[alsa-devel] Verification of settings

[Rob Nertney]

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?

Thanks,
Rob