drm: Add a new connector atomic property for link status

At the time userspace does setcrtc, we've already promised the mode
would work. The promise is based on the theoretical capabilities of
the link, but it's possible we can't reach this in practice. The DP
spec describes how the link should be reduced, but we can't reduce
the link below the requirements of the mode. Black screen follows.

One idea would be to have setcrtc return a failure. However, it
already should not fail as the atomic checks have passed. It would
also conflict with the idea of making setcrtc asynchronous in the
future, returning before the actual mode setting and link training.

Another idea is to train the link "upfront" at hotplug time, before
pruning the mode list, so that we can do the pruning based on
practical not theoretical capabilities. However, the changes for link
training are pretty drastic, all for the sake of error handling and
DP compliance, when the most common happy day scenario is the current
approach of link training at mode setting time, using the optimal
parameters for the mode. It is also not certain all hardware could do
this without the pipe on; not even all our hardware can do this. Some
of this can be solved, but not trivially.

Both of the above ideas also fail to address link degradation *during*
operation.

The solution is to add a new "link-status" connector property in order
to address link training failure in a way that:
a) changes the current happy day scenario as little as possible, to
avoid regressions, b) can be implemented the same way by all drm
drivers, c) is still opt-in for the drivers and userspace, and opting
out doesn't regress the user experience, d) doesn't prevent drivers
from implementing better or alternate approaches, possibly without
userspace involvement. And, of course, handles all the issues presented.
In the usual happy day scenario, this is always "good". If something
fails during or after a mode set, the kernel driver can set the link
status to "bad" and issue a hotplug uevent for userspace to have it
re-check the valid modes through GET_CONNECTOR IOCTL, and try modeset
again. If the theoretical capabilities of the link can't be reached,
the mode list is trimmed based on that.

v7 by Jani:
* Rebase, simplify set property while at it, checkpatch fix
v6:
* Fix a typo in kernel doc (Sean Paul)
v5:
* Clarify doc for silent rejection of atomic properties by driver (Daniel Vetter)
v4:
* Add comments in kernel-doc format (Daniel Vetter)
* Update the kernel-doc for link-status (Sean Paul)
v3:
* Fixed a build error (Jani Saarinen)
v2:
* Removed connector->link_status (Daniel Vetter)
* Set connector->state->link_status in drm_mode_connector_set_link_status_property
(Daniel Vetter)
* Set the connector_changed flag to true if connector->state->link_status changed.
* Reset link_status to GOOD in update_output_state (Daniel Vetter)
* Never allow userspace to set link status from Good To Bad (Daniel Vetter)

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tony Cheng <tony.cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Eric Anholt <eric@anholt.net> (for the -modesetting patch)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/0182487051aa9f1594820e35a4853de2f8747b4e.1481883920.git.jani.nikula@intel.com

3 files changed, 28 insertions, 0 deletions

diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
index e5e1eddd19fb..f08aa5dfc9d7 100644
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -90,6 +90,17 @@ enum subpixel_order {
 };
 
 /**
+ * enum drm_link_status - connector's link_status property value
+ *
+ * This enum is used as the connector's link status property value.
+ * It is set to the values defined in uapi.
+ */
+enum drm_link_status {
+	DRM_LINK_STATUS_GOOD = DRM_MODE_LINK_STATUS_GOOD,
+	DRM_LINK_STATUS_BAD = DRM_MODE_LINK_STATUS_BAD,
+};
+
+/**
  * struct drm_display_info - runtime data about the connected sink
  *
  * Describes a given display (e.g. CRT or flat panel) and its limitations. For
@@ -243,6 +254,12 @@ struct drm_connector_state {
 
 	struct drm_encoder *best_encoder;
 
+	/**
+	 * @link_status: Connector link_status to keep track of whether link is
+	 * GOOD or BAD to notify userspace if retraining is necessary.
+	 */
+	enum drm_link_status link_status;
+
 	struct drm_atomic_state *state;
 
 	struct drm_tv_connector_state tv;
@@ -837,6 +854,8 @@ int drm_mode_connector_set_path_property(struct drm_connector *connector,
 int drm_mode_connector_set_tile_property(struct drm_connector *connector);
 int drm_mode_connector_update_edid_property(struct drm_connector *connector,
 					    const struct edid *edid);
+void drm_mode_connector_set_link_status_property(struct drm_connector *connector,
+						 uint64_t link_status);
 
 /**
  * struct drm_tile_group - Tile group metadata
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index f220ee909bdb..ea169a90b3c4 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -439,6 +439,11 @@ struct drm_mode_config {
 	 */
 	struct drm_property *tile_property;
 	/**
+	 * @link_status_property: Default connector property for link status
+	 * of a connector
+	 */
+	struct drm_property *link_status_property;
+	/**
 	 * @plane_type_property: Default plane property to differentiate
 	 * CURSOR, PRIMARY and OVERLAY legacy uses of planes.
 	 */
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index ce7efe2e8a5e..8c67fc03d53d 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -123,6 +123,10 @@ extern "C" {
 #define DRM_MODE_DIRTY_ON       1
 #define DRM_MODE_DIRTY_ANNOTATE 2
 
+/* Link Status options */
+#define DRM_MODE_LINK_STATUS_GOOD	0
+#define DRM_MODE_LINK_STATUS_BAD	1
+
 struct drm_mode_modeinfo {
 	__u32 clock;
 	__u16 hdisplay;