[Mesa-dev] [PATCH 13/30] i965/miptree: Add an explicit format parameter to create_for_dri_image

[Daniel Stone]

Hi,

On 28 June 2017 at 16:35, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Wed, Jun 28, 2017 at 4:06 AM, Daniel Stone <daniel at fooishbar.org> wrote:
>> On 28 June 2017 at 02:05, Jason Ekstrand <jason at jlekstrand.net> wrote:
>> > The long answer is that the DRI formats do not specify a colorspace.
>>
>> Also, strictly speaking, the DRI_IMAGE_FORMAT_* tokens don't specify a
>> colourspace, nor do the DRM FourCC tokens. DRI_IMAGE_FOURCC_* is
>> equivalent to the latter, bar the addition of a special and unique
>> SARGB8 token, i.e. ARGB8888 with the sRGB transfer function (and
>> presumably primaries?). The rest are presumed UNORM.
>
> Wha?  What's the difference between SARGB8 and ARGB8888 then?  My
> understanding was that scanout basically treats everything as sRGB anyway.
> Clearly, my sRGB knowledge is imperfect.

GBM_FORMAT_ARGB8888 (aka DRI_IMAGE_FOURCC_ARGB8888), gets mapped to
DRI_IMAGE_FORMAT_ARGB8888, which gets mapped to
MESA_FORMAT_B8G8R8X8_UNORM (dri_util.c). Only
DRI_IMAGE_{FORMAT,FOURCC}_SARGB8 (no defined GBM token, but you can
pass it through the GBM API and it'll work sometimes) gets mapped to a
MESA_FORMAT_*_SRGB. So AFAICT, to get an sRGB scanout buffer from
Mesa/GBM, you'd need to allocate UNORM and do inverse-gamma in your
frag shader.

Wayland similarly never maps anything to sRGB.

X11 always imports EGLImages as UNORM, so blending would be broken in
a composited environment if we were actually allocating sRGB.

i965 tries pretty hard to allocate sRGB images in the pre-DRIImage,
DRI2 (as in the X11 protocol named 'DRI2') codepath, but this isn't
used by Wayland, GBM, or DRI3.

So no, not for pretty much any externally-visible images AFAICT. Even
if it were true for scanout, the client would need to tell KMS, so KMS
could send a HDMI infoframe telling the display.

Colourspaces \_o_/

> As for enums, sure, that can probably happen.  GL and ISL both have enums
> for colorspace that we could re-use.

Yes, having too few format tokens is not a problem we have. We seem to
have about as many of those as we have things called 'DRI2'.

Cheers,
Daniel