Switched from setting the period size and count to setting the buffer size...

Switched from setting the period size and count to setting the buffer size directly, based on feedback from the ALSA development list:
http://mailman.alsa-project.org/pipermail/alsa-devel/2009-October/022267.html

This has the nice side effect of reducing latency on my SBLive! card.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%404080

src/audio/alsa/SDL_alsa_audio.c

@@ -46,6 +46,10 @@
 /* The default ALSA audio driver */
 #define DEFAULT_DEVICE	"default"
 
+/* Whether we should set the buffer size or the period size */
+/*#define SET_PERIOD_SIZE*/
+/*#define DEBUG_PERIOD_SIZE*/
+
 /* Audio driver functions */
 static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec);
 static void ALSA_WaitAudio(_THIS);
@@ -78,13 +82,13 @@ static int (*SDL_NAME(snd_pcm_hw_params_set_period_size_near))(snd_pcm_t *pcm, s
 static int (*SDL_NAME(snd_pcm_hw_params_get_period_size))(const snd_pcm_hw_params_t *params, snd_pcm_uframes_t *frames, int *dir);
 static int (*SDL_NAME(snd_pcm_hw_params_set_periods_near))(snd_pcm_t *pcm, snd_pcm_hw_params_t *params, unsigned int *val, int *dir);
 static int (*SDL_NAME(snd_pcm_hw_params_get_periods))(const snd_pcm_hw_params_t *params, unsigned int *val, int *dir);
+static int (*SDL_NAME(snd_pcm_hw_params_set_buffer_size_near))(snd_pcm_t *pcm, snd_pcm_hw_params_t *params, snd_pcm_uframes_t *val);
 static int (*SDL_NAME(snd_pcm_hw_params_get_buffer_size))(const snd_pcm_hw_params_t *params, snd_pcm_uframes_t *val);
 static int (*SDL_NAME(snd_pcm_hw_params))(snd_pcm_t *pcm, snd_pcm_hw_params_t *params);
 /*
 */
 static int (*SDL_NAME(snd_pcm_sw_params_current))(snd_pcm_t *pcm, snd_pcm_sw_params_t *swparams);
 static int (*SDL_NAME(snd_pcm_sw_params_set_start_threshold))(snd_pcm_t *pcm, snd_pcm_sw_params_t *params, snd_pcm_uframes_t val);
-static int (*SDL_NAME(snd_pcm_sw_params_set_avail_min))(snd_pcm_t *pcm, snd_pcm_sw_params_t *params, snd_pcm_uframes_t val);
 static int (*SDL_NAME(snd_pcm_sw_params))(snd_pcm_t *pcm, snd_pcm_sw_params_t *params);
 static int (*SDL_NAME(snd_pcm_nonblock))(snd_pcm_t *pcm, int nonblock);
 #define snd_pcm_hw_params_sizeof SDL_NAME(snd_pcm_hw_params_sizeof)
@@ -114,11 +118,11 @@ static struct {
 	{ "snd_pcm_hw_params_get_period_size",	(void**)(char*)&SDL_NAME(snd_pcm_hw_params_get_period_size)	},
 	{ "snd_pcm_hw_params_set_periods_near",	(void**)(char*)&SDL_NAME(snd_pcm_hw_params_set_periods_near)	},
 	{ "snd_pcm_hw_params_get_periods",	(void**)(char*)&SDL_NAME(snd_pcm_hw_params_get_periods)	},
+	{ "snd_pcm_hw_params_set_buffer_size_near",	(void**)(char*)&SDL_NAME(snd_pcm_hw_params_set_buffer_size_near) },
 	{ "snd_pcm_hw_params_get_buffer_size",	(void**)(char*)&SDL_NAME(snd_pcm_hw_params_get_buffer_size) },
 	{ "snd_pcm_hw_params",	(void**)(char*)&SDL_NAME(snd_pcm_hw_params)	},
 	{ "snd_pcm_sw_params_current",	(void**)(char*)&SDL_NAME(snd_pcm_sw_params_current)	},
 	{ "snd_pcm_sw_params_set_start_threshold",	(void**)(char*)&SDL_NAME(snd_pcm_sw_params_set_start_threshold)	},
-	{ "snd_pcm_sw_params_set_avail_min",	(void**)(char*)&SDL_NAME(snd_pcm_sw_params_set_avail_min)	},
 	{ "snd_pcm_sw_params",	(void**)(char*)&SDL_NAME(snd_pcm_sw_params)	},
 	{ "snd_pcm_nonblock",	(void**)(char*)&SDL_NAME(snd_pcm_nonblock)	},
 };
@@ -368,8 +372,10 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 	snd_pcm_format_t     format;
 	snd_pcm_uframes_t    frames;
 	unsigned int         rate;
+#ifdef SET_PERIOD_SIZE
 	unsigned int         periods;
-	unsigned int 		 channels;
+#endif
+	unsigned int 	     channels;
 	Uint16               test_format;
 
 	/* Open the audio device */
@@ -464,10 +470,11 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 	spec->freq = rate;
 
 	/* Set the buffer size, in samples */
+#ifdef SET_PERIOD_SIZE
 	frames = spec->samples;
 	status = SDL_NAME(snd_pcm_hw_params_set_period_size_near)(pcm_handle, hwparams, &frames, NULL);
 	if ( status < 0 ) {
-		SDL_SetError("Couldn't set audio frequency: %s", SDL_NAME(snd_strerror)(status));
+		SDL_SetError("Couldn't set period size: %s", SDL_NAME(snd_strerror)(status));
 		ALSA_CloseAudio(this);
 		return(-1);
 	}
@@ -475,7 +482,16 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 	spec->samples = frames;
 
 	periods = 2;
-	SDL_NAME(snd_pcm_hw_params_set_periods_near)(pcm_handle, hwparams, &periods, NULL);
+	status = SDL_NAME(snd_pcm_hw_params_set_periods_near)(pcm_handle, hwparams, &periods, NULL);
+	if ( status < 0 ) {
+		SDL_SetError("Couldn't set period count: %s", SDL_NAME(snd_strerror)(status));
+		ALSA_CloseAudio(this);
+		return(-1);
+	}
+#else
+	frames = spec->samples * 2;
+	status = SDL_NAME(snd_pcm_hw_params_set_buffer_size_near)(pcm_handle, hwparams, &frames);
+#endif
 
 	/* "set" the hardware with the desired parameters */
 	status = SDL_NAME(snd_pcm_hw_params)(pcm_handle, hwparams);
@@ -486,7 +502,7 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 	}
 
 /* This is useful for debugging... */
-#if 0
+#ifdef DEBUG_PERIOD_SIZE
 { snd_pcm_uframes_t bufsize; snd_pcm_sframes_t persize; unsigned int periods; int dir;
    SDL_NAME(snd_pcm_hw_params_get_buffer_size)(hwparams, &bufsize);
    SDL_NAME(snd_pcm_hw_params_get_period_size)(hwparams, &persize, &dir);
@@ -510,12 +526,6 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 		ALSA_CloseAudio(this);
 		return(-1);
 	}
-	status = SDL_NAME(snd_pcm_sw_params_set_avail_min)(pcm_handle, swparams, frames);
-	if ( status < 0 ) {
-		SDL_SetError("Couldn't set avail min: %s", SDL_NAME(snd_strerror)(status));
-		ALSA_CloseAudio(this);
-		return(-1);
-	}
 	status = SDL_NAME(snd_pcm_sw_params)(pcm_handle, swparams);
 	if ( status < 0 ) {
 		SDL_SetError("Couldn't set software audio parameters: %s", SDL_NAME(snd_strerror)(status));