[PATCH v3] Documentation: gpu: Mention the requirements for new properties

[Daniel Vetter]

On Tue, Jun 15, 2021 at 2:09 PM Simon Ser <contact at emersion.fr> wrote:
>
> On Tuesday, June 15th, 2021 at 12:16, Pekka Paalanen <ppaalanen at gmail.com> wrote:
>
> > Good reminder about CRCs. CRCs have zero tolerance, so they are not
> > useful for testing properties that have any leeway, are they?
>
> IIRC, IGT's alpha blending test currently computes the CRC for all
> possible roundings, then checks that the hw returns one of the
> acceptable CRCs.
>
> With more complex color management properties, this approach might not
> be possible and write-back support in hw drivers would really help.

Yeah CRC based tests have severe limits, and even if you only try to
test the extreme stuff there's enough busted hw out there that they
will fail. E.g. when scaling I'm sure there's hw that bleeds in pixels
from outside the bounding box and can't be fixed, or there's some
intel hw where the alpha blending gets the mapping between [0, 0xff]
<-> [0.0, 1.0] and you get a nice faint ghost instead of full
transparency or opaqueness.

I think for those broken hw is just broken, nothing we can do.

Also writeback isn't supported by enough display hw, and not everyone
has access to chamelium or similar.

I think best we can do is that relevant igt have an interactive mode
(built-in for crc tests, so you can pause and look at the screen) and
then perhaps compare with the vkms reference implementation manually.
That's not great, and vkms is also not anywhere near close enough for
this either (and like currently you can't even look at it, we'd first
need a v4l output mode).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch