Unverified Commit a66da873 authored by Simon Ser's avatar Simon Ser
Browse files

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/fbe61f529737191d0920521946a575bd55f00fbe



Signed-off-by: default avatarSimon 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: default avatarDaniel Vetter <daniel@ffwll.ch>
Acked-by: default avatarPekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/kFylMmeRMGJk-oi8f-Td8A7GNa1C-GsK23-vjKg77VhWfwpkLJg7QxFlQ_g9KdVxZiyWl9eJWpUGa5PYasR9YcyvIbuBmHVfKeHb4rH0yTM=@emersion.fr
parent 8a070524

drivers/gpu/drm/drm_connector.c

+26 −0
Original line number Diff line number Diff line
@@ -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