drm/edid: Add 6 bpc quirk for display AEO model 0.

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 DP panel with faulty EDID.

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

That commit triggers new regressions in precision for DP->DVI and
DP->VGA displays. A patch is out to revert that commit, but it 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.

Tested by injecting the edid from the fdo bug into a DP connector
via drm_kms_helper.edid_firmware and verifying the 6 bpc + dithering
is selected.

This patch should be backported to stable.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: stable@vger.kernel.org
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

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 },
@@ -4082,6 +4087,9 @@ 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;