_FORMAT_YVU444,		.depth = 0,  .num_planes = 3, .cpp = { 1, 1, 1 }, .hsub = 1, .vsub = 1, .is_yuv = true },
+		{ .format = DRM_FORMAT_Y8,		.depth = 8,  .num_planes = 1, .cpp = { 1, 0, 0 }, .hsub = 1, .vsub = 1, .is_yuv = true },
 		{ .format = DRM_FORMAT_NV12,		.depth = 0,  .num_planes = 2, .cpp = { 1, 2, 0 }, .hsub = 2, .vsub = 2, .is_yuv = true },
 		{ .format = DRM_FORMAT_NV21,		.depth = 0,  .num_planes = 2, .cpp = { 1, 2, 0 }, .hsub = 2, .vsub = 2, .is_yuv = true },
 		{ .format = DRM_FORMAT_NV16,		.depth = 0,  .num_planes = 2, .cpp = { 1, 2, 0 }, .hsub = 2, .vsub = 1, .is_yuv = true },

include/uapi/drm/drm_fourcc.h

@@ -441,6 +441,15 @@ extern "C" {
 #define DRM_FORMAT_YUV444	fourcc_code('Y', 'U', '2', '4') /* non-subsampled Cb (1) and Cr (2) planes */
 #define DRM_FORMAT_YVU444	fourcc_code('Y', 'V', '2', '4') /* non-subsampled Cr (1) and Cb (2) planes */
 
+/*
+ * Y-only (greyscale) formats
+ *
+ * The Y-only formats are handled similarly to the YCbCr formats in the display
+ * pipeline, with the Cb and Cr implicitly neutral (0.0 in nominal values). This
+ * also means that COLOR_RANGE property applies to the Y-only formats.
+ */
+
+#define DRM_FORMAT_Y8		fourcc_code('G', 'R', 'E', 'Y')  /* 8-bit Y-only */
 
 /*
  * Format Modifiers: