On 06/12/2013 07:38 AM, Vinod Koul wrote:
On Mon, Jun 10, 2013 at 07:29:46PM +0200, Lars-Peter Clausen wrote:
On 05/30/2013 08:46 PM, Lars-Peter Clausen wrote:
+static int jz4740_dma_alloc_chan_resources(struct dma_chan *c) +{
- struct jz4740_dmaengine_chan *chan = to_jz4740_dma_chan(c);
- chan->jz_chan = jz4740_dma_request(chan, NULL);
- if (!chan->jz_chan)
return -EBUSY;- jz4740_dma_set_complete_cb(chan->jz_chan, jz4740_dma_complete_cb);
- return 0;
Sorry, I didnt reply on this one. The API expects you to allocate a pool of descriptors. These descriptors are to be used in .device_prep_xxx calls later.
The size of the descriptor is not fixed, so they can not be pre-allocated. And this is nothing new either, most of the more recently added dmaengine drivers allocate their descriptors on demand.
Vinod, are you ok with this explanation?
Sorry, I was travelling...
Can you explain more of a bit when you say size is not fixed.
This is the function that allocates the descriptor:
static struct jz4740_dma_desc *jz4740_dma_alloc_desc(unsigned int num_sgs) { return kzalloc(sizeof(struct jz4740_dma_desc) + sizeof(struct jz4740_dma_sg) * num_sgs, GFP_ATOMIC); }
So the size depends on the entries in the sg list.
Why would it be issue if we allocate descriptors at the alloc_chan. The idea is that you preallocated pool at alloc_chan and since the .device_prep_xxx calls can be called from atomic context as well, you dont need to do this later. You can use use these descriptors at that time. The idea is keep rotating the descriptors from free poll to used one
Yes, I know all that. And it makes sense to use a pool in certain situations, e.g. if the hardware only supports a limited set of physical descriptors. But in this case the descriptors are completely virtual. Forcing the driver to use a pool would make it more complex, use more memory and also a bit slower (although probably not noticeable).
- Lars