drm/sched: improve docs around drm_sched_entity

I found a few too many things that are tricky and not documented, so I
started typing.

I found a few more things that looked broken while typing, see the
varios FIXME in drm_sched_entity.

Also some of the usual logics:
- actually include sched_entity.c declarations, that was lost in the
  move here: 620e762 ("drm/scheduler: move entity handling into
  separate file")

- Ditch the kerneldoc for internal functions, keep the comments where
  they're describing more than what the function name already implies.

- Switch drm_sched_entity to inline docs.

Acked-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Emma Anholt <emma@anholt.net>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805104705.862416-7-daniel.vetter@ffwll.ch

Documentation/gpu/drm-mm.rst

@@ -504,3 +504,6 @@ Scheduler Function References
 
 .. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
    :export:
+
+.. kernel-doc:: drivers/gpu/drm/scheduler/sched_entity.c
+   :export:

drivers/gpu/drm/scheduler/sched_entity.c

@@ -45,8 +45,14 @@
  * @guilty: atomic_t set to 1 when a job on this queue
  *          is found to be guilty causing a timeout
  *
- * Note: the sched_list should have at least one element to schedule
- *       the entity
+ * Note that the &sched_list must have at least one element to schedule the entity.
+ *
+ * For changing @priority later on at runtime see
+ * drm_sched_entity_set_priority(). For changing the set of schedulers
+ * @sched_list at runtime see drm_sched_entity_modify_sched().
+ *
+ * An entity is cleaned up by callind drm_sched_entity_fini(). See also
+ * drm_sched_entity_destroy().
  *
  * Returns 0 on success or a negative error code on failure.
  */
@@ -92,6 +98,11 @@ EXPORT_SYMBOL(drm_sched_entity_init);
  * @sched_list: the list of new drm scheds which will replace
  *		 existing entity->sched_list
  * @num_sched_list: number of drm sched in sched_list
+ *
+ * Note that this must be called under the same common lock for @entity as
+ * drm_sched_job_arm() and drm_sched_entity_push_job(), or the driver needs to
+ * guarantee through some other means that this is never called while new jobs
+ * can be pushed to @entity.
  */
 void drm_sched_entity_modify_sched(struct drm_sched_entity *entity,
 				    struct drm_gpu_scheduler **sched_list,
@@ -104,13 +115,6 @@ void drm_sched_entity_modify_sched(struct drm_sched_entity *entity,
 }
 EXPORT_SYMBOL(drm_sched_entity_modify_sched);
 
-/**
- * drm_sched_entity_is_idle - Check if entity is idle
- *
- * @entity: scheduler entity
- *
- * Returns true if the entity does not have any unscheduled jobs.
- */
 static bool drm_sched_entity_is_idle(struct drm_sched_entity *entity)
 {
 	rmb(); /* for list_empty to work without lock */
@@ -123,13 +127,7 @@ static bool drm_sched_entity_is_idle(struct drm_sched_entity *entity)
 	return false;
 }
 
-/**
- * drm_sched_entity_is_ready - Check if entity is ready
- *
- * @entity: scheduler entity
- *
- * Return true if entity could provide a job.
- */
+/* Return true if entity could provide a job. */
 bool drm_sched_entity_is_ready(struct drm_sched_entity *entity)
 {
 	if (spsc_queue_peek(&entity->job_queue) == NULL)
@@ -192,14 +190,7 @@ long drm_sched_entity_flush(struct drm_sched_entity *entity, long timeout)
 }
 EXPORT_SYMBOL(drm_sched_entity_flush);
 
-/**
- * drm_sched_entity_kill_jobs_cb - helper for drm_sched_entity_kill_jobs
- *
- * @f: signaled fence
- * @cb: our callback structure
- *
- * Signal the scheduler finished fence when the entity in question is killed.
- */
+/* Signal the scheduler finished fence when the entity in question is killed. */
 static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f,
 					  struct dma_fence_cb *cb)
 {
@@ -224,14 +215,6 @@ drm_sched_job_dependency(struct drm_sched_job *job,
 	return NULL;
 }
 
