[Intel-gfx] DP compliance failure due to dithering for 18bpp video pattern

[Ville Syrjälä]

On Wed, Jan 11, 2017 at 05:09:16PM +0200, Jani Nikula wrote:
> On Tue, 10 Jan 2017, Manasi Navare <manasi.d.navare at intel.com> wrote:
> > Hi All,
> >
> > We are seeing CRC check failures in some of the 18bpp video pattern
> > DP Compliance tests causing the tests to fail. On further investigation, it is
> > rootcaused to dithering that the i915 driver enables in case of 18bpp pipe
> > configuration that messes up the CRC and causes the test to fail.
> 
> The CTS spec actually accounts for CRC failures caused by dithering and
> color space conversions. See section 3.2.1. However, it would be
> preferrable to be able to automate this.
> 
> > Some of the approaches that can solve this problem are:
> > 1.  Add a new method in intel_dp.c to request the compliance test state.
> > Call this new method in intel_display.c to not enable dithering during a
> > compliance test. Issue with this is it makes the general portion of the driver
> > compliance aware.
> >
> > 2.  Move the dithering enable to compute_config methods in all encoder source
> > files. Issue: Lot of duplicate code and DP is the only encoder that uses 18bpc.
> >
> > 3.  Disable dithering at all times in the driver. However this can cause image
> > quality issue with 8bpc plane and 6 bit pipe.
> >
> > Any suggestions on which approach can be implemented in order to pass
> > compliance?
> 
> I can't find any mention in the specs that we couldn't enable/disable
> dithering on the fly. It's PIPE_MISC for BDW+ and PIPE_CONF for the
> rest. So I'm wondering about doing...
> 
> 4. Disable dithering at intel_dp_sink_crc_start() and enable it again
>    (according to config->dither) at intel_dp_sink_crc_stop(). It's
>    similar to the hsw_disable_ips() and hsw_enable_ips() calls, but
>    would have to cover more platforms.
> 
> Ville, thoughts on changing dithering on the fly?

Should be fine I think.

BTW see 
https://lists.freedesktop.org/archives/intel-gfx/2016-December/115186.html
if you intend to add more crc workaround type of things. There I'm
changing the IPS w/a to force a full modeset because it was the easiest
way to do things, and the current thing is just broken.

-- 
Ville Syrjälä
Intel OTC