[Intel-gfx] Enabling 30-bit depth with Intel Graphics

[Marvin Pribadi]

[resending because my previous msg didn't get through]

Hi devs,

I would like to get 10-bit color depth working on my system for proper
playback of HEVC Main10 profile. I'm writing some software to do this, but
for initial test purposes I'm using readily available applications. So far
I'm not able to get proper 10-bit color output. My system is an Intel NUC
NUC5i7rYH (with Iris Graphics 6100) running Ubuntu 15.10 with Intel
Graphics Installer 1.4.0. The output is HDMI to a Dell U3011 monitor and I
also have an Astro VA-1838 HDMI Protocol Analyzer.

After installation of the Intel Graphics Installer 1.4.0, I can confirm
that the output now shows as HDMI 12-bit RGB @1920x1080p60 (using the Astro
VA-1838). However, display of gradient test images and video clips that
have 10-bit color depth or more, are either down sampled to 8-bit depth or
dithered when using XVideo output (confirmed by reading pixel values using
the Astro). These tests were done using combinations of Gimp 2.9.2,
ImageMagick 6.8.9.9, VLC 2.2.1, ffmpeg 2.7.6, and x265 10-bit 1.9.

I noticed that the xdpyinfo utility shows that the X server display only
supports depths 24, 1 , 4, 8, 15, 16, and 32 bpp. I read online that a
custom xorg.conf file can be created to force the default depth to 30 bpp.
I tried this and it does then make xdpyinfo report support for 30 bpp,
however it causes several issues, such as the Ubuntu Unity Plugin failing
to start, VLC reporting that there is no available XVideo output adapter,
and sluggish VLC playback with X11 video output. Furthermore VLC playback
using the X11 video output is still down sampled to 8-bit.

My questions are:

   - Is Intel Graphics for Linux supposed to support 10-bit color depth (30
   bpp) in X?
   - If so, what do I need to do to enable it?
   - If not, can you clarify what is supported and provide some advice on
   how I can get 10-bit color output using Intel Graphics?

Thanks,
-Marvin
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