On Mon, 2 Feb 2004, James Courtier-Dutton wrote:
> James Courtier-Dutton wrote:
> >
> > Once thing I have noticed, is that with the alc650, we used to have VRA
> > (alsa 0.9.8), but the 1.0.2 intel8x0 driver ignores the VRA and fixes
> > itself at 48000.
> > So, before I never needed the sample rate converters, but as it now
> > fixes the rate at 48000, I need the sample rate converters, and they
> > don't actually work.
> >
> >
>
> I have found out what the problem is with the resamplers not working.
> I was using: -
> err = snd_pcm_sw_params_set_start_threshold(handle, swparams,
> buffer_size); <- note buffer_size.
>
> So, the state will stay in SND_PCM_STATE_PREPARED state until the buffer
> is full.
> Inside snd_pcm_write_areas() [in alsa-lib] we have a snd_pcm_wait
> statement before we write frames into the buffer.
> if (((snd_pcm_uframes_t)avail < pcm->avail_min && size >
> (snd_pcm_uframes_t)avail) || (size >= pcm->xfer_align &&
> (snd_pcm_uframes_t)avail < pcm->xfer_align)) {
> then it calls snd_pcm_wait()
>
> So, this will call snd_pcm_wait() until avail >= pcm->avail_min.
> This will only become true if we are in SND_PCM_STATE_RUNNING state.
>
> below the snd_pcm_wait(), we do try to execute snd_pcm_start() to get us
> into SND_PCM_STATE_RUNNING, but this only gets executed if: -
> if (state == SND_PCM_STATE_PREPARED && hw_avail >= (snd_pcm_sframes_t)
> pcm->start_threshold) {
>
> So, we will only reach a running state if the buffer is FULL, but the
> buffer will NEVER become full, because of the previous snd_pcm_wait();
> CATCH22!!!
>
> I think a fix to this would be to limit as follows: -
> start_threshold <= (buffer_size - avail_min)
>
> I have fixed the issue for now, just by setting the "start_threshold" to
> something other than buffer_size. But I think some work needs to be done
> with alsa-lib to prevent this deadlock situation ever happening in alsa-lib.
>
> From the application writers point of view, if the buffer is X bytes
> long, and one used snd_pcm_writei() to write X+10 bytes, the buffer
> should become full. Currently, that is not always the case, and alsa-lib
> instead locks in snd_pcm_wait().
>
> I attach a patch that fixes alsa-lib.
> All it does it call snd_pcm_start() just before snd_pcm_wait() and due
> to the if statement just above it, will only happen when the buffer is
> going to be filled, therefore ensuring that even if "start_threshold" is
> set too high, snd_pcm_start() will always be called if the buffer is
> full, or about to become full.
I don't think that this is a right fix. The programmer must know what is
doing and the manual start is still alternative. You must set
start_threshold to (buffer_size / period_size) * period_size in your case.
Jaroslav
-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs
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