[PATCH v2] drm/edid: fixup EDID 1.4 judge reduced-blanking timings logic

[allen]

According to VESA ENHANCED EXTENDED DISPLAY IDENTIFICATION DATA STANDARD
(Defines EDID Structure Version 1, Revision 4) page: 39
How to determine whether the monitor support RB timing or not?
EDID 1.4
First:  read detailed timing descriptor and make sure byte 0 = 0x00,
	byte 1 = 0x00, byte 2 = 0x00 and byte 3 = 0xFD
Second: read EDID bit 0 in feature support byte at address 18h = 1
	and detailed timing descriptor byte 10 = 0x04
Third:  if EDID bit 0 in feature support byte = 1 &&
	detailed timing descriptor byte 10 = 0x04
	then we can check byte 15, if bit 4 in byte 15 = 1 is support RB
        if EDID bit 0 in feature support byte != 1 ||
	detailed timing descriptor byte 10 != 0x04,
	then byte 15 can not be used

The linux code is_rb function not follow the VESA's rule

Signed-off-by: Allen Chen <allen.chen at ite.com.tw>
Reported-by: kbuild test robot <lkp at intel.com>
---
 drivers/gpu/drm/drm_edid.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index f5926bf..826ed78 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -2017,13 +2017,21 @@ struct drm_display_mode *drm_mode_find_dmt(struct drm_device *dev,
 	}
 }
 
+static bool
+is_display_descriptor(const u8 *r, u8 tag)
+{
+	return (!r[0] && !r[1] && !r[2] && r[3] == tag) ? true : false;
+}
+
 static void
 is_rb(struct detailed_timing *t, void *data)
 {
 	u8 *r = (u8 *)t;
-	if (r[3] == EDID_DETAIL_MONITOR_RANGE)
-		if (r[15] & 0x10)
-			*(bool *)data = true;
+
+	if (is_display_descriptor(r, EDID_DETAIL_MONITOR_RANGE)) {
+		if (r[10] == BIT(2))
+			*(int *)data = !!(r[15] & 0x10);
+	}
 }
 
 /* EDID 1.4 defines this explicitly.  For EDID 1.3, we guess, badly. */
@@ -2031,9 +2039,13 @@ struct drm_display_mode *drm_mode_find_dmt(struct drm_device *dev,
 drm_monitor_supports_rb(struct edid *edid)
 {
 	if (edid->revision >= 4) {
-		bool ret = false;
-		drm_for_each_detailed_block((u8 *)edid, is_rb, &ret);
-		return ret;
+		if (edid->features & BIT(0)) {
+			int ret = -1;
+
+			drm_for_each_detailed_block((u8 *)edid, is_rb, &ret);
+			if (ret != -1)
+				return ret;
+		}
 	}
 
 	return ((edid->input & DRM_EDID_INPUT_DIGITAL) != 0);
-- 
1.9.1