Fixed bug 4710 - audio/alsa: avoid configuring hardware parameters with only a single period

Anthony Pesch

The previous code first configured the period size using snd_pcm_hw_par-
ams_set_period_size_near. Then, it further narrowed the configuration
space by calling snd_pcm_hw_params_set_buffer_size_near using a buffer
size of 2 times the _requested_ period size in order to try and get a
configuration with only 2 periods. If the configured period size was
larger than the requested size, the second call could inadvertently
narrow the configuration space to contain only a single period.

Rather than fixing the call to snd_pcm_hw_params_set_buffer_size_near
to use a size of 2 times the configured period size, the code has been
changed to use snd_pcm_hw_params_set_periods_min in order to more
clearly explain the intent.

src/audio/alsa/SDL_alsa_audio.c

@@ -72,7 +72,9 @@ static int (*ALSA_snd_pcm_hw_params_set_period_size_near)
   (snd_pcm_t *, snd_pcm_hw_params_t *, snd_pcm_uframes_t *, int *);
 static int (*ALSA_snd_pcm_hw_params_get_period_size)
   (const snd_pcm_hw_params_t *, snd_pcm_uframes_t *, int *);
-static int (*ALSA_snd_pcm_hw_params_set_periods_near)
+static int (*ALSA_snd_pcm_hw_params_set_periods_min)
+  (snd_pcm_t *, snd_pcm_hw_params_t *, unsigned int *, int *);
+static int (*ALSA_snd_pcm_hw_params_set_periods_first)
   (snd_pcm_t *, snd_pcm_hw_params_t *, unsigned int *, int *);
 static int (*ALSA_snd_pcm_hw_params_get_periods)
   (const snd_pcm_hw_params_t *, unsigned int *, int *);
@@ -148,7 +150,8 @@ load_alsa_syms(void)
     SDL_ALSA_SYM(snd_pcm_hw_params_set_rate_near);
     SDL_ALSA_SYM(snd_pcm_hw_params_set_period_size_near);
     SDL_ALSA_SYM(snd_pcm_hw_params_get_period_size);
-    SDL_ALSA_SYM(snd_pcm_hw_params_set_periods_near);
+    SDL_ALSA_SYM(snd_pcm_hw_params_set_periods_min);
+    SDL_ALSA_SYM(snd_pcm_hw_params_set_periods_first);
     SDL_ALSA_SYM(snd_pcm_hw_params_get_periods);
     SDL_ALSA_SYM(snd_pcm_hw_params_set_buffer_size_near);
     SDL_ALSA_SYM(snd_pcm_hw_params_get_buffer_size);
@@ -462,14 +465,14 @@ ALSA_set_buffer_size(_THIS, snd_pcm_hw_params_t *params)
 {
     int status;
     snd_pcm_hw_params_t *hwparams;
-    snd_pcm_uframes_t bufsize;
     snd_pcm_uframes_t persize;
+    unsigned int periods;
 
     /* Copy the hardware parameters for this setup */
     snd_pcm_hw_params_alloca(&hwparams);
     ALSA_snd_pcm_hw_params_copy(hwparams, params);
 
-    /* Prioritize matching the period size to the requested buffer size */
+    /* Attempt to match the period size to the requested buffer size */
     persize = this->spec.samples;
     status = ALSA_snd_pcm_hw_params_set_period_size_near(
                 this->hidden->pcm_handle, hwparams, &persize, NULL);
@@ -477,10 +480,16 @@ ALSA_set_buffer_size(_THIS, snd_pcm_hw_params_t *params)
         return(-1);
     }
 
-    /* Next try to restrict the parameters to having only two periods */
-    bufsize = this->spec.samples * 2;
-    status = ALSA_snd_pcm_hw_params_set_buffer_size_near(
-                    this->hidden->pcm_handle, hwparams, &bufsize);
+    /* Need to at least double buffer */
+    periods = 2;
+    status = ALSA_snd_pcm_hw_params_set_periods_min(
+                this->hidden->pcm_handle, hwparams, &periods, NULL);
+    if ( status < 0 ) {
+        return(-1);
+    }
+
+    status = ALSA_snd_pcm_hw_params_set_periods_first(
+                this->hidden->pcm_handle, hwparams, &periods, NULL);
     if ( status < 0 ) {
         return(-1);
     }
@@ -495,9 +504,9 @@ ALSA_set_buffer_size(_THIS, snd_pcm_hw_params_t *params)
 
     /* This is useful for debugging */
     if ( SDL_getenv("SDL_AUDIO_ALSA_DEBUG") ) {
-        unsigned int periods = 0;
+        snd_pcm_uframes_t bufsize;
 
-        ALSA_snd_pcm_hw_params_get_periods(hwparams, &periods, NULL);
+        ALSA_snd_pcm_hw_params_get_buffer_size(hwparams, &bufsize);
 
         fprintf(stderr,
             "ALSA: period size = %ld, periods = %u, buffer size = %lu\n",