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

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Date:	Wed, 18 May 2016 23:57:10 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Gustavo Padovan <gustavo@...ovan.org>
Cc:	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,
	Christian König <christian.koenig@....com>,
	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

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.

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.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre