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

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On 06/14/2016 01:05 PM, Daniel Vetter wrote:
On Thu, May 26, 2016 at 4:39 PM, Mario Kleiner
<mario.kleiner.de@xxxxxxxxx> 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@xxxxxxxxx>
Acked-by: Jani Nikula <jani.nikula@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>

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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