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Date:	Sat, 12 Jul 2014 09:29:05 +0200
From:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
To:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
Cc:	Rob Clark <robdclark@...il.com>, David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] drm: rework flip-work helpers to avoid calling
 func when the FIFO is full

On Sat, 12 Jul 2014 09:00:08 +0200
Boris BREZILLON <boris.brezillon@...e-electrons.com> wrote:

> 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@...e-electrons.com>
> ---
>  drivers/gpu/drm/drm_flip_work.c | 96 ++++++++++++++++++++++++++++++-----------
>  include/drm/drm_flip_work.h     | 29 +++++++++----
>  2 files changed, 93 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_flip_work.c b/drivers/gpu/drm/drm_flip_work.c
> index f9c7fa3..7441aa8 100644
> --- a/drivers/gpu/drm/drm_flip_work.c
> +++ b/drivers/gpu/drm/drm_flip_work.c
> @@ -25,6 +25,44 @@
>  #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
>   * @val: the value to queue
> @@ -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 +148,11 @@ 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);

is missing here.

>  	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 +167,6 @@ 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);
> diff --git a/include/drm/drm_flip_work.h b/include/drm/drm_flip_work.h
> index 9eed34d..0a5acff 100644
> --- a/include/drm/drm_flip_work.h
> +++ b/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 +33,7 @@
>   *
>   * 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 safe.
>   */
>  
>  struct drm_flip_work;
> @@ -51,22 +50,36 @@ 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);



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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