[alsa-devel] [PATCH] ASoC: davinci-pcm: comments for the conversion to BATCH mode

[Liam Girdwood]

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 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
> --- 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);

Acked-by: Liam Girdwood <lrg at ti.com>