Re: [alsa-devel] [PATCH v2 3/6] dma: Add a jz4740 dmaengine driver

[Vinod Koul <vinod.koul@intel.com>]

On Wed, Jun 12, 2013 at 02:07:10PM +0200, Lars-Peter Clausen wrote:
> 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).
Okay, I am going to try applying these now

--
~Vinod