[igt-dev] [PATCH 3/3] tests/kms_writeback: support DRM_FORMAT_XRGB2101010 for writeback

[Maxime Ripard]

On Thu, Aug 17, 2023 at 10:15:04AM -0400, Harry Wentland wrote:
> 
> 
> On 2023-08-17 03:50, Maxime Ripard wrote:
> > On Thu, Aug 17, 2023 at 01:09:44AM -0600, Alex Hung wrote:
> >>
> >>
> >> On 2023-08-17 00:32, Maxime Ripard wrote:
> >>> Hi,
> >>>
> >>> On Wed, Aug 16, 2023 at 02:53:16PM -0600, Alex Hung wrote:
> >>>> Allow kms_writeback to run with DRM_FORMAT_XRGB8888 if supported
> >>>> or to try DRM_FORMAT_XRGB2101010 if DRM_FORMAT_XRGB8888 is not
> >>>> available.
> >>>
> >>> XRGB8888 is always supposed to be available, if it's not, it's a driver
> >>> issue. Which means that XRGB2101010 will never actually be tested.
> >>
> >> It is up to a device driver to support specific format(s), not necessary an
> >> issue. At least for now amdgpu won't support XRGB8888 but XRGB2101010.
> > 
> > I guess it's unrelated to that patch in particular, but it's not ok from
> > a KMS standpoint. It was poorly documented before, but see
> > 
> > https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_fb_helper.c#L430
> > https://lore.kernel.org/dri-devel/20230807140317.26146-2-jfalempe@redhat.com/
> > 
> 
> Our HW writeback can't do 8888. This would be a silly reason to not be
> able to use it for testing purposes.

Ah, right, sorry for the brainfart.

> I don't even understand what "SW Color Conversion" means here. Is it
> expected that drivers use CPU code in kernel to walk the buffer and
> "color convert" from XRGB8888 to whatever the HW supports (and the
> inverse for the writeback buffer)? That seems a bit ridiculous.

That's exactly what that means. We have a bunch of helpers to do that
already, used for example by simpledrm if it's backed by a different
format. See:
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_format_helper.c#L914

Maxime
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