drm: document how user-space should use link-status

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)

v3:

- Fix description of what happens when link-status isn't reset to
  "GOOD", and when link-status is reset without ALLOW_MODESET (Daniel,
  Ville)
- Changing link-status to "BAD" is a no-op
- Clearly state that "BAD" means black screen (Manasi)

[1]: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/commit/fbe61f529737191d0920521946a575bd55f00fbeSigned-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/kFylMmeRMGJk-oi8f-Td8A7GNa1C-GsK23-vjKg77VhWfwpkLJg7QxFlQ_g9KdVxZiyWl9eJWpUGa5PYasR9YcyvIbuBmHVfKeHb4rH0yTM=@emersion.fr

drivers/gpu/drm/drm_connector.c

@@ -994,6 +994,32 @@ 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 (e.g. the screen
+ *      becomes completely black). 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).
+ *
+ *      When user-space performs an atomic commit on a connector with a "BAD"
+ *      link-status without resetting the property to "GOOD", the sink may
+ *      still not receive pixels. When user-space performs an atomic commit
+ *      which resets the link-status property to "GOOD" without the
+ *      ALLOW_MODESET flag set, it might fail because a modeset is required.
+ *
+ *      User-space can only change link-status to "GOOD", changing it to "BAD"
+ *      is a no-op.
+ *
+ *      For backwards compatibility with non-atomic userspace the kernel
+ *      tries to automatically set the link-status back to "GOOD" in the
+ *      SETCRTC IOCTL. This might fail if the mode is no longer valid, similar
+ *      to how it might fail if a different screen has been connected in the
+ *      interim.
  * non_desktop:
  * 	Indicates the output should be ignored for purposes of displaying a
  * 	standard desktop environment or console. This is most likely because