[PATCH] drm/edid: Select DMT timing if EDID's display feature not support GTF.

[Lee Shawn C]

Refer to EDID 1.3 spec, display FEATURE (byte 18h) bit #0 said
"If this bit is set to 1, the display supports timings based on the
GTF standard using default GTF parameter values".

And EDID 1.4 spec shows "If bit 0 is set to 0, then the display
is noncontinuous frequency (multi-mode) and is only specified to accept
the video timing formats that are listed in BASE EDID and certain
EXTENSION Blocks.

When display feature did not support CVT or GFT2 and monitor's EDID version
greater than or equal to "1.2". DRM driver would select GTF as default
for standard timing calculation. It may generated some video timing
that can't display properly by external monitor.

For example. When driver retrieved "0xD1 0xFC" (FHD, 120Hz) and
"0xD1 0xE8" (FHD, 100Hz) from "Standard Timings". GTF formula
would generate video timing like below. It already over monitor's
spec to cause black screen issue.
"1920x1080" 120 368881 1920 2072 2288 2656 1080 1081 1084 1157 0x0 0x6
"1920x1080" 100 301992 1920 2072 2280 2640 1080 1081 1084 1144 0x0 0x6

Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Adam Jackson <ajax at redhat.com>
Cc: Cooper Chiou <cooper.chiou at intel.com>
Signed-off-by: Lee Shawn C <shawn.c.lee at intel.com>
---
 drivers/gpu/drm/drm_edid.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 82a4ceed3fcf..f6a3d2d993c7 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -2089,7 +2089,9 @@ static int standard_timing_level(struct edid *edid)
 			return LEVEL_CVT;
 		if (drm_gtf2_hbreak(edid))
 			return LEVEL_GTF2;
-		return LEVEL_GTF;
+		if (edid->revision == 3 && (edid->features & DRM_EDID_FEATURE_DEFAULT_GTF))
+			return LEVEL_GTF;
+
 	}
 	return LEVEL_DMT;
 }
-- 
2.17.1