Hi,
On Fri, 14 Aug 2009, Clemens Ladisch wrote:
snd_pcm_delay() returns the filled part of the buffer plus
the additional delay imposed by the device.
[...]
The additional delay can be set by the driver in
pcm_substream->runtime->delay. (Currently, only
the USB audio driver bothers to set this.)
aa, now that's interesting, and I think that's just the missing piece I'm
looking for. I was looking at the 2.6.30.4 kernel, and couldn't find any
traces of 'runtime->delay', but oh yes, 2.6.31 tree indeed has
it - yay! :)
It would seem this was added by Takashi with commit "ALSA: Add extra delay
count in PCM" on 2009-05-05, so no wonder not too many drivers set it yet.
Hmm, but isn't it so that this extra delay is added properly to
snd_pcm_delay() result, when 'sync_ptr_ioctl==false' property is set for
the pcm (i.e. use SYNC_PTR ioctl instead of mmap control structures).
Attached is an alsa-lib patch attempting to fix this (as sufficient info
is not available in the mmap control block, always go via IOCTL_DELAY).
But otherwise this does look good and is just what I was looking for. When
using snd_pcm_status(), application can get all the information (avail,
delay) in one go. Of course one limitation is that the runtime->delay
updates are synced to driver events, not to application calling
snd_pcm_delay (unlike query of hw_ptr status via pointer driver callback),
but maybe this is not even needed in the end. Have to think about this
some more...
Thanks Clemens, James and Mark for the quick replies!
commit fc98ec2e631a6afa85e7505bc553824d7f6f56cc
Author: Kai Vehmanen <kvehmanen@xxxxxx>
Date: Sat Aug 15 02:05:02 2009 +0300
pcm_hw: Always use SNDRV_PCM_IOCTL_DELAY
As the kernel drivers may adjust the delay with 'runtime->delay',
the SNDRV_PCM_IOCTL_DELAY ioctl must be always used when
calculating the snd_pcm_delay result.
diff --git a/src/pcm/pcm_hw.c b/src/pcm/pcm_hw.c
index c46d14f..4491099 100644
--- a/src/pcm/pcm_hw.c
+++ b/src/pcm/pcm_hw.c
@@ -523,11 +523,6 @@ static int snd_pcm_hw_delay(snd_pcm_t *pcm, snd_pcm_sframes_t *delayp)
default:
return -EBADFD;
}
- if (pcm->stream == SND_PCM_STREAM_PLAYBACK)
- *delayp = snd_pcm_mmap_playback_hw_avail(pcm);
- else
- *delayp = snd_pcm_mmap_capture_avail(pcm);
- return 0;
}
if (ioctl(fd, SNDRV_PCM_IOCTL_DELAY, delayp) < 0) {
err = -errno;
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