On Thu, Feb 19, 2009 at 08:42:15AM +0200, Takashi Iwai wrote:
> At Thu, 19 Feb 2009 13:20:16 +0800,
> Wu Fengguang wrote:
> >
> > Hi Takashi,
> >
> > I noticed that 'appl_ptr' is always 0 during playback, whether it be
> > mplayer or aplay, T61 or DG45ID. Is this a bug?
>
> No, it's a feature. When you use dmix, the appl_ptr isn't updated
> in the driver side but the driver is running in a "free-wheel" mode.
> OTOH, if you use pulseaudio or use HDMI out, the device is opened in a
> normal mode, thus appl_ptr is handled in the driver side.
Ah thanks. This dismissed my questions :)
The dmix plugin always returns error code or negative delay:
alsa-lib/src/pcm/pcm_dmix.c
477 static int snd_pcm_dmix_delay(snd_pcm_t *pcm, snd_pcm_sframes_t *delayp)
478 {
479 snd_pcm_direct_t *dmix = pcm->private_data;
480 int err;
481
482 switch(dmix->state) {
483 case SNDRV_PCM_STATE_DRAINING:
484 case SNDRV_PCM_STATE_RUNNING:
485 err = snd_pcm_dmix_sync_ptr(pcm);
486 if (err < 0)
487 return err;
488 /* fallthru */
489 case SNDRV_PCM_STATE_PREPARED:
490 case SNDRV_PCM_STATE_SUSPENDED:
491 case STATE_RUN_PENDING:
492 *delayp = snd_pcm_mmap_playback_hw_avail(pcm);
493 return 0;
494 case SNDRV_PCM_STATE_XRUN:
495 return -EPIPE;
496 case SNDRV_PCM_STATE_DISCONNECTED:
497 return -ENODEV;
498 default:
499 return -EBADFD;
500 }
501 }
That explains why mplayer always do snd_pcm_forward() after snd_pcm_delay():
snd_pcm_delay(0x122edc0, 0x7fff0c2201e0, 0x5f4ca4, 0, 0x11e1588) = 0
snd_pcm_forward(0x122edc0, 1756, 0xc0000000000026dc, 9948, 0x11e1588) = 0
snd_pcm_delay(0x122edc0, 0x7fff0c2201c0, 24000, 0x122da98, 0x11e1588) = 0
snd_pcm_forward(0x122edc0, 8, 0xc000000000002008, 8200, 0x11e1588) = 0x3ffffffffffff924
The mplayer get_delay() code reads:
MPlayer-1.0rc2/libao2/ao_alsa.c
869 /* delay in seconds between first and last sample in buffer */
870 static float get_delay(void)
871 {
872 if (alsa_handler) {
873 snd_pcm_sframes_t delay;
874
875 if (snd_pcm_delay(alsa_handler, &delay) < 0)
876 return 0;
877
878 if (delay < 0) {
879 /* underrun - move the application pointer forward to catch
up */
880 #if SND_LIB_VERSION >= 0x000901 /* snd_pcm_forward() exists since
0.9.0rc8 */
881 snd_pcm_forward(alsa_handler, -delay);
882 #endif
883 delay = 0;
884 }
885 return (float)delay / (float)ao_data.samplerate;
886 } else {
887 return(0);
888 }
889 }
Which is called in the following places:
3 mplayer.c playing_audio_pts 1601 audio_out->get_delay();
4 mplayer.c fill_audio_out_buffers 1823 else if (audio_eof && mpctx->audio_out->get_delay() < .04) {
5 mplayer.c sleep_until_update 1841 float delay = mpctx->audio_out->get_delay();
6 mplayer.c update_video 2026 float delay = playback_speed*mpctx->audio_out->get_delay();
It happen to not cause trouble because
1) in mplayer, the negative delay will be reset to 0 before being returned
2) in ALSA, snd_pcm_playback_forward() will do nothing if avail is negative
Thanks,
Fengguang
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