Hardware initialization fails (Device or resource is busy)

[hauptmech]

pyalsaaudio hardware initialization fails because it is written with a misunderstanding of how alsa hardware initialization works. It needs to be rewritten...

Issues #61, #60, #47, #46, #43, #34 are probably due to this.

From the alsa_lib documentation (ie code):

/** \brief Install one PCM hardware configuration chosen from a configuration space and #snd_pcm_prepare it
 * \param pcm PCM handle
 * \param params Configuration space definition container
 * \return 0 on success otherwise a negative error code
 *
 * The configuration is chosen fixing single parameters in this order:
 * first access, first format, first subformat, min channels, min rate, 
 * min period time, max buffer size, min tick time. If no mutually
 * compatible set of parameters can be chosen, a negative error code
 * will be returned.
 *
 * After this call, #snd_pcm_prepare() is called automatically and
 * the stream is brought to \c #SND_PCM_STATE_PREPARED state.
 *
 * The hardware parameters cannot be changed when the stream is
 * running (active). The software parameters can be changed
 * at any time.
 *
 * The configuration space will be updated to reflect the chosen
 * parameters.
 */
int snd_pcm_hw_params(snd_pcm_t *pcm, snd_pcm_hw_params_t *params)

pyalsaaudio is trying to set the hardware configuration multiple times, and getting -EBUSY (Device or resource busy) because the state of the audio device is SND_PCM_STATE_PREPARED and configuration is no longer possible.

Basically you must set all of the hardware configuration as you like it, and then call snd_pcm_hw_params() once and only once at the end.

I don't have time to offer a patch but the following diff shows my approach:

diff --git a/alsaaudio.c b/alsaaudio.c
index 76c0ffe..db81f96 100644
--- a/alsaaudio.c
+++ b/alsaaudio.c
@@ -360,13 +359,17 @@ alsapcm_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     int pcmmode = 0;
     char *device = "default";
     char *card = NULL;
+    unsigned int rate = 48000;
+    unsigned int latency = 500000;
+    unsigned int channels = 2;
+    snd_pcm_format_t format = SND_PCM_FORMAT_S16_LE;
     int cardidx = -1;
     char hw_device[128];
-    char *kw[] = { "type", "mode", "device", "cardindex", "card", NULL };
+    char *kw[] = { "type", "mode", "device", "cardindex", "card", "rate", "latency", "fmt", "channels", NULL };
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|Oisiz", kw,
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|Oisiziiii", kw,
                                      &pcmtypeobj, &pcmmode, &device,
-                                     &cardidx, &card))
+                                     &cardidx, &card, &rate, &latency, &format, &channels))
         return NULL;
 
     if (cardidx >= 0) {
@@ -410,18 +413,28 @@ alsapcm_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     self->handle = 0;
     self->pcmtype = pcmtype;
     self->pcmmode = pcmmode;
-    self->channels = 2;
-    self->rate = 44100;
-    self->format = SND_PCM_FORMAT_S16_LE;
-    self->periodsize = 32;
+    self->channels = channels;
+    self->rate = rate;
+    self->format = format;
+
 
     res = snd_pcm_open(&(self->handle), device, self->pcmtype,
                        self->pcmmode);
 
-    if (res >= 0) {
-        res = alsapcm_setup(self);
+    if (res >= 0){
+     snd_pcm_hw_params_malloc(&(self->hwparams));
+    }
+    res = snd_pcm_set_params(self->handle, format, 
+      SND_PCM_ACCESS_RW_INTERLEAVED, channels, rate, 1, latency);
+    if (res) {
+            PyErr_Format(ALSAAudioError, "%s [%s]", snd_strerror(res), device);
     }
-    
+
+    //Populate framesize and periodsize
+    snd_pcm_hw_params_current(self->handle, self->hwparams);
+    self->framesize = channels * snd_pcm_hw_params_get_sbits(self->hwparams)/8;
+    snd_pcm_hw_params_get_period_size(self->hwparams,&self->periodsize,0);
+
     if (res >= 0) {
         self->cardname = strdup(device);
     }

[larsimmisch]

From the alsa_lib documentation (ie code)

Well, I think I understand what you're getting at. Good catch - I never managed to reproduce the E_BUSY (I must admit I didn't try hard) and this is a somewhat subtle problem..

Maybe I'll find some time the week after next to put something like this in. If you manage to create a patch, that would be much appreciated.

[riverar]

Hi @larsimmisch @hauptmech, any further work in this area?

Seems like the fundamental design and API needs changing, or perhaps don't finalize hw params (snd_pcm_hw_params) until the first read()? Not sure if that's ideal.

[robmsmt]

I am having this same problem, I tried to implement the approach listed above however get the following:

alsaaudio.c: In function ‘alsapcm_new’:
alsaaudio.c:435:37: error: ‘alsapcm_t {aka struct <anonymous>}’ has no member named ‘hwparams’
      snd_pcm_hw_params_malloc(&(self->hwparams));

[hauptmech]

The diff above was not complete, only meant to communicate the idea. Attached are my files.
pyalsaaudio.zip

[larsimmisch]

In the end, I decided to make rate, channels, 'formatandperiodsizekeyword arguments to thePCMconstructor and deprecatesetchannels, setrate, setformatandsetperiodsize`. I hope that is the best way to move forward.

[ossilator]

pedantically, nothing prevents reconfiguring a prepared stream, so @hauptmech's point is technically invalid, though i suppose bugs might exist. also, it's not very efficient.

but one could also avoid the problem without deprecating the old api, at the cost of a behavior change: switch to lazy setup, invoking it only upon the first read/write/etc., or when requested explicitly via a new function.

anyway, what's done is done.