[PATCH] drm/edid: fix CEA extension byte #3 parsing

[Jani Nikula]

Only an EDID CEA extension has byte #3, while the CTA DisplayID Data
Block does not. Don't interpret bogus data for color formats.

For most displays it's probably an unlikely scenario you'd have a CTA
DisplayID Data Block without a CEA extension, but they do exist.

Fixes: e28ad544f462 ("drm/edid: parse CEA blocks embedded in DisplayID")
Cc: <stable at vger.kernel.org> # v4.15
Cc: Shawn C Lee <shawn.c.lee at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>

---

commit e28ad544f462 was merged in v5.3, but it has Cc: stable for v4.15.

This is also fixed in my CEA data block iteration series [1], but we'll
want the simple fix for stable first.

Hum, CTA is formerly CEA, I and the code seem to use both, should we use
only one or the other?

[1] https://patchwork.freedesktop.org/series/101659/
---
 drivers/gpu/drm/drm_edid.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 561f53831e29..ccf7031a6797 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -5187,10 +5187,14 @@ static void drm_parse_cea_ext(struct drm_connector *connector,
 
 	/* The existence of a CEA block should imply RGB support */
 	info->color_formats = DRM_COLOR_FORMAT_RGB444;
-	if (edid_ext[3] & EDID_CEA_YCRCB444)
-		info->color_formats |= DRM_COLOR_FORMAT_YCBCR444;
-	if (edid_ext[3] & EDID_CEA_YCRCB422)
-		info->color_formats |= DRM_COLOR_FORMAT_YCBCR422;
+
+	/* CTA DisplayID Data Block does not have byte #3 */
+	if (edid_ext[0] == CEA_EXT) {
+		if (edid_ext[3] & EDID_CEA_YCRCB444)
+			info->color_formats |= DRM_COLOR_FORMAT_YCBCR444;
+		if (edid_ext[3] & EDID_CEA_YCRCB422)
+			info->color_formats |= DRM_COLOR_FORMAT_YCBCR422;
+	}
 
 	if (cea_db_offsets(edid_ext, &start, &end))
 		return;
-- 
2.30.2