Question about sRGB framebuffer support

[Ville Syrjälä]

On Wed, May 06, 2020 at 12:25:00PM +0300, Artem Mygaiev wrote:
> 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

That doesn't really make sense to me. You never feed linear data to
actual displays.

> 
> > >
> > > 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

-- 
Ville Syrjälä
Intel