[alsa-devel] [PATCH] ASoC: davinci-pcm: comments for the conversion to BATCH mode
bengardiner at nanometrics.ca
Wed May 25 15:27:22 CEST 2011
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
Add descriptive comments of the phase offset with and without ping-pong
Signed-off-by: Ben Gardiner <bengardiner at 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
@@ -605,6 +605,18 @@ static int davinci_pcm_prepare(struct snd_pcm_substream *substream)
+ * 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)
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.
asp_count = prtd->period - 2;
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