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ASoC: davinci-pcm: comments for the conversion to BATCH mode

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In the previous commit 'ASoC: davinci-pcm: convert to BATCH mode', the phase
offset of 2 was mentioned in the commit message but not well commented in the
source.

Add descriptive comments of the phase offset with and without ping-pong
buffers enabled.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
This commit is contained in:
Ben Gardiner 2011-05-25 09:27:22 -04:00 committed by Mark Brown
parent 52e2c5d38e
commit bb5b5fd4d4
1 changed files with 19 additions and 0 deletions
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19
sound/soc/davinci/davinci-pcm.c
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@ -605,6 +605,18 @@ static int davinci_pcm_prepare(struct snd_pcm_substream *substream)
print_buf_info(prtd->asp_link[0], "asp_link[0]");
print_buf_info(prtd->asp_link[1], "asp_link[1]");
/*
* There is a phase offset of 2 periods between the position
* used by dma setup and the position reported in the pointer
* function.
*
* The phase offset, when not using ping-pong buffers, is due to
* the two consecutive calls to davinci_pcm_enqueue_dma() below.
*
* Whereas here, with ping-pong buffers, the phase is due to
* there being an entire buffer transfer complete before the
* first dma completion event triggers davinci_pcm_dma_irq().
*/
davinci_pcm_period_elapsed(substream);
davinci_pcm_period_elapsed(substream);
@ -631,6 +643,13 @@ davinci_pcm_pointer(struct snd_pcm_substream *substream)
int asp_count;
unsigned int period_size = snd_pcm_lib_period_bytes(substream);
/*
* There is a phase offset of 2 periods between the position used by dma
* setup and the position reported in the pointer function. Either +2 in
* the dma setup or -2 here in the pointer function (with wrapping,
* both) accounts for this offset -- choose the latter since it makes
* the first-time setup clearer.
*/
spin_lock(&prtd->lock);
asp_count = prtd->period - 2;
spin_unlock(&prtd->lock);