Comment 2 for bug 1819968

Probably related:

https://patchwork.kernel.org/patch/9716467/

Since the introduction of kernel 4.8 I've been experiencing color
banding on my Lenovo G50-80 notebook. I also had reports of the same
symptoms on the G50-70 model.

I figured out that the problem had been introduced by commit
210a021dab639694600450c14b877bf3e3240adc

The G50-80's LCD panel supports 6bpc, but is now set to 8bpc,
and the system probably falls back to 6bpc
without any dithering, which result in heavily banded gradients.

I fixed it for myself by adding an EDID quirk for that particular LCD
panel (see the attached patch). It's unknown to me, however, if there
are any other panels affected by the issue, so maybe a more general
solution might be desirable.

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index fad3d44e4642..ee3124bcae87 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -147,6 +147,9 @@ static const struct edid_quirk {
  /* Medion MD 30217 PG */
  { "MED", 0x7b8, EDID_QUIRK_PREFER_LARGE_75 },

+ /* Fix for Lenovo G50 */
+ { "SDC", 18514, EDID_QUIRK_FORCE_6BPC },
+
  /* Panel in Samsung NP700G7A-S01PL notebook reports 6bpc */
  { "SEC", 0xd033, EDID_QUIRK_FORCE_8BPC },