[PATCH 2/3] dma: mxs-dma: Track number of irqs for callback

[mpa@pengutronix.de (Markus Pargmann)]

On Thu, Sep 26, 2013 at 08:26:46PM +0100, Russell King - ARM Linux wrote:
> On Thu, Sep 26, 2013 at 05:06:35PM +0200, Markus Pargmann wrote:
> > Currently there is one tasklet scheduled for all interrupts between two
> > mxs_dma_tasklet calls for one channel. With more than one interrupts,
> > the tasklet is only called once.
> > 
> > Using low latency audio playback, can lead to problems. When using
> > sound/soc/soc-dmaengine-pcm.c, the dmaengine_pcm_dma_complete relies on
> > being called for each interrupt that occures. This function is the
> > callback which is called from the mxs-dma driver in mxs_dma_tasklet.
> > This can cause wrong position counters and can lead to a wrong DMA
> > buffer offset. In this situation the DMA engine and pcm lib may write
> > and read from the same buffer segment.
> 
> I've actually covered this before.  DMAengine drivers (or infact the
> entire system) makes no guarantees how many callbacks will happen for
> a cyclic DMA.  Think about what happens if you lose an interrupt because
> of USB taking a very long time to service your mouse (yes, that really
> happens.)
> 
> You will notice that the soc dmaengine layer has two pcm ops, one for
> implementations with residue reporting, which can cope with missed
> interrupts, and another for those which provide no residue.  I strongly
> recommend that you ensure that your DMAengine driver correctly reports
> the residue, and use the soc dmaengine driver in a mode which uses that
> rather than implementing these fragile schemes.
> 

Thanks, I didn't know about this before. I implemented correct residue
reporting for mxs-dma cyclic buffers and it works without this tasklet
callback loop now.

Regards,

Markus Pargmann


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