[alsa-devel] [PATCH 11/11] ASoC: omap-pcm: Convert to use dmaengine

[Takashi Iwai]

At Wed, 12 Sep 2012 13:00:28 +0100,
Russell King - ARM Linux wrote:
> 
> On Wed, Sep 12, 2012 at 02:47:07PM +0300, Peter Ujfalusi wrote:
> > 2. Pause/Resume
> >    OMAP DMA engine backend does not support pausing and resuming
> >    an in-progress transfer.  It is unclear from the specs what
> >    effect clearing the enable bit has on the DMA position of a
> >    destination synchronized transfer, and whether the transfer
> >    can be restarted from the exact point that it was paused (or
> >    whether the data in the FIFO read from memory is simply
> >    discarded.)
> 
> It's worth noting that this comment (which was in my original patch)
> is there to spark _comment_ and _discussion_ and should not make its
> way into the final version of these patches.

I agree this can be regarded as a sort of regression...

> Given that suspend/resume is important on OMAP platforms, it's something
> that needs to be resolved - in a way that complies with what ALSA expects.
> I do not believe that the way the existing drivers do this is compliant
> as the manuals imply that stopping memory->peripheral transfers results
> in data being discarded from the DMA's FIFOs.  As I understand it, ALSA
> requires no data to be discarded.

... but in general you don't need to implement the full PAUSE and
RESUME operations for PCM.  For example, when SNDRV_PCM_INFO_RESUME
bit isn't set, user-space is supposed to do a clean restart of the
stream at wake up.  So, it shouldn't break the apps severely in
theory (but in practice, who knows :)

> As we have no way to know how much data may be discarded from the DMA
> FIFO...

Yeah, the FIFO and the amount of other on flight data can be better
managed.


Takashi