linux-kernel - Re: [PATCH v7 03/11] drm/fourcc: Add DRM_FORMAT

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Message-ID: <20260128130948.GA3210848@killaraus>
Date: Wed, 28 Jan 2026 15:09:48 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Vishal Sagar <vishal.sagar@....com>,
	Anatoliy Klymenko <anatoliy.klymenko@....com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Michal Simek <michal.simek@....com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
	Pekka Paalanen <ppaalanen@...il.com>,
	Pekka Paalanen <pekka.paalanen@...labora.com>,
	Dmitry Baryshkov <lumag@...nel.org>
Subject: Re: [PATCH v7 03/11] drm/fourcc: Add DRM_FORMAT_Y8

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 <dmitry.baryshkov@...aro.org>
> >> Reviewed-by: Pekka Paalanen <pekka.paalanen@...labora.com>
> >> Reviewed-by: Vishal Sagar <vishal.sagar@....com>
> >> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> >> ---
> >>  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