Question about sRGB framebuffer support

[Artem Mygaiev]

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].

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