Re: [PATCH 1/2] drm/i915/dp: Revert "drm/i915/dp: fall back to 18 bpp when sink capability is unknown"

[Mario Kleiner <mario.kleiner.de@gmail.com>]

On 06/14/2016 01:05 PM, Daniel Vetter wrote:
> On Thu, May 26, 2016 at 4:39 PM, Mario Kleiner
> <mario.kleiner.de@gmail.com> wrote:
>> This reverts commit 013dd9e03872
>> ("drm/i915/dp: fall back to 18 bpp when sink capability is unknown")
>>
>> This commit introduced a regression into stable kernels,
>> as it reduces output color depth to 6 bpc for any video
>> sink connected to a Displayport connector if that sink
>> doesn't report a specific color depth via EDID, or if
>> our EDID parser doesn't actually recognize the proper
>> bpc from EDID.
>>
>> Affected are active DisplayPort->VGA converters and
>> active DisplayPort->DVI converters. Both should be
>> able to handle 8 bpc, but are degraded to 6 bpc with
>> this patch.
>>
>> The reverted commit was meant to fix
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=105331
>>
>> A followup patch implements a fix for that specific bug,
>> which is caused by a faulty EDID of the affected DP panel
>> by adding a new EDID quirk for that panel.
>>
>> DP 18 bpp fallback handling and other improvements to
>> DP sink bpc detection will be handled for future
>> kernels in a separate series of patches.
>>
>> Please backport to stable.
>>
>> Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
>> Acked-by: Jani Nikula <jani.nikula@intel.com>
>> Cc: stable@vger.kernel.org
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> I wonder whether we shouldn't just move this into the DP code, and
> instead of looking at the edid (which is just pass-through for dp->vga
> dongles) we should only look at dpcd values? Or maybe only look at the
> edid value if the sink is native DP, and not when it's a dongle.
>
> That would probably also avoid the quirk, and that quirk seems a bit fishy.
> -Daniel
>

This patch is just a simple fix for the color depth regression which 
affects stable kernels. It can be back-ported easily to affected stable 
kernels, as Jani advised me.

I wanted to clean up and resubmit that DP helper function which looks at 
dpcd values and might be a bit too much for stable, once this fix is in.

-mario