[PATCH v3] dmaengine: mxs-dma: add dma support for i.MX23/28

[vinod.koul@intel.com (Koul, Vinod)]

On Wed, 2011-02-23 at 15:51 +0530, Russell King - ARM Linux wrote:
> On Wed, Feb 23, 2011 at 10:05:47AM +0000, Russell King - ARM Linux wrote:
> > On Wed, Feb 23, 2011 at 11:25:16AM +0530, Koul, Vinod wrote:
> > > Well in that case you are calling the callback from irq handler which is
> > > not so good. The tasklet is other extreme (slow).
> > 
> > tasklets are run on the exit path of interrupts.
> > 
> > > Given that this is
> > > also intended for audio I am not sure if calling period elapsed from
> > > tasklet callback will not cause any audio over/underruns, last I tried
> > > it used to break quite often in stressed situations.
> > 
> > Nevertheless, the DMA engine API specifies that callbacks shall be called
> > from tasklet context.
> 
> Note also that the DMA engine API is supposed to work as follows:
> 
> 1. You submit DMA jobs to it.
> 2. The DMA engine starts the first job
> 3. When the first job is complete, it retires it to a retired list and
>    marks the tasklet to run
> 4. If there is another job waiting, the DMA engine starts the next job
> 5. Repeat 3 until no further jobs are queued
> 
> The tasklet then processes the list of retired jobs, cleaning them
> up and calling their callbacks. 
> 
> So the way to efficiently handle audio is to initially submit several
> 'period' sized buffers for the DMA engine.  As each buffer completes,
> you call snd_pcm_elapsed() to tell ALSA that a period has finished
> and submit the next buffer.  As there's always several buffers submitted,
> the DMA engine never waits for the callback to complete.  And ALSA is
> capable of handling latency in snd_pcm_elapsed().
Russel,
agree with you on this. But in case you have few periods say 2 and small
period sizes (10ms), or similar scenarios where client is waiting to do
something (process data) on callback notification, faster the callback
better it is.

I know DMA engine species callback to be called from tasklet context,
but do we need this for slave-dma or not? If this is posing some
limitation then I am okay to discuss on ways to improve :) 

Thoughts??

-- 
~Vinod