Re: a11y: howto reduce latency with alsa's dmix plugin?

[Halim Sahin <halim.sahin@freenet.de>]

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On Di, Sep 01 2009, Takashi Iwai wrote:

> At best, we need a small C code that reproduces the behavior.
Ok here is a small example.
It's a modified version of pcm_min.c.
Simply press ctrl+c to reproduce the problematic behaviour.
on my machine it takes about an half sek to stop with an usb head which
uses dmix.
Using an sblive card (without dmix) stopps immediately.

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/*
 *  This extra small demo sends a random samples to your speakers.
 */

#include <alsa/asoundlib.h>
#include <signal.h>
#include <unistd.h>
static char *device = "default";

snd_output_t *output = NULL;

unsigned char buffer[16 * 1024];        /* some random data */

snd_pcm_t *handle;

void suspend (int signum)
{
  int err;

  printf ("trying to stop\n");
  if ((err = snd_pcm_drop (handle)) < 0)
    printf ("cannot pause\n");

  sleep (1);
  snd_pcm_close (handle);

  exit (1);
}

int main (void)
{
  int err;

  unsigned int i;

  snd_pcm_sframes_t frames;

  signal (SIGINT, suspend);
  for (i = 0; i < sizeof (buffer); i++)
    buffer[i] = random () & 0xff;

  if ((err = snd_pcm_open (&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0)
   {
     printf ("Playback open error: %s\n", snd_strerror (err));
     exit (EXIT_FAILURE);
   }
  if ((err = snd_pcm_set_params (handle,
                                 SND_PCM_FORMAT_U8,
                                 SND_PCM_ACCESS_RW_INTERLEAVED,
                                 1, 48000, 1, 500000)) < 0)
   {                            /* 0.5sec */
     printf ("Playback open error: %s\n", snd_strerror (err));
     exit (EXIT_FAILURE);
   }

  for (i = 0; i < 16; i++)
   {
     frames = snd_pcm_writei (handle, buffer, sizeof (buffer));
     if (frames < 0)
       frames = snd_pcm_recover (handle, frames, 0);
     if (frames < 0)
      {
        printf ("snd_pcm_writei failed: %s\n", snd_strerror (err));
        break;
      }
     if (frames > 0 && frames < (long) sizeof (buffer))
       printf ("Short write (expected %li, wrote %li)\n",
               (long) sizeof (buffer), frames);
   }

  snd_pcm_close (handle);
  return 0;
}

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Hope you can help.
Kind regards
Halim

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