[PATCH] drm/edid: fixup EDID 1.3 and 1.4 judge reduced-blanking timings logic

[allen <allen.chen@ite.com.tw>]

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@ite.com.tw>
Reported-by: kbuild test robot <lkp@intel.com>
---
 drivers/gpu/drm/drm_edid.c | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index f5926bf..e11e585 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -93,6 +93,12 @@ struct detailed_mode_closure {
 	int modes;
 };
 
+struct edid_support_rb_closure {
+	struct edid *edid;
+	bool valid_support_rb;
+	bool support_rb;
+};
+
 #define LEVEL_DMT	0
 #define LEVEL_GTF	1
 #define LEVEL_GTF2	2
@@ -2017,23 +2023,41 @@ 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;
+	struct edid_support_rb_closure *closure = data;
+	struct edid *edid = closure->edid;
+
+	if (is_display_descriptor(r, EDID_DETAIL_MONITOR_RANGE)) {
+		if (edid->features & BIT(0) && r[10] == BIT(2)) {
+			closure->valid_support_rb = true;
+			closure->support_rb = (r[15] & 0x10) ? true : false;
+		}
+	}
 }
 
 /* EDID 1.4 defines this explicitly.  For EDID 1.3, we guess, badly. */
 static bool
 drm_monitor_supports_rb(struct edid *edid)
 {
+	struct edid_support_rb_closure closure = {
+		.edid = edid,
+		.valid_support_rb = false,
+		.support_rb = false,
+	};
+
 	if (edid->revision >= 4) {
-		bool ret = false;
-		drm_for_each_detailed_block((u8 *)edid, is_rb, &ret);
-		return ret;
+		drm_for_each_detailed_block((u8 *)edid, is_rb, &closure);
+		if (closure.valid_support_rb)
+			return closure.support_rb;
 	}
 
 	return ((edid->input & DRM_EDID_INPUT_DIGITAL) != 0);
-- 
1.9.1