[Intel-gfx] rfc: breaking old userspace gamma for 10-bit support

Eric Anholt eric at anholt.net
Fri Jul 23 22:13:48 CEST 2010

On Fri, 23 Jul 2010 14:00:30 -0400, Andrew Lutomirski <luto at mit.edu> wrote:
> AFAICT intel hardware wants a 129-entry LUT when using high precision
> gamma ramps.  Rather than hacking some kind of decimation into the
> kernel driver (and thus silently breaking DirectColor), I'd like to
> teach userspace how to deal with variable gamma sizes.
> gnome-color-manager already more-or-less supports arbitrary gamma ramp
> sizes (supposedly), dispwin ought to do it, and there might not be any
> other software that really cares.  gnome-screensaver saves and
> restores the gamma ramp, and I haven't checked if it works right for
> funny sizes.
> The worst problem we'll have is that current xf86-drv-intel can't
> handle non-256 gamma sizes at all.  So if we change the kernel we'll
> break it completely.
> One option is to have the kernel report gamma_size = 129 but still
> accept 256 and decimate itself.  That might cause current userspace to
> keep working (except for DirectColor).
> Any thoughts?

The kernel doesn't get to break old userspace.  The kernel could support
new userspace that only asks for 129 slots and set a mode that has
better precision in that case.  New interfaces would probably be
required to communicate that up front -- I haven't looked into it, but I
just want to make sure you don't spend a bunch of time going down a path
that will be rejected.
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