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

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Message-ID: <20260128160352.GA3225981@killaraus>
Date: Wed, 28 Jan 2026 18:03:52 +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 05:38:11PM +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.
> > 
> >> 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.
> 
> I'll drop.
> 
> >> + */
> >> +
> >> +#define DRM_FORMAT_Y8		fourcc_code('G', 'R', 'E', 'Y')  /* 8-bit Y-only */
> > 
> > I would have gone for 'Y', '8', ' ', ' '
>
> Missed these comments earlier...
> 
> Yes, "Y8  " makes sense. But I was trying to be nice, as there are
> already users for Y8 ("GREY") with BSP kernels, and changing the fourcc
> code would break their userspace...

That's a more compeling argument than matching the V4L2 4CC. I have a
slight preference for 'Y8  ' for how it would appear in debug messages,
but I'm also fine being nice :-)

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

-- 
Regards,

Laurent Pinchart