[PATCH] drm: document that user-space should avoid parsing EDIDs

[Brian Starkey]

On Fri, Oct 09, 2020 at 09:24:01AM +0000, Simon Ser wrote:
> User-space should avoid parsing EDIDs for metadata already exposed via
> other KMS interfaces and properties. For instance, user-space should not
> try to extract a list of modes from the EDID: the kernel might mutate
> the mode list (because of link capabilities or quirks for instance).
> 
> Other metadata not exposed by KMS can be parsed by user-space. This
> includes for instance monitor identification (make/model/serial) and
> supported color-spaces.
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Suggested-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> Cc: Ville Syrj�l� <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_connector.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 717c4e7271b0..00e58fd2d292 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -960,6 +960,10 @@ static const struct drm_prop_enum_list dp_colorspaces[] = {
>   * 	drm_connector_update_edid_property(), usually after having parsed
>   * 	the EDID using drm_add_edid_modes(). Userspace cannot change this
>   * 	property.
> + *
> + * 	User-space should not parse the EDID to obtain information exposed via
> + * 	other KMS properties. For instance, user-space should not try to parse
> + * 	mode lists from the EDID.

I assume that this is so that the kernel can apply quirks/limits in
cases where it knows it needs to? I think it would be nice to put at
least a few words indicating "why", otherwise this feels like an
arbitrary commandment with no justification.

Cheers,
-Brian

>   * DPMS:
>   * 	Legacy property for setting the power state of the connector. For atomic
>   * 	drivers this is only provided for backwards compatibility with existing
> -- 
> 2.28.0
> 
> 
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