Question about sRGB framebuffer support

[Ville Syrjälä]

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).

> 
> 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