linux-kernel - Re: [PATCH 1/2] dma-buf/fence: add fence

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Date:	Thu, 19 May 2016 09:46:47 +0200
From:	Christian König <christian.koenig@....com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Gustavo Padovan <gustavo@...ovan.org>,
	<dri-devel@...ts.freedesktop.org>, <marcheu@...gle.com>,
	Daniel Stone <daniels@...labora.com>, <seanpaul@...gle.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	<linux-kernel@...r.kernel.org>,
	<laurent.pinchart@...asonboard.com>,
	"Alex Deucher" <alexander.deucher@....com>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	John Harrison <John.C.Harrison@...el.com>,
	<m.chehab@...sung.com>
Subject: Re: [PATCH 1/2] dma-buf/fence: add fence_collection fences

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
>