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
