drm/panfrost: Simplify the reset serialization logic

Now that we can pass our own workqueue to drm_sched_init(), we can use
an ordered workqueue on for both the scheduler timeout tdr and our own
reset work (which we use when the reset is not caused by a fault/timeout
on a specific job, like when we have AS_ACTIVE bit stuck). This
guarantees that the timeout handlers and reset handler can't run
concurrently which drastically simplifies the locking.

v5:
* Don't call cancel_delayed_timeout() in the reset path (those works
  are canceled in drm_sched_stop())

v4:
* Actually pass the reset workqueue to drm_sched_init()
* Don't call cancel_work_sync() in panfrost_reset(). It will deadlock
  since it might be called from the reset work, which is executing and
  cancel_work_sync() will wait for the handler to return. Checking the
  reset pending status should avoid spurious resets

v3:
* New patch
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-10-boris.brezillon@collabora.com

drivers/gpu/drm/panfrost/panfrost_device.h

@@ -109,6 +109,7 @@ struct panfrost_device {
 	struct mutex sched_lock;
 
 	struct {
+		struct workqueue_struct *wq;
 		struct work_struct work;
 		atomic_t pending;
 	} reset;
@@ -247,9 +248,8 @@ const char *panfrost_exception_name(u32 exception_code);
 static inline void
 panfrost_device_schedule_reset(struct panfrost_device *pfdev)
 {
-	/* Schedule a reset if there's no reset in progress. */
-	if (!atomic_xchg(&pfdev->reset.pending, 1))
-		schedule_work(&pfdev->reset.work);
+	atomic_set(&pfdev->reset.pending, 1);
+	queue_work(pfdev->reset.wq, &pfdev->reset.work);
 }
 
 #endif

drivers/gpu/drm/panfrost/panfrost_job.c

@@ -25,17 +25,8 @@
 #define job_write(dev, reg, data) writel(data, dev->iomem + (reg))
 #define job_read(dev, reg) readl(dev->iomem + (reg))
 
-enum panfrost_queue_status {
-	PANFROST_QUEUE_STATUS_ACTIVE,
-	PANFROST_QUEUE_STATUS_STOPPED,
-	PANFROST_QUEUE_STATUS_STARTING,
-	PANFROST_QUEUE_STATUS_FAULT_PENDING,
-};
-
 struct panfrost_queue_state {
 	struct drm_gpu_scheduler sched;
-	atomic_t status;
-	struct mutex lock;
 	u64 fence_context;
 	u64 emit_seqno;
 };
@@ -368,57 +359,67 @@ void panfrost_job_enable_interrupts(struct panfrost_device *pfdev)
 	job_write(pfdev, JOB_INT_MASK, irq_mask);
 }
 
-static bool panfrost_scheduler_stop(struct panfrost_queue_state *queue,
+static void panfrost_reset(struct panfrost_device *pfdev,
 			   struct drm_sched_job *bad)
 {
-	enum panfrost_queue_status old_status;
-	bool stopped = false;
+	unsigned int i;
+	bool cookie;
+
+	if (!atomic_read(&pfdev->reset.pending))
+		return;
+
+	/* Stop the schedulers.
+	 *
+	 * FIXME: We temporarily get out of the dma_fence_signalling section
+	 * because the cleanup path generate lockdep splats when taking locks
+	 * to release job resources. We should rework the code to follow this
+	 * pattern:
+	 *
+	 *	try_lock
+	 *	if (locked)
+	 *		release
+	 *	else
+	 *		schedule_work_to_release_later
+	 */
+	for (i = 0; i < NUM_JOB_SLOTS; i++)
+		drm_sched_stop(&pfdev->js->queue[i].sched, bad);
 
-	mutex_lock(&queue->lock);
-	old_status = atomic_xchg(&queue->status,
-				 PANFROST_QUEUE_STATUS_STOPPED);
-	if (old_status == PANFROST_QUEUE_STATUS_STOPPED)
-		goto out;
+	cookie = dma_fence_begin_signalling();
 
-	WARN_ON(old_status != PANFROST_QUEUE_STATUS_ACTIVE);
-	drm_sched_stop(&queue->sched, bad);
 	if (bad)
 		drm_sched_increase_karma(bad);
 
-	stopped = true;
+	spin_lock(&pfdev->js->job_lock);
+	for (i = 0; i < NUM_JOB_SLOTS; i++) {
+		if (pfdev->jobs[i]) {
+			pm_runtime_put_noidle(pfdev->dev);
+			panfrost_devfreq_record_idle(&pfdev->pfdevfreq);
+			pfdev->jobs[i] = NULL;
+		}
+	}
+	spin_unlock(&pfdev->js->job_lock);
 
-	/*
-	 * Set the timeout to max so the timer doesn't get started
-	 * when we return from the timeout handler (restored in
-	 * panfrost_scheduler_start()).
-	 */
-	queue->sched.timeout = MAX_SCHEDULE_TIMEOUT;
+	panfrost_device_reset(pfdev);
 
-out:
-	mutex_unlock(&queue->lock);
+	/* GPU has been reset, we can clear the reset pending bit. */
+	atomic_set(&pfdev->reset.pending, 0);
 
-	return stopped;
-}
+	/* Now resubmit jobs that were previously queued but didn't have a
+	 * chance to finish.
+	 * FIXME: We temporarily get out of the DMA fence signalling section
+	 * while resubmitting jobs because the job submission logic will
+	 * allocate memory with the GFP_KERNEL flag which can trigger memory
+	 * reclaim and exposes a lock ordering issue.
+	 */
+	dma_fence_end_signalling(cookie);
+	for (i = 0; i < NUM_JOB_SLOTS; i++)
+		drm_sched_resubmit_jobs(&pfdev->js->queue[i].sched);
+	cookie = dma_fence_begin_signalling();
 
-static void panfrost_scheduler_start(struct panfrost_queue_state *queue)
-{
-	enum panfrost_queue_status old_status;
-
-	mutex_lock(&queue->lock);
-	old_status = atomic_xchg(&queue->status,
-				 PANFROST_QUEUE_STATUS_STARTING);
-	WARN_ON(old_status != PANFROST_QUEUE_STATUS_STOPPED);
-
-	/* Restore the original timeout before starting the scheduler. */
-	queue->sched.timeout = msecs_to_jiffies(JOB_TIMEOUT_MS);
-	drm_sched_resubmit_jobs(&queue->sched);
-	drm_sched_start(&queue->sched, true);
-	old_status = atomic_xchg(&queue->status,
-				 PANFROST_QUEUE_STATUS_ACTIVE);
-	if (old_status == PANFROST_QUEUE_STATUS_FAULT_PENDING)
-		drm_sched_fault(&queue->sched);
-
-	mutex_unlock(&queue->lock);
+	for (i = 0; i < NUM_JOB_SLOTS; i++)
+		drm_sched_start(&pfdev->js->queue[i].sched, true);
+
+	dma_fence_end_signalling(cookie);
 }
 
 static enum drm_gpu_sched_stat panfrost_job_timedout(struct drm_sched_job
@@ -443,11 +444,8 @@ static enum drm_gpu_sched_stat panfrost_job_timedout(struct drm_sched_job
 		job_read(pfdev, JS_TAIL_LO(js)),
 		sched_job);
 
-	/* Scheduler is already stopped, nothing to do. */
-	if (!panfrost_scheduler_stop(&pfdev->js->queue[js], sched_job))
-		return DRM_GPU_SCHED_STAT_NOMINAL;
-
-	panfrost_device_schedule_reset(pfdev);
+	atomic_set(&pfdev->reset.pending, 1);
+	panfrost_reset(pfdev, sched_job);
 
 	return DRM_GPU_SCHED_STAT_NOMINAL;
 }
@@ -474,8 +472,6 @@ static void panfrost_job_handle_irq(struct panfrost_device *pfdev, u32 status)
 		job_write(pfdev, JOB_INT_CLEAR, mask);
 
 		if (status & JOB_INT_MASK_ERR(j)) {
-			enum panfrost_queue_status old_status;
-
 			job_write(pfdev, JS_COMMAND_NEXT(j), JS_COMMAND_NOP);
 
 			dev_err(pfdev->dev, "js fault, js=%d, status=%s, head=0x%x, tail=0x%x",
@@ -483,18 +479,6 @@ static void panfrost_job_handle_irq(struct panfrost_device *pfdev, u32 status)
 				panfrost_exception_name(job_read(pfdev, JS_STATUS(j))),
 				job_read(pfdev, JS_HEAD_LO(j)),
 				job_read(pfdev, JS_TAIL_LO(j)));
-
-			/*
-			 * When the queue is being restarted we don't report
-			 * faults directly to avoid races between the timeout
-			 * and reset handlers. panfrost_scheduler_start() will
-			 * call drm_sched_fault() after the queue has been
-			 * started if status == FAULT_PENDING.
-			 */
-			old_status = atomic_cmpxchg(&pfdev->js->queue[j].status,
-						    PANFROST_QUEUE_STATUS_STARTING,
-						    PANFROST_QUEUE_STATUS_FAULT_PENDING);
-			if (old_status == PANFROST_QUEUE_STATUS_ACTIVE)
 			drm_sched_fault(&pfdev->js->queue[j].sched);
 		}
 
@@ -550,56 +534,13 @@ static irqreturn_t panfrost_job_irq_handler(int irq, void *data)
 	return IRQ_WAKE_THREAD;
 }
 
-static void panfrost_reset(struct work_struct *work)
+static void panfrost_reset_work(struct work_struct *work)
 {
 	struct panfrost_device *pfdev = container_of(work,
 						     struct panfrost_device,
 						     reset.work);
-	unsigned int i;
-	bool cookie;
-
-	cookie = dma_fence_begin_signalling();
-	for (i = 0; i < NUM_JOB_SLOTS; i++) {
-		/*
-		 * We want pending timeouts to be handled before we attempt
-		 * to stop the scheduler. If we don't do that and the timeout
-		 * handler is in flight, it might have removed the bad job
-		 * from the list, and we'll lose this job if the reset handler
-		 * enters the critical section in panfrost_scheduler_stop()
-		 * before the timeout handler.
-		 *
-		 * Timeout is set to MAX_SCHEDULE_TIMEOUT - 1 because we need
-		 * something big enough to make sure the timer will not expire
-		 * before we manage to stop the scheduler, but we can't use
-		 * MAX_SCHEDULE_TIMEOUT because drm_sched_get_cleanup_job()
-		 * considers that as 'timer is not running' and will dequeue
-		 * the job without making sure the timeout handler is not
-		 * running.
-		 */
-		pfdev->js->queue[i].sched.timeout = MAX_SCHEDULE_TIMEOUT - 1;
-		cancel_delayed_work_sync(&pfdev->js->queue[i].sched.work_tdr);
-		panfrost_scheduler_stop(&pfdev->js->queue[i], NULL);
-	}
-
-	/* All timers have been stopped, we can safely reset the pending state. */
-	atomic_set(&pfdev->reset.pending, 0);
-
-	spin_lock(&pfdev->js->job_lock);
-	for (i = 0; i < NUM_JOB_SLOTS; i++) {
-		if (pfdev->jobs[i]) {
-			pm_runtime_put_noidle(pfdev->dev);
-			panfrost_devfreq_record_idle(&pfdev->pfdevfreq);
-			pfdev->jobs[i] = NULL;
-		}
-	}
-	spin_unlock(&pfdev->js->job_lock);
 
-	panfrost_device_reset(pfdev);
-
-	for (i = 0; i < NUM_JOB_SLOTS; i++)
-		panfrost_scheduler_start(&pfdev->js->queue[i]);
-
-	dma_fence_end_signalling(cookie);
+	panfrost_reset(pfdev, NULL);
 }
 
 int panfrost_job_init(struct panfrost_device *pfdev)
@@ -607,7 +548,7 @@ int panfrost_job_init(struct panfrost_device *pfdev)
 	struct panfrost_job_slot *js;
 	int ret, j, irq;
 
-	INIT_WORK(&pfdev->reset.work, panfrost_reset);
+	INIT_WORK(&pfdev->reset.work, panfrost_reset_work);
 
 	pfdev->js = js = devm_kzalloc(pfdev->dev, sizeof(*js), GFP_KERNEL);
 	if (!js)
@@ -629,15 +570,18 @@ int panfrost_job_init(struct panfrost_device *pfdev)
 		return ret;
 	}
 
-	for (j = 0; j < NUM_JOB_SLOTS; j++) {
-		mutex_init(&js->queue[j].lock);
+	pfdev->reset.wq = alloc_ordered_workqueue("panfrost-reset", 0);
+	if (!pfdev->reset.wq)
+		return -ENOMEM;
 
+	for (j = 0; j < NUM_JOB_SLOTS; j++) {
 		js->queue[j].fence_context = dma_fence_context_alloc(1);
 
 		ret = drm_sched_init(&js->queue[j].sched,
 				     &panfrost_sched_ops,
 				     1, 0,
-				     msecs_to_jiffies(JOB_TIMEOUT_MS), NULL,
+				     msecs_to_jiffies(JOB_TIMEOUT_MS),
+				     pfdev->reset.wq,
 				     NULL, "pan_js");
 		if (ret) {
 			dev_err(pfdev->dev, "Failed to create scheduler: %d.", ret);
@@ -653,6 +597,7 @@ int panfrost_job_init(struct panfrost_device *pfdev)
 	for (j--; j >= 0; j--)
 		drm_sched_fini(&js->queue[j].sched);
 
+	destroy_workqueue(pfdev->reset.wq);
 	return ret;
 }
 
@@ -665,9 +610,10 @@ void panfrost_job_fini(struct panfrost_device *pfdev)
 
 	for (j = 0; j < NUM_JOB_SLOTS; j++) {
 		drm_sched_fini(&js->queue[j].sched);
-		mutex_destroy(&js->queue[j].lock);
 	}
 
+	cancel_work_sync(&pfdev->reset.work);
+	destroy_workqueue(pfdev->reset.wq);
 }
 
 int panfrost_job_open(struct panfrost_file_priv *panfrost_priv)