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    SubjectRe: [alsa-devel] [PATCH v2 3/3] ASoC: dmaengine-pcm: Add support for querying stream position from DMA driver
    On Mon, Jun 11, 2012 at 08:11:42PM +0200, Lars-Peter Clausen wrote:
    > Currently the sound dmaengine pcm helper functions implement the pcm_pointer
    > callback by trying to count the number of elapsed periods. This is done by
    > advancing the stream position in the dmaengine callback by one period.
    > Unfortunately there is no guarantee that the callback will be called for each
    > elapsed period. It may be possible that under high system load it is only called
    > once for multiple elapsed periods. This patch addresses the issue by
    > implementing support for querying the current stream position directly from the
    > dmaengine driver. Since not all dmaengine drivers support reporting the stream
    > position yet the old period counting implementation is kept for now.
    >
    > Furthermore the new mechanism allows to report the stream position with a
    > sub-period granularity, given that the dmaengine driver supports this.
    >
    > Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
    >
    > ---
    > Changes since v1:
    > * Don't implement fallback support in snd_dmaengine_pcm_pointer, instead it
    > is now moved to its own function. This was done to make it more explicit
    > that implementing residue reporting for cyclic is required and that this
    > won't be optional for new drivers.
    > ---
    > include/sound/dmaengine_pcm.h | 1 +
    > sound/soc/soc-dmaengine-pcm.c | 29 ++++++++++++++++++++++++++++-
    > 2 files changed, 29 insertions(+), 1 deletion(-)
    >
    > diff --git a/include/sound/dmaengine_pcm.h b/include/sound/dmaengine_pcm.h
    > index ea57915..b877334 100644
    > --- a/include/sound/dmaengine_pcm.h
    > +++ b/include/sound/dmaengine_pcm.h
    > @@ -38,6 +38,7 @@ void *snd_dmaengine_pcm_get_data(struct snd_pcm_substream *substream);
    > int snd_hwparams_to_dma_slave_config(const struct snd_pcm_substream *substream,
    > const struct snd_pcm_hw_params *params, struct dma_slave_config *slave_config);
    > int snd_dmaengine_pcm_trigger(struct snd_pcm_substream *substream, int cmd);
    > +snd_pcm_uframes_t snd_dmaengine_pcm_pointer(struct snd_pcm_substream *substream);
    > snd_pcm_uframes_t snd_dmaengine_pcm_pointer_no_residue(struct snd_pcm_substream *substream);
    >
    > int snd_dmaengine_pcm_open(struct snd_pcm_substream *substream,
    > diff --git a/sound/soc/soc-dmaengine-pcm.c b/sound/soc/soc-dmaengine-pcm.c
    > index 7c0877e..2995334 100644
    > --- a/sound/soc/soc-dmaengine-pcm.c
    > +++ b/sound/soc/soc-dmaengine-pcm.c
    > @@ -30,6 +30,7 @@
    >
    > struct dmaengine_pcm_runtime_data {
    > struct dma_chan *dma_chan;
    > + dma_cookie_t cookie;
    >
    > unsigned int pos;
    >
    > @@ -153,7 +154,7 @@ static int dmaengine_pcm_prepare_and_submit(struct snd_pcm_substream *substream)
    >
    > desc->callback = dmaengine_pcm_dma_complete;
    > desc->callback_param = substream;
    > - dmaengine_submit(desc);
    > + prtd->cookie = dmaengine_submit(desc);
    >
    > return 0;
    > }
    > @@ -213,6 +214,32 @@ snd_pcm_uframes_t snd_dmaengine_pcm_pointer_no_residue(struct snd_pcm_substream
    > }
    > EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_pointer_no_residue);
    >
    > +/**
    > + * snd_dmaengine_pcm_pointer - dmaengine based PCM pointer implementation
    > + * @substream: PCM substream
    > + *
    > + * This function can be used as the PCM pointer callback for dmaengine based PCM
    > + * driver implementations.
    > + */
    > +snd_pcm_uframes_t snd_dmaengine_pcm_pointer(struct snd_pcm_substream *substream)
    > +{
    > + struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
    > + struct dma_tx_state state;
    > + enum dma_status status;
    > + unsigned int buf_size;
    > + unsigned int pos = 0;
    > +
    > + status = dmaengine_tx_status(prtd->dma_chan, prtd->cookie, &state);
    > + if (status == DMA_IN_PROGRESS || status == DMA_PAUSED) {
    > + buf_size = snd_pcm_lib_buffer_bytes(substream);
    > + if (state.residue > 0 && state.residue <= buf_size)
    > + pos = buf_size - state.residue;
    One question i still not very clear, for the case when one buffer size of cyclic dma
    transfer completes, the dma residue may be buf_size( or 0 as Russell said), then
    pos here is 0. So is it correct that call bytes_to_frames(substream->runtime, 0)
    for this case?

    Regards
    Dong Aisheng



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