[PATCH v2] drm: document how user-space should use link-status

[Pekka Paalanen]

On Tue, 02 Jun 2020 19:18:15 +0000
Simon Ser <contact at emersion.fr> wrote:

> Describe what a "BAD" link-status means for user-space and how it should
> handle it. The logic described has been implemented in igt [1].
> 
> v2:
> 
> - Change wording to avoid "enabled" (Daniel)
> - Add paragraph about multiple connectors sharing the same CRTC (Pekka)
> - Add paragraph about performing an atomic commit on a connector without
>   updating the link-status property (Daniel)
> 
> [1]: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/commit/fbe61f529737191d0920521946a575bd55f00fbe
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Manasi Navare <manasi.d.navare at intel.com>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> ---
>  drivers/gpu/drm/drm_connector.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index f2b20fd66319..829b21124048 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -994,6 +994,21 @@ static const struct drm_prop_enum_list dp_colorspaces[] = {
>   *      after modeset, the kernel driver may set this to "BAD" and issue a
>   *      hotplug uevent. Drivers should update this value using
>   *      drm_connector_set_link_status_property().
> + *
> + *      When user-space receives the hotplug uevent and detects a "BAD"
> + *      link-status, the sink doesn't receive pixels anymore. The list of
> + *      available modes may have changed. User-space is expected to pick a new
> + *      mode if the current one has disappeared and perform a new modeset with
> + *      link-status set to "GOOD" to re-enable the connector.
> + *
> + *      If multiple connectors share the same CRTC and one of them gets a "BAD"
> + *      link-status, the other are unaffected (ie. the sinks still continue to
> + *      receive pixels).
> + *

Hi,

looks good up to here.

> + *      When user-space performs an atomic commit on a connector with a "BAD"
> + *      link-status without resetting the property to "GOOD", it gets
> + *      implicitly reset. This might make the atomic commit fail if the modeset
> + *      is unsuccessful.

I think this was what Daniel was saying that the kernel should require
ALLOW_MODESET to be set for the automatic reset, right?

I'm fine with how the doc is written now. But if ALLOW_MODESET becomes
a requirement for the automatic reset, I suspect there is a risk to
regress Weston, assuming the automatic reset used to be successful.

I understand this doc describes the current situation and it answers my
questions, so:

Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>

Thanks,
pq

>   * non_desktop:
>   * 	Indicates the output should be ignored for purposes of displaying a
>   * 	standard desktop environment or console. This is most likely because

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