Question about sRGB framebuffer support

[Artem Mygaiev]

On Wed, May 6, 2020 at 12:18 PM Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
>
> On Wed, May 06, 2020 at 12:04:22PM +0300, Artem Mygaiev wrote:
> > Hello Ville
> >
> > On Wed, May 6, 2020 at 10:45 AM Ville Syrjälä
> > <ville.syrjala at linux.intel.com> wrote:
> > >
> > > On Tue, May 05, 2020 at 01:24:16PM +0300, Artem Mygaiev wrote:
> > > > Hello all
> > > >
> > > > I am currently working on DRM/KMS driver for Fresco Logic FL2000 USB display
> > > > controller [1]. I have already implemented a POC driver [2] which is working for
> > > > me, although there are still plenty of things to improve or fix, of course.
> > > >
> > > > So far I have one thing that I somehow cannot find in DRM/KMS documentation or
> > > > existing drivers: how to tell the system that HW expects sRGB (i.e. non-linear)
> > > > color encoding in framebuffers? This is a HW limitation that I cannot influence
> > > > by configuration.
> > >
> > > Does it do something to process the data that requires linearization
> > > or why does it care about the gamma applied to the data? In a typical
> > > use case the data is just passed through unless the user asks otherwise,
> > > so it doesn't matter much what gamma was used. Though most displays
> > > probably expect something resembling sRGB gamma by default, so that's
> > > presumably what most things generate, and images/videos/etc. pretty
> > > much always have gamma already applied when they are produced.
> > >
> >
> > Unfortunately the HW was designed in a way that when it is configured to 24-bit
> > RGB888 it expects sRGB and applies degamma automatically. It is not possible to
> > disable this, I've asked vendor and they confirmed this [1].
>
> So it always does degamma+gamma for no real reason? That shouldn't
> really matter (apart from potentially losing some precision in those
> conversions).
>

It always does only degamma (sRGB -> linear), so if you supply linear RGB it
will totally corrupt picture colors, e.g. this is how kmscube looks like:
https://github.com/klogg/fl2000_drm/issues/15

> >
> > The only workaround I could implement now is to switch it to 16-bit RGB565 and
> > perform framebuffer conversions in driver, similar to what
> > rm_fb_xrgb8888_to_rgb565() alike helpers do; but it would be still great to
> > understand whether it is possible to support sRGB.
> >
> > [1] https://github.com/FrescoLogic/FL2000/issues/42
> >
> > > >
> > > > Any pointers are greatly appreciated.
> > > >
> > > > [1] www.frescologic.com/product/single/fl2000
> > > > [2] https://github.com/klogg/fl2000_drm
> > > >
> > > > Best regards,
> > > >  -- Artem
> > > > _______________________________________________
> > > > dri-devel mailing list
> > > > dri-devel at lists.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > >
> > > --
> > > Ville Syrjälä
> > > Intel
> >
> > Best regards,
> > Artem Mygaiev
>
> --
> Ville Syrjälä
> Intel

Best regards,
Artem Mygaiev