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

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Message-ID: <6bf4347b-ce70-4325-965f-40f81b24a0d1@ideasonboard.com>
Date: Wed, 28 Jan 2026 14:31:57 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Laurent Pinchart <laurent.pinchart@...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

Hi,

On 28/01/2026 13:49, Laurent Pinchart wrote:
> Hi Tomi,
> 
> Thank you for the patch.
> 
> 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.

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.

==


 Tomi

>> 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:
>