EDID/DP color precision fixes on Intel hw for stable

[Mario Kleiner]

On 05/06/2016 08:27 PM, Ville Syrjälä wrote:
> On Mon, Mar 28, 2016 at 01:52:44AM +0200, Mario Kleiner wrote:
>> Bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=105331
>>
>> received a potential fix that was backported to stable. While that
>> patch itself is correct for treating DP video sinks with "unknown
>> color depth", it uncovered some lack in our general EDID 1.3
>> handling, and in how we treat DP->DVI/VGA, causing the fall back
>> of Intel DP to 6 bpc / 18 bpp in cases where it shouldn't fall
>> back. That leads to unhappy neuroscience/medical users of Intel gpus
>> which need their DP->DVI or DP->VGA display devices to operate at at
>> least 8 bpc without dithering.
>>
>> The following three patches try to improve our EDID handling and
>> Intel DP to try harder to detect the proper bpc to avoid these
>> regressions for DP-DVI and DP-VGA. The third patch tries to fix
>> FDO bug 105331 without causing general unhappiness of other users.
>

Thanks for the feedback.

> It would seem simpler to me to move the 18bpp fallback into intel_dp.c
> and only do it for native DP sinks/downstream ports. That way we should
> avoid the need for any EDID quirks IIUC.
>

I think that specific EDID quirk in patch 3/3 for that FDO bug we'd 
always need, because that specific panels EDID reports 8 bpc capability 
by setting the "DFP 1.x compliant TMDS" bit in its EDID 1.3, but 
according to the FDO bug it needs to be driven with 6 bpc + dithering 
for good results.

Do you agree with patch 1/3? That would avoid kms drivers needing to 
work out DFP compliant displays.I think we could probably make the 
assumption that anything that has EDID 1.3 is 8 bpc capable? DVI spec 
seems to require that for anything DVI, and  i'd assume any VGA DAC 
manufactured in the last 20 years would have at least 8 bpc?

Wrt. 18 bpp fallback you mean putting it into intel_dp_legacy_bpc() from 
patch 2/3 or similar and checking that the sink is really not an active 
DVI or VGA converter?

I tried to keep these patches relatively simple/conservative to allow 
safe backporting to stable kernels that are affected by the regression.

> The downstream port caps we should still check I suppose. Later
> versions of the spec extend the information for pretty much all port
> types. I started to write something similar [1] a while back, and by the
> looks of things I was probably basing that on the DP 1.2 spec since 1.3
> has even more stuff there. Anyways we should put that logic into the
> DP helper so that other drivers migth use it as well.
>
> [1] git://github.com/vsyrjala/linux.git dp_downstream_ports
>

Have to look at that. I don't have official access to the latest specs, 
just to whatever i could find floating in the internet.

thanks,
-mario