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

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>

1 files changed, 19 insertions, 0 deletions

diff --git a/sound/soc/davinci/davinci-pcm.c b/sound/soc/davinci/davinci-pcm.c
index fa8fc617c0db..c9e0320f6289 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);