Change ALSA buffer size and period size

With some hardware, the previously used value (200ms/50ms) led to buffer underruns. We increase the buffer size to 0.5s and the period to be 1/16th. This push the minimum amount of allocated RAM to ALSA hardware buffer to 128kB (default is 64kB). This will lead to extra verbosity in the log, but if it was working with 200ms then those warnings/errors can be ignored

mythtv/libs/libmyth/audiooutputalsa.cpp

@@ -423,8 +423,8 @@ bool AudioOutputALSA::OpenDevice()
             return false;
     }
 
-    period_time = 50000;  // aim for an interrupt every 50ms
-    buffer_time = period_time << 2; // buffer 200ms worth of samples
+    buffer_time = 500000; // buffer 0.5s worth of samples
+    period_time = 16;     // aim for an interrupt every (1/16th of buffer_time)
 
     err = SetParameters(pcm_handle, format, channels, samplerate,
                         buffer_time, period_time);
@@ -617,11 +617,11 @@ int AudioOutputALSA::SetParameters(snd_pcm_t *handle, snd_pcm_format_t format,
                                    uint channels, uint rate, uint buffer_time,
                                    uint period_time)
 {
-    int err, dir;
-    snd_pcm_hw_params_t *params;
-    snd_pcm_sw_params_t *swparams;
-    snd_pcm_uframes_t buffer_size;
-    snd_pcm_uframes_t period_size;
+    int err;
+    snd_pcm_hw_params_t  *params;
+    snd_pcm_sw_params_t  *swparams;
+    snd_pcm_uframes_t     period_size, period_size_min, period_size_max;
+    snd_pcm_uframes_t     buffer_size, buffer_size_min, buffer_size_max;
 
     VBAUDIO(QString("SetParameters(format=%1, channels=%2, rate=%3, "
                     "buffer_time=%4, period_time=%5)")
@@ -678,14 +678,25 @@ int AudioOutputALSA::SetParameters(snd_pcm_t *handle, snd_pcm_format_t format,
     }
 
     /* set the buffer time */
-    dir = 0;
+    err = snd_pcm_hw_params_get_buffer_size_min(params, &buffer_size_min);
+    err = snd_pcm_hw_params_get_buffer_size_max(params, &buffer_size_max);
+    err = snd_pcm_hw_params_get_period_size_min(params, &period_size_min, NULL);
+    err = snd_pcm_hw_params_get_period_size_max(params, &period_size_max, NULL);
+    VBAUDIO(QString("Buffer size range from %1 to %2")
+            .arg(buffer_size_min)
+            .arg(buffer_size_max));
+    VBAUDIO(QString("Period size range from %1 to %2")
+            .arg(period_size_min)
+            .arg(period_size_max));
+
+    /* set the buffer time */
     uint original_buffer_time = buffer_time;
     err = snd_pcm_hw_params_set_buffer_time_near(handle, params,
-                                                 &buffer_time, &dir);
+                                                 &buffer_time, NULL);
     CHECKERR(QString("Unable to set buffer time %1").arg(buffer_time));
 
     /* See if we need to increase the prealloc'd buffer size
-      If buffer_time is too small we could underrun - make 10% difference ok */
+       If buffer_time is too small we could underrun - make 10% difference ok */
     if ((buffer_time * 1.10f < (float)original_buffer_time) && pbufsize < 0)
     {
         VBAUDIO(QString("Requested %1us got %2 buffer time")
@@ -696,11 +707,10 @@ int AudioOutputALSA::SetParameters(snd_pcm_t *handle, snd_pcm_format_t format,
     VBAUDIO(QString("Buffer time = %1 us").arg(buffer_time));
 
     /* set the period time */
-    dir = 1;
-    err = snd_pcm_hw_params_set_period_time_near(handle, params,
-                                                 &period_time, &dir);
+    err = snd_pcm_hw_params_set_periods_near(handle, params,
+                                             &period_time, NULL);
     CHECKERR(QString("Unable to set period time %1").arg(period_time));
-    VBAUDIO(QString("Period time = %1 us").arg(period_time));
+    VBAUDIO(QString("Period time = %1 periods").arg(period_time));
 
     /* write the parameters to device */
     err = snd_pcm_hw_params(handle, params);
@@ -720,7 +730,7 @@ int AudioOutputALSA::SetParameters(snd_pcm_t *handle, snd_pcm_format_t format,
     CHECKERR("Unable to get current swparams");
 
     /* start the transfer after period_size */
-    err = snd_pcm_sw_params_set_start_threshold(handle, swparams, buffer_size);
+    err = snd_pcm_sw_params_set_start_threshold(handle, swparams, period_size);
     CHECKERR("Unable to set start threshold");
 
     /* allow the transfer when at least period_size samples can be processed */