[PATCH 2/3] drm/vc4: Force ->x_scaling[1] should never be set to VC4_SCALING_NONE

[Dave Stevenson]

Hi Boris & Eric.

On Thu, 8 Nov 2018 at 15:12, Eric Anholt <eric at anholt.net> wrote:
>
> Boris Brezillon <boris.brezillon at bootlin.com> writes:
>
> > On Thu, 08 Nov 2018 06:52:44 -0800
> > Eric Anholt <eric at anholt.net> wrote:
> >
> >> Boris Brezillon <boris.brezillon at bootlin.com> writes:
> >>
> >> > For the YUV conversion to work properly, ->x_scaling[0,1] should never
> >> > be set to VC4_SCALING_NONE, but vc4_get_scaling_mode() might return
> >> > VC4_SCALING_NONE if the horizontal scaling ratio exactly matches the
> >> > horizontal subsampling factor. Add a test to turn VC4_SCALING_NONE
> >> > into VC4_SCALING_PPF when that happens.
> >> >
> >> > Fixes: fc04023fafec ("drm/vc4: Add support for YUV planes.")
> >> > Signed-off-by: Boris Brezillon <boris.brezillon at bootlin.com>
> >>
> >> I couldn't find a spec justification for this -- did you have a testcase
> >> that fails?
> >
> > Yep. Just set the downscaling ratio to 0.5 with an NV12 format and
> > you'll hit the issue (I used modetest to do that):
> >
> > # modetest -M vc4 -s 29:1920x1080-60  -P 96 at 95:1920x1080*0.5 at NV12
>
> I found that the firmware has a similar behavior to your patch ("if Y is
> !unity (x or scaling) and UV is unity, set UV to HPPF/VPPF scaling").
> They also select the unity flag after the YUV scaling fixup.
>
> Regardless, if this works, it's got my reviewed-by.
>
> Hopefully we can do some IGT with writeback or chamelium testing all of
> the X/Y scaling options with a focus on hitting these 1:1 ratios.  The
> state space is big and the docs are just ambiguous enough.

Great timing as I've hit exactly this when playing back a 1080P video
on a 1080P screen. The colours were very muted in this situation,
whilst playing any other resolution or any RGB format was fine. Took
me a while to realise it wasn't the conversion matrices being set
incorrectly :-/ Applying this patch sorts the problem.
This was on the downstream 4.19 kernel, and the v2 of this set
backported fairly easily. Can I request that for stable? Otherwise we
can cherry-pick it for downstream.

Thanks
  Dave