[PATCH v4 03/11] drm/fourcc: Add DRM_FORMAT_Y8

[Geert Uytterhoeven]

Hi Pekka,

On Thu, 27 Mar 2025 at 16:59, Pekka Paalanen
<pekka.paalanen at haloniitty.fi> wrote:
> On Thu, 27 Mar 2025 16:21:16 +0200
> Tomi Valkeinen <tomi.valkeinen at ideasonboard.com> wrote:
> > On 27/03/2025 11:20, Pekka Paalanen wrote:
> > > On Wed, 26 Mar 2025 15:55:18 +0200
> > > Tomi Valkeinen <tomi.valkeinen at ideasonboard.com> wrote:
> > >> On 26/03/2025 15:52, Geert Uytterhoeven wrote:
> > >>> On Wed, 26 Mar 2025 at 14:23, Tomi Valkeinen
> > >>> <tomi.valkeinen at ideasonboard.com> wrote:
> > >>>> Add greyscale Y8 format.
> > >>>>
> > >>>> Acked-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> > >>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
> > >>>
> > >>> Thanks for your patch!
> > >>>
> > >>>> --- a/include/uapi/drm/drm_fourcc.h
> > >>>> +++ b/include/uapi/drm/drm_fourcc.h
> > >>>> @@ -405,6 +405,9 @@ 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 */
> > >>>>
> > >>>> +/* Greyscale formats */
> > >>>> +
> > >>>> +#define DRM_FORMAT_Y8          fourcc_code('G', 'R', 'E', 'Y')  /* 8-bit Y-only */
> > >>>
> > >>> This format differs from e.g. DRM_FORMAT_R8, which encodes
> > >>> the number of bits in the FOURCC format. What do you envision
> > >>> for e.g. DRM_FORMAT_Y16? fourcc_code('G', 'R', '1', '6')?
> > >>
> > >> I wanted to use the same fourcc as on V4L2 side. Strictly speaking it's
> > >> not required, but different fourccs for the same formats do confuse.
> > >>
> > >> So, generally speaking, I'd pick an existing fourcc from v4l2 side if
> > >> possible, and if not, invent a new one.
> > >
> > > what's the actual difference between DRM_FORMAT_R8 and DRM_FORMAT_Y8?
> > >
> > > Is the difference that when R8 gets expanded to RGB, it becomes (R, 0,
> > > 0), but Y8 gets expanded to (c1 * Y, c2 * Y, c3 * Y) where c1..c3 are
> > > defined by MatrixCoefficients (H.273 terminology)?
> > >
> > > That would be my intuitive assumption following how YCbCr is handled.
> > > Is it obvious enough, or should there be a comment to that effect?
> >
> > You raise an interesting point. Is it defined how a display driver, that
> > supports R8 as a format, shows R8 on screen? I came into this in the
> > context of grayscale formats, so I thought R8 would be handled as (R, R,
> > R) in RGB. But you say (R, 0, 0), which... also makes sense.
>
> That is a good question too. I based my assumption on OpenGL behavior
> of R8.
>
> Single channel displays do exist I believe, but being single-channel,
> expansion on the other channels is likely meaningless. Hm, but for the
> KMS color pipeline, it would be meaningful, like with a CTM.
> Interesting.
>
> I don't know. Maybe Geert does?

I did some digging, and was a bit surprised that it was you who told
me to use R8 instead of Y8?
https://lore.kernel.org/all/20220202111954.6ee9a10c@eldfell

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds