drm: rework flip-work helpers to avoid calling func when the FIFO is full

Make use of lists instead of kfifo in order to dynamically allocate
task entry when someone require some delayed work, and thus preventing
drm_flip_work_queue from directly calling func instead of queuing this
call.
This allow drm_flip_work_queue to be safely called even within irq
handlers.

Add new helper functions to allocate a flip work task and queue it when
needed. This prevents allocating data within irq context (which might
impact the time spent in the irq handler).

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_flip_work.c

@@ -24,6 +24,44 @@
 #include "drmP.h"
 #include "drm_flip_work.h"
 
+/**
+ * drm_flip_work_allocate_task - allocate a flip-work task
+ * @data: data associated to the task
+ * @flags: allocator flags
+ *
+ * Allocate a drm_flip_task object and attach private data to it.
+ */
+struct drm_flip_task *drm_flip_work_allocate_task(void *data, gfp_t flags)
+{
+	struct drm_flip_task *task;
+
+	task = kzalloc(sizeof(*task), flags);
+	if (task)
+		task->data = data;
+
+	return task;
+}
+EXPORT_SYMBOL(drm_flip_work_allocate_task);
+
+/**
+ * drm_flip_work_queue_task - queue a specific task
+ * @work: the flip-work
+ * @task: the task to handle
+ *
+ * Queues task, that will later be run (passed back to drm_flip_func_t
+ * func) on a work queue after drm_flip_work_commit() is called.
+ */
+void drm_flip_work_queue_task(struct drm_flip_work *work,
+			      struct drm_flip_task *task)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&work->lock, flags);
+	list_add_tail(&task->node, &work->queued);
+	spin_unlock_irqrestore(&work->lock, flags);
+}
+EXPORT_SYMBOL(drm_flip_work_queue_task);
+
 /**
  * drm_flip_work_queue - queue work
  * @work: the flip-work
@@ -34,10 +72,14 @@
  */
 void drm_flip_work_queue(struct drm_flip_work *work, void *val)
 {
-	if (kfifo_put(&work->fifo, val)) {
-		atomic_inc(&work->pending);
+	struct drm_flip_task *task;
+
+	task = drm_flip_work_allocate_task(val,
+				drm_can_sleep() ? GFP_KERNEL : GFP_ATOMIC);
+	if (task) {
+		drm_flip_work_queue_task(work, task);
 	} else {
-		DRM_ERROR("%s fifo full!\n", work->name);
+		DRM_ERROR("%s could not allocate task!\n", work->name);
 		work->func(work, val);
 	}
 }
@@ -56,9 +98,12 @@ EXPORT_SYMBOL(drm_flip_work_queue);
 void drm_flip_work_commit(struct drm_flip_work *work,
 		struct workqueue_struct *wq)
 {
-	uint32_t pending = atomic_read(&work->pending);
-	atomic_add(pending, &work->count);
-	atomic_sub(pending, &work->pending);
+	unsigned long flags;
+
+	spin_lock_irqsave(&work->lock, flags);
+	list_splice_tail(&work->queued, &work->commited);
+	INIT_LIST_HEAD(&work->queued);
+	spin_unlock_irqrestore(&work->lock, flags);
 	queue_work(wq, &work->worker);
 }
 EXPORT_SYMBOL(drm_flip_work_commit);
@@ -66,14 +111,26 @@ EXPORT_SYMBOL(drm_flip_work_commit);
 static void flip_worker(struct work_struct *w)
 {
 	struct drm_flip_work *work = container_of(w, struct drm_flip_work, worker);
-	uint32_t count = atomic_read(&work->count);
-	void *val = NULL;
+	struct list_head tasks;
+	unsigned long flags;
 
-	atomic_sub(count, &work->count);
+	while (1) {
+		struct drm_flip_task *task, *tmp;
 
