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>]

Any news on this one?

Thanks a bunch,
-mario

On 06/14/2016 04:12 PM, Mario Kleiner wrote:
> 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
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