ALSA - DMA ping-pong

Ran,

I think DMA usage depends on which ALSA function is used. DMA is involved only when there is data transfer started (playback or record).

BR
Pavel

Hi Pavel,

It seems according to code that DMA is always used whenever there is data transfer (without care for the specific alsa function for the data transfer).

davinci_pcm.c:

davinci_pcm_open() -> davinci_pcm_dma_request(substream) -> link = prtd->asp_channel = edma_alloc_channel(params->channel)

I also see that without configuring dma correct in kernel then davinci_pcm_open will return error.

 

Best Regards,

Ran

Hi Pavel,

I am mainly interetsed in working with dma in ping-pong mode.

I see in davinci_pcm.c a comment "not used with ping/pong" :

/*

  * Not used with ping/pong

  */

 static void davinci_pcm_enqueue_dma(struct snd_pcm_substream *substream)

 {

.....

         period_size = snd_pcm_lib_period_bytes(substream);

         dma_offset = prtd->period * period_size;

         dma_pos = runtime->dma_addr + dma_offset;

....

if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
src = dma_pos;
dst = prtd->params->dma_addr;
src_bidx = data_type;
dst_bidx = 0;
src_cidx = data_type * fifo_level;
dst_cidx = 0;
} else {
....

acnt = prtd->params->acnt;
edma_set_src(link, src, INCR, W8BIT);
edma_set_dest(link, dst, INCR, W8BIT);

}

But this comment is not well understood becuase you can see that dma with offset/length according to alsa period parameter.

So what is the meaning of the comment "not used with ping/pong" ?

Regards,

Ran

Ran,

Ran Shalit said:

It seems according to code that DMA is always used whenever there is data transfer (without care for the specific alsa function for the data transfer).

davinci_pcm.c:

davinci_pcm_open() -> davinci_pcm_dma_request(substream) -> link = prtd->asp_channel = edma_alloc_channel(params->channel)

Which ALSA API function invokes davinci_pcm_open() ?

BR
Pavel

Hi Pavel,

Is see that /sound/core/pcm* files use substream->open() for pcm.
The thing we are main concrened with now, is to support ping/pong with DMA.
From what I read about dma&alsa (there is no much info about dma & alsa), I think that the period parameter should be used for the ping-pong.
and it also seems reasonable in davinci code, becuase the dma offset is determined according to period:

dma_offset = prtd->period * period_size;
dma_pos = runtime->dma_addr + dma_offset;

Is there any reason for the commened "Not used with ping/pong" in the header of:

static void davinci_pcm_enqueue_dma(struct snd_pcm_substream *substream)

Do you think that ping-pong is already supported ?

Regards,
Ran

In addition, to this I will add that
arecord utility will also invoke this pcm open :
arecord -> snd_pcm_open -> davinci_pcm_open

Regards,
Ran

Ran,

Linking can be performed in order to reload the same transfer or a completely different transfer. Often systems employ a ping-pong buffering system. In this case, when the ping transfer is done, the EDMA can auto reload the pong transfer and on and on it goes.

See also the below e2e thread:
e2e.ti.com/.../252002

Check also DM814x TRM, chapter EDMA

Hi Pavel,

Thank you very much for the hints.
But Isn't using period with dma_pos inside dma buffer is already a ping-pong method (which is already embedded in davinci code) ?
dma_offset = prtd->period * period_size;
dma_pos = runtime->dma_addr + dma_offset;

Regards,
Ran

Ran Shalit said:

But Isn't using period with dma_pos inside dma buffer is already a ping-pong method (which is already embedded in davinci code) ?

I don't think so.

Ping-pong buffering is a simple technique that allows the CPU activity to be distanced from the EDMA activity. This means that there are multiple (usually two) sets of data buffers for all incoming and outgoing data streams. While the EDMA transfers the data into and out of the ping buffers, the CPU manipulates the data in the pong buffers. When both CPU and EDMA activity completes, they switch. The EDMA then writes over the old input data and transfers the new output data.

To change the continuous operation example, such that a ping-pong buffering scheme is used, the EDMA channels need only a moderate change. Instead of one parameter set, there are two; one for transferring data to/from the ping buffers and one for transferring data to/from the pong buffers. As soon as one transfer completes, the channel loads the PaRAM set for the other and the data transfers continue.

davinci_pcm_enqueue_dma() use only asp_link[0]. While for ping-pong, asp_link[0] and asp_link[1] should be used, see:

ping_pong_dma_setup()

request_ping_pong()

BR
Pavel