Re: [PATCH 1/2] dma-buf/fence: add fence_collection fences

[Christian KÃnig]

Am 19.05.2016 um 00:57 schrieb Chris Wilson:
> On Wed, May 18, 2016 at 05:59:52PM -0300, Gustavo Padovan wrote:
> > +static void collection_check_cb_func(struct fence *fence, struct fence_cb *cb)
> > +{
> > +	struct fence_collection_cb *f_cb;
> > +	struct fence_collection *collection;
> > +
> > +	f_cb = container_of(cb, struct fence_collection_cb, cb);
> > +	collection = f_cb->collection;
> > +
> > +	if (atomic_dec_and_test(&collection->num_pending_fences))
> > +		fence_signal(&collection->base);
> > +}
> > +
> > +static bool fence_collection_enable_signaling(struct fence *fence)
> > +{
> > +	struct fence_collection *collection = to_fence_collection(fence);
> > +	int i;
> > +
> > +	for (i = 0 ; i < collection->num_fences ; i++) {
> > +		if (fence_add_callback(collection->fences[i].fence,
> > +				       &collection->fences[i].cb,
> > +				       collection_check_cb_func)) {
> > +			atomic_dec(&collection->num_pending_fences);
> > +		}
> > +	}
> We don't always have a convenient means to preallocate an array of
> fences to use. Keeping a list of fences in addition to the array would
> be easier to user in many circumstances.

I agree that there is use for such an implementation as well, but as 
mentioned in the last review cycle we intentionally chose an array 
instead of a more complex implementation here.

This way the array can be passed to function like 
fence_wait_any_timeout() as well.

I also suggested to rename it to fence_array to make that difference 
clear and allow for another implementation to live side by side with this.

My crux at the moment is that I need both for the amdgpu driver, an 
array based implementation and a collection like one.

Gustavo would you mind if I take your patches and work a bit on this?

> Just means we need a
>
> struct fence_collection_entry {
> 	struct fence *fence;
> 	struct list_head link;
> };
>
> int fence_collection_add(struct fence *_collection,
> 			 struct fence *fence)
> {
> 	struct fence_collection *collection =
> 		to_fence_collection(_collection);
> 	struct fence_collection_entry *entry;
>
> 	entry = kmalloc(sizeof(*entry), GFP_KERNEL);
> 	if (!entry)
> 		return -ENOMEM;
>
> 	entry->fence = fence_get(fence);
> 	list_add_tail(&entry->link, &collection->fence_list);
> 	atomic_inc(&collection->num_pending_fences);
>
> 	return 0;
> }
>
> and a couple of list iterations as well as walking the arrays.
>
> (This fence_collection_add() needs to be documented to only be valid
> from the constructing thread before the fence is sealed for export/use.)

As suggested by Daniel as well I would prefer that the the array 
implementation only gets the fences as already filled array in the 
constructor. This is much more fail save.

Christian.

> -Chris