On 25/05/11 14:27, Ben Gardiner wrote:
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().*/@@ -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.*/
Acked-by: Liam Girdwood lrg@ti.com