linux-kernel - Re: [PATCH v2 04/10] drm/fourcc: Add DRM_FORMAT_Y10

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Message-ID: <CAMuHMdXrdV4YRo1nMCVMTga=M+16GT8GkWe+Rk1DkgDZ=abGag@mail.gmail.com>
Date: Wed, 15 Jan 2025 15:08:19 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
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>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>, Michal Simek <michal.simek@....com>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 04/10] drm/fourcc: Add DRM_FORMAT_Y10_LE32

Hi Tomi,

On Wed, Jan 15, 2025 at 2:46 PM Tomi Valkeinen
<tomi.valkeinen@...asonboard.com> wrote:
> On 15/01/2025 14:52, Geert Uytterhoeven wrote:
> > On Wed, Jan 15, 2025 at 1:42 PM Tomi Valkeinen
> > <tomi.valkeinen@...asonboard.com> wrote:
> >> On 15/01/2025 14:33, Geert Uytterhoeven wrote:
> >>> On Wed, Jan 15, 2025 at 12:11 PM Tomi Valkeinen
> >>> <tomi.valkeinen@...asonboard.com> wrote:
> >>>> On 15/01/2025 12:33, Geert Uytterhoeven wrote:
> >>>>> On Wed, Jan 15, 2025 at 10:04 AM Tomi Valkeinen
> >>>>> <tomi.valkeinen@...asonboard.com> wrote:
> >>>>>> Add Y10_LE32, a 10 bit greyscale format, with 3 pixels packed into
> >>>>>> 32-bit container.
> >>>>>>
> >>>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> >>>>>
> >>>>> Thanks for your patch!
> >>>>>
> >>>>>> --- a/include/uapi/drm/drm_fourcc.h
> >>>>>> +++ b/include/uapi/drm/drm_fourcc.h
> >>>>>> @@ -408,6 +408,7 @@ extern "C" {
> >>>>>>     /* Greyscale formats */
> >>>>>>
> >>>>>>     #define DRM_FORMAT_Y8          fourcc_code('G', 'R', 'E', 'Y')  /* 8-bit Y-only */
> >>>>>> +#define DRM_FORMAT_Y10_LE32    fourcc_code('Y', 'P', 'A', '4')  /* [31:0] x:Y2:Y1:Y0 2:10:10:10 little endian */
> >>>>>
> >>>>> R10_LE32? Or R10_PA4?
> >>>>
> >>>> Can we discuss the "R" vs "Y" question under the cover letter? There's
> >>>> some more context about it in there.
> >>>
> >>> Sorry, hadn't read the cover letter. I got attracted by "Y8" and "Y10".
> >>>
> >>>> I took the "LE32" from Gstreamer's format. Maybe it's a bit pointless.
> >>>>
> >>>> I don't know if it makes sense to add the fourcc to the DRM format name.
> >>>> The fourcc is very limited. Rather, we could, say, have
> >>>> DRM_FORMAT_Y10_PACKED_32 (or "R", if you insist =).
> >>>>
> >>>>> Does LE32 have a meaning?  My first guess just reading the subject
> >>>>> was wrong ("little endian  32-bit" ;-)
> >>>>
> >>>> I'm not sure I follow. It's little-endian. The pixel group/unit is a
> >>>> 32-bit number, where the leftmost pixel on the screen is in bits 9-0,
> >>>> and the padding is in bits 31-30, and stored in memory as little-endian.
> >>>
> >>> Ah, the "LE" applies to the pixels inside each word.
> >>
> >> No, to the 32-bit container.
> >>
> >>> DRM formats stored in memory are always little-endian, unless the
> >>> DRM_FORMAT_BIG_ENDIAN bit is set, which is what I was hinting
> >>> at...
> >>
> >> Indeed you're right. The "LE" is pointless. So back to the bike-shedding
> >> about the name =).
> >
> > As the order inside the container is Y2:Y1:Y0, it _is_ little endian.
> > Cfr.
> >
> > #define DRM_FORMAT_YUYV  fourcc_code('Y', 'U', 'Y', 'V') /* [31:0]
> > Cr0:Y1:Cb0:Y0 8:8:8:8 little endian */
>
> Hmm, I see. I hadn't thought LE in that context, but I think it makes
> sense when there are multiple pixels in one container. So the "little
> endian" above would refer to the order of Y1 and Y0. So is Y1 the
> least-significant-pixel? =)

No, Y0 is the least-significant member of the container, which
corresponds to the first pixel ("little end first").

> But, say, in
>
> #define DRM_FORMAT_RG88         fourcc_code('R', 'G', '8', '8') /* [15:0] R:G
> 8:8 little endian */
>
> the "little endian" refers to the 16-bit value itself? Which is not
> necessary, as the default assumption is little endian.

I think so.

> In any case, when considering your latest point... "LE" in the name
> makes sense? But with a quick look I didn't find any formats that would
> have "big endian pixel order", so maybe we can just assume little endian
> pixel order too.

[CDR][124] have. See the descriptions of the commits that introduced
them for the rationale behind this:

b92db7e4fe740daa drm/fourcc: Add DRM_FORMAT_D[1248]
d093100b425df6fe drm/fourcc: Add DRM_FORMAT_R[124]
e5bd7e3e4a68f0be drm/fourcc: Add DRM_FORMAT_C[124]

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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