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Commit bb5b5fd4 authored by Ben Gardiner's avatar Ben Gardiner Committed by Mark Brown
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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: default avatarBen Gardiner <bengardiner@nanometrics.ca>
Acked-by: default avatarLiam Girdwood <lrg@ti.com>
Signed-off-by: default avatarMark Brown <broonie@opensource.wolfsonmicro.com>
parent 52e2c5d3

sound/soc/davinci/davinci-pcm.c

+19 −0
Original line number Diff line number Diff line
@@ -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);