[PATCH] Update imx-sdma cyclic handling to report residue

[Shawn Guo]

On Wed, Jun 25, 2014 at 01:00:33PM +0100, Russell King - ARM Linux wrote:
> I received a report this morning from one of the Novena developers that
> the behaviour of the iMX6 ASoC codec driver (using imx-pcm-dma.c) was
> sub-optimal under high system load.
> 
> While there are issues relating to system load remaining, upon reviewing
> the ASoC imx-pcm-dma.c driver, it was noticed that it not using the
> residue support, because SDMA doesn't support it.  This has the effect
> that SDMA has to make multiple calls into the ASoC and ALSA code, one
> for each period.
> 
> Since ALSA's snd_pcm_elapsed() does not need to be called multiple times
> and it is entirely sufficient to call it once to update ALSA with the
> current buffer position via the pointer method, we can do better here.
> We can also avoid stopping the DMA entirely, just like real cyclic DMA
> implementations behave.  While this means that we replay some old samples,
> this is a nicer behaviour than having audio stop and restart.
> 
> The changes to achieve this are relatively minor - imx-sdma.c can track
> where the DMA is to the nearest descriptor boundary - it does this
> already when deciding how many callbacks to issue.  In doing this,
> buf_tail always points at the descriptor which will complete next.
> 
> The residue is defined by the bytes remaining to the end of the buffer,
> when the buffer is viewed as a single block of memory [start...end].
> So, when we start out, there's a full buffer worth of residue, and this
> counts down as we approach the end of the buffer, eventually becoming
> zero at the end, before returning to the full buffer worth when we
> wrap back to the start.
> 
> Moving the walking of the descriptors into the interrupt handler means
> that we can update the BD_DONE flag at interrupt time, thus avoiding
> a delayed tasklet stopping the cyclic DMA.
> 
> This means that the residue can be calculated from (total descriptors -
> buf_tail) * descriptor size.  This is what the change below does.  We
> update imx-pcm-dma.c to remove the NO_RESIDUE flag since we now provide
> the residue.
> 
> Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>

For all the audio test cases I usually run, nothing breaks, so

Tested-by: Shawn Guo <shawn.guo at linaro.org>