-/**
- * drm_sched_entity_kill_jobs - Make sure all remaining jobs are killed
- *
- * @entity: entity which is cleaned up
- *
- * Makes sure that all remaining jobs in an entity are killed before it is
- * destroyed.
- */
 static void drm_sched_entity_kill_jobs(struct drm_sched_entity *entity)
 {
 	struct drm_sched_job *job;
@@ -273,9 +256,11 @@ static void drm_sched_entity_kill_jobs(struct drm_sched_entity *entity)
  *
  * @entity: scheduler entity
  *
- * This should be called after @drm_sched_entity_do_release. It goes over the
- * entity and signals all jobs with an error code if the process was killed.
+ * Cleanups up @entity which has been initialized by drm_sched_entity_init().
  *
+ * If there are potentially job still in flight or getting newly queued
+ * drm_sched_entity_flush() must be called first. This function then goes over
+ * the entity and signals all jobs with an error code if the process was killed.
  */
 void drm_sched_entity_fini(struct drm_sched_entity *entity)
 {
@@ -315,10 +300,10 @@ EXPORT_SYMBOL(drm_sched_entity_fini);
 
 /**
  * drm_sched_entity_destroy - Destroy a context entity
- *
  * @entity: scheduler entity
  *
- * Calls drm_sched_entity_do_release() and drm_sched_entity_cleanup()
+ * Calls drm_sched_entity_flush() and drm_sched_entity_fini() as a
+ * convenience wrapper.
  */
 void drm_sched_entity_destroy(struct drm_sched_entity *entity)
 {
@@ -327,9 +312,7 @@ void drm_sched_entity_destroy(struct drm_sched_entity *entity)
 }
 EXPORT_SYMBOL(drm_sched_entity_destroy);
 
-/*
- * drm_sched_entity_clear_dep - callback to clear the entities dependency
- */
+/* drm_sched_entity_clear_dep - callback to clear the entities dependency */
 static void drm_sched_entity_clear_dep(struct dma_fence *f,
 				       struct dma_fence_cb *cb)
 {
@@ -371,11 +354,7 @@ void drm_sched_entity_set_priority(struct drm_sched_entity *entity,
 }
 EXPORT_SYMBOL(drm_sched_entity_set_priority);
 
-/**
- * drm_sched_entity_add_dependency_cb - add callback for the entities dependency
- *
- * @entity: entity with dependency
- *
+/*
  * Add a callback to the current dependency of the entity to wake up the
  * scheduler when the entity becomes available.
  */
@@ -423,13 +402,6 @@ static bool drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity)
 	return false;
 }
 
-/**
- * drm_sched_entity_pop_job - get a ready to be scheduled job from the entity
- *
- * @entity: entity to get the job from
- *
- * Process all dependencies and try to get one job from the entities queue.
- */
 struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
 {
 	struct drm_sched_job *sched_job;
@@ -465,14 +437,6 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
 	return sched_job;
 }
 
-/**
- * drm_sched_entity_select_rq - select a new rq for the entity
- *
- * @entity: scheduler entity
- *
- * Check all prerequisites and select a new rq for the entity for load
- * balancing.
- */
 void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
 {
 	struct dma_fence *fence;
@@ -520,7 +484,8 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
  *
  * Note: To guarantee that the order of insertion to queue matches the job's
  * fence sequence number this function should be called with drm_sched_job_arm()
- * under common lock.
+ * under common lock for the struct drm_sched_entity that was set up for
+ * @sched_job in drm_sched_job_init().
  *
  * Returns 0 for success, negative error code otherwise.
  */

