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

Peter Ujfalusi
2012-09-12T12:53:13Z

On 09/12/2012 03:00 PM, 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.
>
> 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.
>
> As we have no way to know how much data may be discarded from the DMA
> FIFO...

I need to look at this, but at first look we do wait for the drain in
omap_stop_dma(). We used to use omap_stop_dma/omap_start_dma for pause/resume
operations.
But sDMA also have a bit: CDPi: PAUSE_LINK_LIST which should do what we are
looking for.
Need to read the relevant parts of the TRM, but AFAIK we are using normal mode
with linked list (self linking).
I already have the patch for this, I just need to test it on HW.

--
Péter