drm/edid: Set 8 bpc color depth for displays with "DFP 1.x compliant TMDS".

According to E-EDID spec 1.3, table 3.9, a digital video sink with the
"DFP 1.x compliant TMDS" bit set is "signal compatible with VESA DFP 1.x
TMDS CRGB, 1 pixel / clock, up to 8 bits / color MSB aligned".

For such displays, the DFP spec 1.0, section 3.10 "EDID support" says:

"If the DFP monitor only supports EDID 1.X (1.1, 1.2, etc.)
 without extensions, the host will make the following assumptions:

 1. 24-bit MSB-aligned RGB TFT
 2. DE polarity is active high
 3. H and V syncs are active high
 4. Established CRT timings will be used
 5. Dithering will not be enabled on the host"

So if we don't know the bit depth of the display from additional
colorimetry info we should assume 8 bpc / 24 bpp by default.

This patch adds info->bpc = 8 assignement for that case.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
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

@@ -3867,6 +3867,20 @@ static void drm_add_display_info(struct edid *edid,
 	/* HDMI deep color modes supported? Assign to info, if so */
 	drm_assign_hdmi_deep_color_info(edid, info, connector);
 
+	/*
+	 * Digital sink with "DFP 1.x compliant TMDS" according to EDID 1.3?
+	 *
+	 * For such displays, the DFP spec 1.0, section 3.10 "EDID support"
+	 * tells us to assume 8 bpc color depth if the EDID doesn't have
+	 * extensions which tell otherwise.
+	 */
+	if ((info->bpc == 0) && (edid->revision < 4) &&
+	    (edid->input & DRM_EDID_DIGITAL_TYPE_DVI)) {
+		info->bpc = 8;
+		DRM_DEBUG("%s: Assigning DFP sink color depth as %d bpc.\n",
+			  connector->name, info->bpc);
+	}
+
 	/* Only defined for 1.4 with digital displays */
 	if (edid->revision < 4)
 		return;