include/drm/gpu_scheduler.h

@@ -53,56 +53,147 @@ enum drm_sched_priority {
  * struct drm_sched_entity - A wrapper around a job queue (typically
  * attached to the DRM file_priv).
  *
- * @list: used to append this struct to the list of entities in the
- *        runqueue.
- * @rq: runqueue on which this entity is currently scheduled.
- * @sched_list: A list of schedulers (drm_gpu_schedulers).
- *              Jobs from this entity can be scheduled on any scheduler
- *              on this list.
- * @num_sched_list: number of drm_gpu_schedulers in the sched_list.
- * @priority: priority of the entity
- * @rq_lock: lock to modify the runqueue to which this entity belongs.
- * @job_queue: the list of jobs of this entity.
- * @fence_seq: a linearly increasing seqno incremented with each
- *             new &drm_sched_fence which is part of the entity.
- * @fence_context: a unique context for all the fences which belong
- *                 to this entity.
- *                 The &drm_sched_fence.scheduled uses the
- *                 fence_context but &drm_sched_fence.finished uses
- *                 fence_context + 1.
- * @dependency: the dependency fence of the job which is on the top
- *              of the job queue.
- * @cb: callback for the dependency fence above.
- * @guilty: points to ctx's guilty.
- * @fini_status: contains the exit status in case the process was signalled.
- * @last_scheduled: points to the finished fence of the last scheduled job.
- * @last_user: last group leader pushing a job into the entity.
- * @stopped: Marks the enity as removed from rq and destined for termination.
- * @entity_idle: Signals when enityt is not in use
- *
  * Entities will emit jobs in order to their corresponding hardware
  * ring, and the scheduler will alternate between entities based on
  * scheduling policy.
  */
 struct drm_sched_entity {
+	/**
+	 * @list:
+	 *
+	 * Used to append this struct to the list of entities in the runqueue
+	 * @rq under &drm_sched_rq.entities.
+	 *
+	 * Protected by &drm_sched_rq.lock of @rq.
+	 */
 	struct list_head		list;
+
+	/**
+	 * @rq:
+	 *
+	 * Runqueue on which this entity is currently scheduled.
+	 *
+	 * FIXME: Locking is very unclear for this. Writers are protected by
+	 * @rq_lock, but readers are generally lockless and seem to just race
+	 * with not even a READ_ONCE.
+	 */
 	struct drm_sched_rq		*rq;
+
+	/**
+	 * @sched_list:
+	 *
+	 * A list of schedulers (struct drm_gpu_scheduler).  Jobs from this entity can
+	 * be scheduled on any scheduler on this list.
+	 *
+	 * This can be modified by calling drm_sched_entity_modify_sched().
+	 * Locking is entirely up to the driver, see the above function for more
+	 * details.
+	 *
+	 * This will be set to NULL if &num_sched_list equals 1 and @rq has been
+	 * set already.
+	 *
+	 * FIXME: This means priority changes through
+	 * drm_sched_entity_set_priority() will be lost henceforth in this case.
+	 */
 	struct drm_gpu_scheduler        **sched_list;
+
+	/**
+	 * @num_sched_list:
+	 *
+	 * Number of drm_gpu_schedulers in the @sched_list.
+	 */
 	unsigned int                    num_sched_list;
+
+	/**
+	 * @priority:
+	 *
+	 * Priority of the entity. This can be modified by calling
+	 * drm_sched_entity_set_priority(). Protected by &rq_lock.
+	 */
 	enum drm_sched_priority         priority;
+
+	/**
+	 * @rq_lock:
+	 *
+	 * Lock to modify the runqueue to which this entity belongs.
+	 */
 	spinlock_t			rq_lock;
 
+	/**
+	 * @job_queue: the list of jobs of this entity.
+	 */
 	struct spsc_queue		job_queue;
 
+	/**
+	 * @fence_seq:
+	 *
+	 * A linearly increasing seqno incremented with each new
+	 * &drm_sched_fence which is part of the entity.
+	 *
+	 * FIXME: Callers of drm_sched_job_arm() need to ensure correct locking,
+	 * this doesn't need to be atomic.
+	 */
 	atomic_t			fence_seq;
+
+	/**
+	 * @fence_context:
+	 *
+	 * A unique context for all the fences which belong to this entity.  The
+	 * &drm_sched_fence.scheduled uses the fence_context but
+	 * &drm_sched_fence.finished uses fence_context + 1.
+	 */
 	uint64_t			fence_context;
 
+	/**
+	 * @dependency:
+	 *
+	 * The dependency fence of the job which is on the top of the job queue.
+	 */
 	struct dma_fence		*dependency;
+
+	/**
+	 * @cb:
+	 *
+	 * Callback for the dependency fence above.
+	 */
 	struct dma_fence_cb		cb;
+
+	/**
+	 * @guilty:
+	 *
+	 * Points to entities' guilty.
+	 */
 	atomic_t			*guilty;
+
+	/**
+	 * @last_scheduled:
+	 *
+	 * Points to the finished fence of the last scheduled job. Only written
+	 * by the scheduler thread, can be accessed locklessly from
+	 * drm_sched_job_arm() iff the queue is empty.
+	 */
 	struct dma_fence                *last_scheduled;
+
+	/**
+	 * @last_user: last group leader pushing a job into the entity.
+	 */
 	struct task_struct		*last_user;
+
+	/**
+	 * @stopped:
+	 *
+	 * Marks the enity as removed from rq and destined for
+	 * termination. This is set by calling drm_sched_entity_flush() and by
+	 * drm_sched_fini().
+	 */
 	bool 				stopped;
+
+	/**
+	 * @entity_idle:
+	 *
+	 * Signals when entity is not in use, used to sequence entity cleanup in
+	 * drm_sched_entity_fini().
+	 */
 	struct completion		entity_idle;
 };