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

[Rodrigo Vivi]

On Thu, Jan 12, 2017 at 3:30 PM, Manasi Navare
<manasi.d.navare at intel.com> wrote:
> On Thu, Jan 12, 2017 at 12:32:09PM +0200, Jani Nikula wrote:
>> On Wed, 11 Jan 2017, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
>> > 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.
>> >>
>
> intel_dp_sink_crc_start() gets called only through the debugfs interface.

Do you really use this sink crc for the compliance tests?
Or are you talking about some other CRC level check like in the compliance box?

> If we disable dithering here, DPR 120 which is calculating the CRC of the
> pixels on the rendered video pattern would already have dithering enabled on it.
> So how will this solve our issue of CRC failures in case of 18bpc test?
>
> Manasi
>
>
>> >> 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.
>>
>> I think forcing a modeset for sink crc would be rather annoying.
>>
>> BR,
>> Jani.
>>
>>
>> --
>> Jani Nikula, Intel Open Source Technology Center
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-- 
Rodrigo Vivi
Blog: http://blog.vivi.eng.br