[alsa-devel] [PATCH v2 1/2] ASoC: dmaengine-pcm: Add support for querying DMA capabilities

Lars-Peter Clausen lars at metafoo.de
Tue Oct 8 18:13:56 CEST 2013

On 10/08/2013 05:06 PM, Vinod Koul wrote:
> On Tue, Oct 08, 2013 at 03:07:59PM +0200, Lars-Peter Clausen wrote:
>> Currently each platform making use the the generic dmaengine PCM driver still
>> needs to provide a custom snd_pcm_hardware struct which specifies the
>> capabilities of the DMA controller, e.g. the maximum period size that can be
>> supported. This patch adds code which uses the newly introduced
>> dma_get_slave_caps() API to query this information from the dmaengine driver.
>> The new code path will only be taken if the 'pcm_hardware' field of the
>> snd_dmaengine_pcm_config struct is NULL.
>> The patch also introduces a new 'fifo_size' field to the
>> snd_dmaengine_dai_dma_data struct which is used to initialize the
>> snd_pcm_hardware 'fifo_size' field and needs to be set by the DAI driver.
>> Signed-off-by: Lars-Peter Clausen <lars at metafoo.de>
>> ---
>> Changes since v1:
>> 	* Use dma_get_max_seg_size() to get the maximum size of one period
>> 	* Assume that a DMA engine driver is able to support an infinite number of
>> 	  periods
>> 	* Increase min_period_bytes from 16 to 256.
>> ---
>>   include/sound/dmaengine_pcm.h         |  2 ++
>>   sound/soc/soc-generic-dmaengine-pcm.c | 55 ++++++++++++++++++++++++++++-------
>>   2 files changed, 47 insertions(+), 10 deletions(-)
>> diff --git a/include/sound/dmaengine_pcm.h b/include/sound/dmaengine_pcm.h
>> index f11c35c..83b2c3e 100644
>> --- a/include/sound/dmaengine_pcm.h
>> +++ b/include/sound/dmaengine_pcm.h
>> @@ -61,6 +61,7 @@ struct dma_chan *snd_dmaengine_pcm_get_chan(struct snd_pcm_substream *substream)
>>    * @slave_id: Slave requester id for the DMA channel.
>>    * @filter_data: Custom DMA channel filter data, this will usually be used when
>>    * requesting the DMA channel.
>> + * @fifo_size: FIFO size of the DAI controller in bytes
> is this supposed to be DMA FIFO or I2S FIFO?

This field is the internal FIFO of the audio core.

> And second question for Takakshi or Mark. I see that this value ends up in
> usermode, what is the intented usage of this?

I think it is supposed to help to calculate the audio delay, but I don't think 
it is actually that widely used at the moment. And yes, in order to make this 
more accurate we should probably add support for incorporating also the fifo 
size of the DMA channel.

>> +	hw.periods_min = 2;
>> +	hw.periods_max = UINT_MAX;
>> +	hw.period_bytes_min = 256;
>> +	hw.period_bytes_max = dma_get_max_seg_size(dma_dev);
> I think you should check if the driver has set this, if not goto some default

I don't think we are supposed to directly peak at the dma_parms field. If the 
DMA driver does not work with the default, and it did not set up a proper 
value, it is a bug in the DMA driver.

- Lars

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