On Wed, Jun 08, 2022 at 10:50:31AM +0300, Jani Nikula wrote:
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
> ---
> drivers/gpu/drm/drm_edid.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 929fc0e46751..c57f6333ea7d 100644
> --- 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;
Not really sure how I came up with this interpretation of the spec.
Looks like I at least fixed up the 18byte descriptor parsing
correctly in commit 7304b9810a73 ("drm/edid: Check the number
of detailed timing descriptors in the CEA ext block")
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> if (iter->end < 4 || iter->end > 127)
> continue;
>
> --
> 2.30.2
--
Ville Syrjälä
Intel
