On Wed, Jan 28, 2026 at 02:31:57PM +0200, Tomi Valkeinen wrote: > On 28/01/2026 13:49, Laurent Pinchart wrote: > > On Mon, Dec 01, 2025 at 02:18:45PM +0200, Tomi Valkeinen wrote: > >> Add greyscale Y8 format. > > > > I would explain here why we need a new format and can't just use > > DRM_FORMAT_R8. You don't need to convince me, but I think it's important > > to summarize the rationale should someone later wonder why we introduced > > this. > > Good point. I can take the text from the cover letter to this commit's > description. > > Would this be fine: > > == > > 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, as: > > 1) We can mark it as 'is_yuv' in the drm_format_info, and this can help > the drivers handle e.g. full/limited range. Probably some hardware > handles grayscale as a value used for all RGB components, in which case > R8 makes sense, but when the hardware handles the Y-only pixels as YCbCr, > where Cb and Cr are "neutral", it makes more sense to consider the > format as an YUV format rather than RGB. > > 2) We can have the same fourcc as in v4l2. While not strictly necessary, > it's a constant source of confusion when the fourccs differ.
I wouldn't consider that as a goal (see my comment below about the 4CC value). V4L2 and DRM 4CCs differ, and applications must handle them separately. Implying we can take shortcuts for a subset of formats will in my opinion generate more harm than good. > 3) It (possibly) makes more sense for the user to use Y8/GREY format > instead of R8, as, in my experience, the documentation usually refers > to gray(scale) format or Y-only format. > > 4) We have other Y-only formats, like the Y10_P32 added in the following > patches, with "Y" in the fourcc name. If those two were the only reasons, I'd tell you to use R8 :-) I would drop 2-4 and only document 1, that's the real reason. > >> Acked-by: Dmitry Baryshkov <[email protected]> > >> Reviewed-by: Pekka Paalanen <[email protected]> > >> Reviewed-by: Vishal Sagar <[email protected]> > >> Signed-off-by: Tomi Valkeinen <[email protected]> > >> --- > >> drivers/gpu/drm/drm_fourcc.c | 1 + > >> include/uapi/drm/drm_fourcc.h | 10 ++++++++++ > >> 2 files changed, 11 insertions(+) > >> > >> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c > >> index b22ef86428a1..a39b9d7a5b62 100644 > >> --- a/drivers/gpu/drm/drm_fourcc.c > >> +++ b/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 > >> }, > >> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h > >> index 6c786701238e..5cfc188c4e72 100644 > >> --- a/include/uapi/drm/drm_fourcc.h > >> +++ b/include/uapi/drm/drm_fourcc.h > >> @@ -459,6 +459,16 @@ 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. > >> + * > > > > Extra blank line. > > > >> + */ > >> + > >> +#define DRM_FORMAT_Y8 fourcc_code('G', 'R', 'E', 'Y') /* > >> 8-bit Y-only */ > > > > I would have gone for 'Y', '8', ' ', ' ' > > > >> > >> /* > >> * Format Modifiers: -- Regards, Laurent Pinchart
