[Intel-gfx] [Regression] "drm/i915: Implement display w/a #1143" breaks HDMI on ASUS GL552VW

[Kai-Heng Feng]

Hi,

There's a regression reported that HDMI output stops working after os upgrade:
https://bugs.launchpad.net/bugs/1871721

Here's the bisect result:
0519c102f5285476d7868a387bdb6c58385e4074 is the first bad commit
commit 0519c102f5285476d7868a387bdb6c58385e4074
Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
Date:   Mon Jan 22 19:41:31 2018 +0200

    drm/i915: Implement display w/a #1143

    Apparently SKL/KBL/CFL need some manual help to get the
    programmed HDMI vswing to stick. Implement the relevant
    workaround (display w/a #1143).

    Note that the relevant chicken bits live in a transcoder register
    even though the bits affect a specific DDI port rather than a
    specific transcoder. Hence we must pick the correct transcoder
    register instance based on the port rather than based on the
    cpu_transcoder.

    Also note that for completeness I included support for DDI A/E
    in the code even though we never have HDMI on those ports.

    v2: CFL needs the w/a as well (Rodrigo and Art)

    Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
    Cc: Art Runyan <arthur.j.runyan at intel.com>
    Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20180122174131.28046-1-ville.syrjala@linux.intel.com
    Reviewed-by: Rodrigo Vivi <rodrigo.vivi at intel.com>


dmesg from drm-tip with drm.debug=0xe can be found here:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1871721/comments/64

Kai-Heng