[pulseaudio-discuss] Why are we checking various sample rates?

[Raymond Yau]

> > >
> > > I'm researching a bug where some set of USB speakers stopped working,
> > > probably in 2.x. The peculiar thing about this sound card is that
seems
> > > to only work at a sample rate of 46875 Hz.
> > >
> > > So in 2.x, the probe fails with
> > > "E: [pulseaudio] alsa-sink.c: Failed to find any supported sample
> > > rates.", whereas in an earlier version, it would just happily work at
> > > 46875 Hz instead, as I understand it.
> > >
> > > So my first question is; why do we check all these rates, when we end
up
> > > just alternating between default-sample-rate (44100 Hz) and
> > > alternate-sample-rate (48000 Hz) anyway?
> >
> > I don't have an answer for that.
> >
> > > And second, can we try not to fail if we can't find any supported
sample
> > > rates, so we can fix this regression?
> >
> > I don't see any fundamental reason why we couldn't support any crazy
> > sample rate.
> >
> > > For reference, this is the bug (with pulse verbose logs in it):
> > >
> > > https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/1074783
> >
> > A related bug: https://bugs.freedesktop.org/show_bug.cgi?id=48109
> >
> > --
> > Tanu
> >
> Hi.
> I played around a bit and found seemingly a solution to this. I'm not sure
> if this is the proper solution but it seems to work for me. Let me know
> if I should clean this up a little further.
>
> Patch attached.
>

+    if ((ret =snd_pcm_hw_params_get_rate_min(hwparams, &rate_min, &dir)) <
0) {
+        pa_log_debug("snd_pcm_hw_params_get_rate_min() failed: %s",
pa_alsa_strerror(ret));
+        goto fallback;
+    }
+
+    if ((ret =snd_pcm_hw_params_get_rate_max(hwparams, &rate_max, &dir)) <
0) {
+        pa_log_debug("snd_pcm_hw_params_get_rate_max() failed: %s",
pa_alsa_strerror(ret));
+        goto fallback;
+    }
+    pa_log_debug("Found min - max rates: %d - %d", rate_min, rate_max);
+
+    for (i = rate_min, n = 0; i <= rate_max; i++) {
+        if (snd_pcm_hw_params_test_rate(pcm, hwparams, i, 0) == 0) {
+            rates = pa_xrealloc(rates, ++n * sizeof(unsigned int));
+            rates[n-1] = i;
+        }
+    }

this create a extreme large array for those drivers which support
SNDRV_PCM_RATE_CONTINUOUS
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