-	while(count--)
-		if (!WARN_ON(!kfifo_get(&work->fifo, &val)))
-			work->func(work, val);
+		INIT_LIST_HEAD(&tasks);
+		spin_lock_irqsave(&work->lock, flags);
+		list_splice_tail(&work->commited, &tasks);
+		INIT_LIST_HEAD(&work->commited);
+		spin_unlock_irqrestore(&work->lock, flags);
+
+		if (list_empty(&tasks))
+			break;
+
+		list_for_each_entry_safe(task, tmp, &tasks, node) {
+			work->func(work, task->data);
+			kfree(task);
+		}
+	}
 }
 
 /**
@@ -91,19 +148,12 @@ static void flip_worker(struct work_struct *w)
 int drm_flip_work_init(struct drm_flip_work *work, int size,
 		const char *name, drm_flip_func_t func)
 {
-	int ret;
-
 	work->name = name;
-	atomic_set(&work->count, 0);
-	atomic_set(&work->pending, 0);
+	INIT_LIST_HEAD(&work->queued);
+	INIT_LIST_HEAD(&work->commited);
+	spin_lock_init(&work->lock);
 	work->func = func;
 
-	ret = kfifo_alloc(&work->fifo, size, GFP_KERNEL);
-	if (ret) {
-		DRM_ERROR("could not allocate %s fifo\n", name);
-		return ret;
-	}
-
 	INIT_WORK(&work->worker, flip_worker);
 
 	return 0;
@@ -118,7 +168,6 @@ EXPORT_SYMBOL(drm_flip_work_init);
  */
 void drm_flip_work_cleanup(struct drm_flip_work *work)
 {
-	WARN_ON(!kfifo_is_empty(&work->fifo));
-	kfifo_free(&work->fifo);
+	WARN_ON(!list_empty(&work->queued) || !list_empty(&work->commited));
 }
 EXPORT_SYMBOL(drm_flip_work_cleanup);

include/drm/drm_flip_work.h

@@ -25,6 +25,7 @@
 #define DRM_FLIP_WORK_H
 
 #include <linux/kfifo.h>
+#include <linux/spinlock.h>
 #include <linux/workqueue.h>
 
 /**
@@ -32,9 +33,9 @@
  *
  * Util to queue up work to run from work-queue context after flip/vblank.
  * Typically this can be used to defer unref of framebuffer's, cursor
- * bo's, etc until after vblank.  The APIs are all safe (and lockless)
- * for up to one producer and once consumer at a time.  The single-consumer
- * aspect is ensured by committing the queued work to a single work-queue.
+ * bo's, etc until after vblank.  The APIs are all thread-safe.
+ * Moreover, drm_flip_work_queue_task and drm_flip_work_queue can be called
+ * in atomic context.
  */
 
 struct drm_flip_work;
@@ -50,23 +51,37 @@ struct drm_flip_work;
  */
 typedef void (*drm_flip_func_t)(struct drm_flip_work *work, void *val);
 
+/**
+ * struct drm_flip_task - flip work task
+ * @node: list entry element
+ * @data: data to pass to work->func
+ */
+struct drm_flip_task {
+	struct list_head node;
+	void *data;
+};
+
 /**
  * struct drm_flip_work - flip work queue
  * @name: debug name
- * @pending: number of queued but not committed items
- * @count: number of committed items
  * @func: callback fxn called for each committed item
  * @worker: worker which calls @func
- * @fifo: queue of committed items
+ * @queued: queued tasks
+ * @commited: commited tasks
+ * @lock: lock to access queued and commited lists
  */
 struct drm_flip_work {
 	const char *name;
-	atomic_t pending, count;
 	drm_flip_func_t func;
 	struct work_struct worker;
-	DECLARE_KFIFO_PTR(fifo, void *);
+	struct list_head queued;
+	struct list_head commited;
+	spinlock_t lock;
 };
 
+struct drm_flip_task *drm_flip_work_allocate_task(void *data, gfp_t flags);
+void drm_flip_work_queue_task(struct drm_flip_work *work,
+			      struct drm_flip_task *task);
 void drm_flip_work_queue(struct drm_flip_work *work, void *val);
 void drm_flip_work_commit(struct drm_flip_work *work,
 		struct workqueue_struct *wq);