[alsa-devel] Verification of settings

[Rob Nertney]

Thanks Lars.

My problem with the DMA is that it doesn't support cyclic mode, but does
support SG mode. I have to create a new descriptor every time, and
requeue/resubmit it. I don't want to drop any frames, so PONG is being
received while PING is being copied and resubmitted to the Linux queue.

On Mon, Jul 11, 2016 at 12:29 PM, Lars-Peter Clausen <lars at metafoo.de>
wrote:

> On 07/11/2016 09:10 PM, Rob Nertney wrote:
> > Hi all,
> >
> > Could I please get a sanity check on my hw_params?
> >
> > I have a DMA which is providing between 1-16 channels of 4 bytes/ch worth
> > of data as a frame. I get an interrupt to my driver every frame's worth
> of
> > data (64 Bytes). The data is S32_LE, 16000Hz.
> >
> > My DMA has 2 buffers, PING and PONG. Each receives an IRQ on a frame
> > length, and these local buffers are the size of a frame length (64Bytes).
> >
> > #define MAX_BUFFER (64 * 2)
> > static struct snd_pcm_hardware my_pcm_hw =
> > {
> >   .info = (SNDRV_PCM_INFO_MMAP |
> >       SNDRV_PCM_INFO_INTERLEAVED |
> >       SNDRV_PCM_INFO_BLOCK_TRANSFER |
> >       SNDRV_PCM_INFO_MMAP_VALID),
> >   .formats          = SNDRV_PCM_FMTBIT_S32,
> >   .rates            = SNDRV_PCM_RATE_16000,
> >   .rate_min         = 16000,
> >   .rate_max         = 16000,
> >   .channels_min     = 1,
> >   .channels_max     = NUM_CHANNELS,
> >   .buffer_bytes_max = MAX_BUFFER,
> >   .period_bytes_min = 4,
> >   .period_bytes_max = 64,
> >   .periods_min      = 2,
> >   .periods_max      = 2,
> > };
> >
> >
> > My understanding is that the MAX_BUFFER needs to be at least twice the
> size
> > of a period so I don't underrun. .periods_max means the maximum number of
> > periods in a the alsa dma_area buffer, right?
> >
> > So when my DMA fires its ISR, I copy from its local PING buffer to the
> > dma_area at offset 0, increment the buf_pos by the frame_length
> (64Bytes),
> > and call snd_pcm_period_elapsed.
> >
> > My DMA fires its ISR for its local PONG buffer, copies to the
> > dma_area+buf_pos, increments buf_pos (now back to 0, since buffer only
> > holds 2 frames/periods), and I call snd_pcm_period_elapsed again,
> correct?
>
> Hi,
>
> In principle sounds OK. But there is not much of a point to use a
> ping-pong buffer if you give the real buffer the same restrictions. You
> might as well use the dma_area buffer as the target of the DMA in that
> case. But since in your case the DMA is very restrictive in what it can
> support it makes more sense to use the ping-pong buffer and broaden the
> restrictions on the real buffer. Allow larger period sizes (only call
> snd_pcm_period_elapsed() every N interrupts), allow more than two
> periods and so on.
>
> - Lars
>
>