[PATCH 3/4] watch_queue: Add a DRM_XE_NOTIFY watch type and export init_watch()

[Thomas Hellström]

Add a DRM_XE_NOTIFY watch type for asynchronous error notifications
from the DRM_XE kernel module.

The reason for not registering a DRM - wide notification type is
that the notification type is 24 bits wide, the subtype is only 8,
If this is a concern one could define the DRM - wide subtypes
to be per driver, not common across DRM.

Also export the init_watch() function for use from kernel drivers.
Use EXPORT_SYMBOL() to align with other exports from the same file.

Assisted-by: GitHub_Copilot:claude-sonnet-4.6
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
 include/uapi/drm/xe_drm.h        |  4 ++--
 include/uapi/linux/watch_queue.h |  3 ++-
 kernel/watch_queue.c             | 13 ++++++++++---
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/uapi/drm/xe_drm.h b/include/uapi/drm/xe_drm.h
index bebb0167bd31..8d5e3f06b8d4 100644
--- a/include/uapi/drm/xe_drm.h
+++ b/include/uapi/drm/xe_drm.h
@@ -2550,8 +2550,8 @@ struct drm_xe_exec_queue_set_property {
  * preempt-fence mode or not restartable.
  *
  * An optional @timestamp_ns can be provided to measure the latency between
- * event delivery and the point the worker is queued; the driver logs this
- * once all sanity checks have passed.
+ * event delivery and locking; the driver logs this regardless of whether the
+ * VM was paused.
  */
 
 /**
diff --git a/include/uapi/linux/watch_queue.h b/include/uapi/linux/watch_queue.h
index c3d8320b5d3a..c800c153989d 100644
--- a/include/uapi/linux/watch_queue.h
+++ b/include/uapi/linux/watch_queue.h
@@ -14,7 +14,8 @@
 enum watch_notification_type {
 	WATCH_TYPE_META		= 0,	/* Special record */
 	WATCH_TYPE_KEY_NOTIFY	= 1,	/* Key change event notification */
-	WATCH_TYPE__NR		= 2
+	WATCH_TYPE_DRM_XE_NOTIFY	= 2,	/* DRM device event notification */
+	WATCH_TYPE__NR		= 3
 };
 
 enum watch_meta_notification_subtype {
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 538520861e8b..701b5c388808 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -445,11 +445,17 @@ static void put_watch(struct watch *watch)
 }
 
 /**
- * init_watch - Initialise a watch
+ * init_watch() - Initialise a watch subscription
  * @watch: The watch to initialise.
- * @wqueue: The queue to assign.
+ * @wqueue: The watch queue (notification pipe) to associate with the watch.
  *
- * Initialise a watch and set the watch queue.
+ * Initialise a newly allocated watch object and associate it with @wqueue.
+ * The caller must subsequently set @watch->id and @watch->info_id before
+ * calling add_watch_to_object() to subscribe the watch to a notification
+ * source.
+ *
+ * The watch queue reference is held internally; call put_watch_queue() if
+ * the watch is not successfully passed to add_watch_to_object().
  */
 void init_watch(struct watch *watch, struct watch_queue *wqueue)
 {
@@ -458,6 +464,7 @@ void init_watch(struct watch *watch, struct watch_queue *wqueue)
 	INIT_HLIST_NODE(&watch->queue_node);
 	rcu_assign_pointer(watch->queue, wqueue);
 }
+EXPORT_SYMBOL(init_watch);
 
 static int add_one_watch(struct watch *watch, struct watch_list *wlist, struct watch_queue *wqueue)
 {
-- 
2.54.0