[PATCH 3/3] drm/edid: Add 6 bpc quirk for display AEO model 0.

[Mario Kleiner]

Bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=105331
reports that the "AEO model 0" display is driven with 8 bpc
without dithering by default, which looks bad because that
panel is apparently a 6 bpc panel.

A fix for this was made by commit 013dd9e03872
("drm/i915/dp: fall back to 18 bpp when sink capability is unknown").

However, that commit, while correct in itself, as a side effect
triggers new regressions in precision for DisplayPort->DVI and
DP->VGA displays. Patches are out to fix those regressions, but
they will revert video output for the AEO model 0 panel to 8 bpc
without dithering.

The EDID 1.3 of that panel, as decoded from the xrandr output
attached to that bugzilla bug report, is somewhat faulty, and beyond
other problems also sets the "DFP 1.x compliant TMDS" bit, which
according to DFP spec means to drive the panel with 8 bpc and
no dithering in absence of other colorimetry information.

Try to make the original bug reporter happy despite the
faulty EDID by adding a quirk to mark that panel as 6 bpc,
so 6 bpc output with dithering creates a nice picture.

As the original patch was backported to stable and the
fixes for regressions caused by that patch are aimed
at stable, this patch should also go to stable to
make everybody happy again.

Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>
Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/drm_edid.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index ff28815..7d10537 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -74,6 +74,8 @@
 #define EDID_QUIRK_FORCE_8BPC			(1 << 8)
 /* Force 12bpc */
 #define EDID_QUIRK_FORCE_12BPC			(1 << 9)
+/* Force 6bpc */
+#define EDID_QUIRK_FORCE_6BPC			(1 << 10)
 
 struct detailed_mode_closure {
 	struct drm_connector *connector;
@@ -100,6 +102,9 @@ static struct edid_quirk {
 	/* Unknown Acer */
 	{ "ACR", 2423, EDID_QUIRK_FIRST_DETAILED_PREFERRED },
 
+	/* AEO model 0 reports 8 bpc, but is a 6 bpc panel */
+	{ "AEO", 0, EDID_QUIRK_FORCE_6BPC },
+
 	/* Belinea 10 15 55 */
 	{ "MAX", 1516, EDID_QUIRK_PREFER_LARGE_60 },
 	{ "MAX", 0x77e, EDID_QUIRK_PREFER_LARGE_60 },
@@ -3950,6 +3955,9 @@ int drm_add_edid_modes(struct drm_connector *connector, struct edid *edid)
 
 	drm_add_display_info(edid, &connector->display_info, connector);
 
+	if (quirks & EDID_QUIRK_FORCE_6BPC)
+		connector->display_info.bpc = 6;
+
 	if (quirks & EDID_QUIRK_FORCE_8BPC)
 		connector->display_info.bpc = 8;
 
-- 
2.7.0