[Bug 99250] [SKL,IVB] HDMI and DP: EDID-supplied BPP is not always properly clamped.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99250

--- Comment #4 from Nicholas Sielicki <sielicki at yandex.com> ---
(In reply to Ville Syrjala from comment #3)
> Let's not be so hasty closing this one. Real world trumps any spec, so I
> think we should make i915 handle this case more gracefully.
> 
> I think what we want to do is explicitly check for the DC_36bit thing. I've
> even written that exact patch once but decided not to send it out since I'd
> not seen any monitor that would violate the spec in this manner.

Any problems with this patch? It isn't specific for i915. Tested on the
ivybridge laptop, does what it's supposed to.

There's already a check for this EDID inconsistency in drm_edid.c, but it only
results in a warning. This patch just moves that check up and returns before
setting anything if the EDID is bad, resulting in 8BPC.

Looking at a quick `grep -r HDMI_DC drivers/gpu/drm`, I have a hunch that if I
plugged this monitor into an AMD card, that they would actually be able to
successfully send it 10BPC, and this patch would make them fall back to 8BPC.
So I can understand if you'd want to move this fix into i915/ instead of here.

That being said, depending on how you/they interpret the spec, falling back to
8BPC when the monitor doesn't send DC_36 might technically be more correct,
anyway.

Thanks again for the help.

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