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

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Message-ID: <6b084e2c-100f-4563-8797-9fd6c862ed0c@ideasonboard.com>
Date: Wed, 15 Jan 2025 14:42:18 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
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,

On 15/01/2025 14:33, Geert Uytterhoeven wrote:
> Hi Tomi,
> 
> 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 =).

  Tomi