Question about sRGB framebuffer support

[Artem Mygaiev]

On Wed, May 6, 2020 at 12:33 PM Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
>
> 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.
>

I have a display with gamma 1.0 (as populated in EDID) which I assume means
linear gamma (am I wrong here?) which is connected to FL2000 dongle, so there is
no gamma applied after de-gamma.

But after further analysis I noticed that the problem is not the colorspace, HW
expects swapped 32-bit dwords in fbuffer. Somehow I completely overlooked this
when was working on original implementation.

Sorry for causing confusion - this is not the area I understand well enough, I
must admit.

> >
> > > >
> > > > 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
> _______________________________________________
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> dri-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

Best regards,
Artem Mygaiev