[Intel-gfx] [BUG] HDMI 12bpc causing occasional flickering and blanking

[Tore Anderson]

Hello,

Since upgrading to the Fedora 4.3 kernel package, the picture on my the
BenQ M2700 HD monitor I've got connected to my HP EliteBook Folio 9470m
laptop began having issues. The monitor is connected with a DP+-to-HDMI
cable (HDMI in the monitor end), and the resolution I'm using is
1920x1080.

The symptoms are brief flickers of which looks like a thin horizontal
line (or maybe more) overlaying the picture for a very brief period.
This is ofte often (but not always) followed by the entire picture
going completely black (kind of like when xscreensaver blanks the
screen). Usually, the picture returns after maybe a second, but
sometimes it remains off for several seconds before returning. It might
also be that these blankings happen without a horizontal line flicker
having occurred immediately before, thought it's hard to be certain
because the flickers are very brief.

The laptop's internal screen is not impacted, the picture there remains
stable while the external screen is flickering/blanking (including if
both screens are set up in xrandr to show the same output).

Most of the time the flickers and blankings happens only occasionally,
and be just a minor annoyance. However, some times it gets really bad,
and the screen spends a majority of the time black - making it
impossible to use the system. I have not determined anything that will
with 100% certainty cause the symptoms to manifest themselves, but I do
have a feeling that it is more likely to happen if the picture is
mostly white/bright (e.g., when displaying a web browser window or an
application such as gitk), and also after having resumed from suspend.
But flickering/blanking certainly does happen if the screen showing
mostly black xterms and without having suspended/resumed too.

I bisected the problem, which resulted in the following verdict:

7a0baa6234468aa387f9b8a1a79dc2a4b4821f67 is the first bad commit
commit 7a0baa6234468aa387f9b8a1a79dc2a4b4821f67
Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
Date:   Tue Jun 30 15:33:54 2015 +0300

    Revert "drm/i915: Disable 12bpc hdmi for now"

    HDMI 12bpc should be working fine now. Let it loose.

    This reverts commit 5e3daaca09f5158eff9c92290faa1d2001ecc6e4.

    v2: Rebased due to CHV/BXT port clock check improvemnts

    Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
    Reviewed-and-tested-by: Imre Deak <imre.deak at intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

I also confirmed that the problem is still present in drm-intel-nightly
as of today (8ed1804). I'm currently running 8ed1804 + 7a0baa6 reverted
and the symptoms are gone.

I've uploaded a kernel logs with drm.debug=0xe here:

http://filebin.net/r3nlwu09y2

One file is from when running 8ed1804 (with some comments of when the symptoms
occurred added by me), the other file is from right now (running
8ed1804 + 7a0baa6 reverted) - I've not yet experienced any issues since
booting the system with this kernel.

Tore