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
--- sound/soc/davinci/davinci-pcm.c | 19 +++++++++++++++++++ 1 files changed, 19 insertions(+), 0 deletions(-)
diff --git a/sound/soc/davinci/davinci-pcm.c b/sound/soc/davinci/davinci-pcm.c index fa8fc61..c9e0320 100644 --- a/sound/soc/davinci/davinci-pcm.c +++ b/sound/soc/davinci/davinci-pcm.c @@ -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);