drm/edid: fix CTA data block collection size for CTA version 3

The CTA Data Block Collection is valid only for CTA extension version
3. In versions 1 and 2, it is a reserved block, which we ignore.

The DTD start offset (byte 2, or d in CTA-861 spec), which determines
the CTA Data Block Collection size, is specified slightly differently
for different versions:

Version 1:
    d = offset for the byte following the reserved data block. If no
    data is provided in the reserved data block, then d=4. If no DTDs
    are provided, then d=0

Version 2:
    d = offset for the byte following the reserved data block. If no
    data is provided in the reserved data block, then d=4. If d=0, then
    no detailed timing descriptors are provided, and no data is provided
    in the reserved data block.

Version 3:
    d = offset for the byte following the data block collection. If no
    data is provided in the data block collection, then d=4. If d=0,
    then no detailed timing descriptors are provided, and no data is
    provided in the data block collection.

Ever since commit 9e50b9d55e9c ("drm: edid: Add some bounds checking"),
we've interpreted 0 to mean there are no DTDs but it's all Data
Blocks. Per the spec, Data Blocks are only valid for version 3, where we
should interpret 0 to mean there are no data blocks.

Follow the spec (and hope the EDIDs follow it too).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2a4c94417f024cbafc5d4ca0a74e4617fc4325d1.1654674560.git.jani.nikula@intel.com

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 929fc0e..c57f633 100644 (file)
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -4498,8 +4498,6 @@ static const void *__cea_db_iter_edid_next(struct cea_db_iter *iter)
 
                iter->index = 4;
                iter->end = ext[2];
-               if (iter->end == 0)
-                       iter->end = 127;
                if (iter->end < 4 || iter->end > 127)
                        continue;