Re: EDID/DP color precision fixes on Intel hw for stable

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, May 06, 2016 at 10:03:06PM +0200, Mario Kleiner wrote:
> 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.

If we just do the fallback for DP, then I don;t think we need any
changes to the EDID parser, and hence no quirk either.

> 
> 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?

Yeah, I'd just check the downstream port type, and do the 18bpp fallback
only for native DP if the sink didn't give us a bpc. For everything else
8bpc seems like a reasonable default (unless the port caps say otherwise,
of course).

> 
> 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

-- 
Ville Syrjälä
Intel OTC
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