output_plugin: replaced method "control()" with "mixer()"

The output plugin shouldn't know any specifics of the mixer API.  Make
it return the mixer object, and let the caller deal with it.

src/audio.c

@@ -62,15 +62,17 @@ void finishAudioConfig(void)
 bool mixer_control_setvol(unsigned int device, int volume, int rel)
 {
 	struct audio_output *output;
+	struct mixer *mixer;
 
 	if (device >= audio_output_count())
 		return false;
 
 	output = audio_output_get(device);
-	if (output->plugin && output->plugin->control) {
+	mixer = ao_plugin_get_mixer(output->plugin, output->data);
+	if (mixer != NULL) {
 		if (rel) {
 			int cur_volume;
-			if (!output->plugin->control(output->data, AC_MIXER_GETVOL, &cur_volume)) {
+			if (!mixer_control(mixer, AC_MIXER_GETVOL, &cur_volume)) {
 				return false;
 			}
 			volume = volume + cur_volume;
@@ -80,7 +82,7 @@ bool mixer_control_setvol(unsigned int device, int volume, int rel)
 		else if (volume < 0)
 			volume = 0;
 
-		return output->plugin->control(output->data, AC_MIXER_SETVOL, &volume);
+		return mixer_control(mixer, AC_MIXER_SETVOL, &volume);
 	}
 	return false;
 }
@@ -88,13 +90,15 @@ bool mixer_control_setvol(unsigned int device, int volume, int rel)
 bool mixer_control_getvol(unsigned int device, int *volume)
 {
 	struct audio_output *output;
+	struct mixer *mixer;
 
 	if (device >= audio_output_count())
 		return false;
 
 	output = audio_output_get(device);
-	if (output->plugin && output->plugin->control) {
-		return output->plugin->control(output->data, AC_MIXER_GETVOL, volume);
-	}
+	mixer = ao_plugin_get_mixer(output->plugin, output->data);
+	if (mixer != NULL)
+		return mixer_control(mixer, AC_MIXER_GETVOL, volume);
+
 	return false;
 }

src/output/alsa_plugin.c

@@ -156,6 +156,14 @@ alsa_finish(void *data)
 	alsa_data_free(ad);
 }
 
+static struct mixer *
+alsa_get_mixer(void *data)
+{
+	struct alsa_data *ad = data;
+
+	return ad->mixer;
+}
+
 static bool
 alsa_test_default_device(void)
 {
@@ -465,21 +473,14 @@ alsa_play(void *data, const char *chunk, size_t size)
 	return true;
 }
 
-static bool
-alsa_control(void *data, int cmd, void *arg)
-{
-	struct alsa_data *ad = data;
-	return mixer_control(ad->mixer, cmd, arg);
-}
-
 const struct audio_output_plugin alsaPlugin = {
 	.name = "alsa",
 	.test_default_device = alsa_test_default_device,
 	.init = alsa_init,
 	.finish = alsa_finish,
+	.get_mixer = alsa_get_mixer,
 	.open = alsa_open,
 	.play = alsa_play,
 	.cancel = alsa_cancel,
 	.close = alsa_close,
-	.control = alsa_control
 };

src/output/oss_plugin.c

@@ -410,6 +410,14 @@ static void oss_finishDriver(void *data)
 	freeOssData(od);
 }
 
+static struct mixer *
+oss_get_mixer(void *data)
+{
+	OssData *od = data;
+
+	return od->mixer;
+}
+
 static int setParam(OssData * od, unsigned param, int *value)
 {
 	int val = *value;
@@ -571,21 +579,14 @@ oss_playAudio(void *data, const char *playChunk, size_t size)
 	return true;
 }
 
-static bool
-oss_control(void *data, int cmd, void *arg)
-{
-	OssData *od = data;
-	return mixer_control(od->mixer, cmd, arg);
-}
-
 const struct audio_output_plugin ossPlugin = {
 	.name = "oss",
 	.test_default_device = oss_testDefault,
 	.init = oss_initDriver,
 	.finish = oss_finishDriver,
+	.get_mixer = oss_get_mixer,
 	.open = oss_openDevice,
 	.play = oss_playAudio,
 	.cancel = oss_dropBufferedAudio,
 	.close = oss_closeDevice,
-	.control = oss_control,
 };

src/output_plugin.h

@@ -63,6 +63,15 @@ struct audio_output_plugin {
 	 */
 	void (*finish)(void *data);
 
+	/**
+	 * Returns the mixer device associated with this audio output.
+	 * This does not actually open the mixer device yet.
+	 *
+	 * @return the mixer object, or NULL if there is no mixer
+	 * attached to this audio output
+	 */
+	struct mixer *(*get_mixer)(void *data);
+
 	/**
 	 * Really open the device.
 	 * @param audio_format the audio format in which data is going
@@ -104,12 +113,6 @@ struct audio_output_plugin {
 	 * for continue to pause
 	 */
 	bool (*pause)(void *data);
-
-	/**
-	 * Control the device. Usualy used for implementing
-	 * set and get mixer levels
-	 */
-	bool (*control)(void *data, int cmd, void *arg);
 };
 
 static inline bool
@@ -135,6 +138,14 @@ ao_plugin_finish(const struct audio_output_plugin *plugin, void *data)
 	plugin->finish(data);
 }
 
+static inline struct mixer *
+ao_plugin_get_mixer(const struct audio_output_plugin *plugin, void *data)
+{
+	return plugin->get_mixer != NULL
+		? plugin->get_mixer(data)
+		: NULL;
+}
+
 static inline bool
 ao_plugin_open(const struct audio_output_plugin *plugin,
 	       void *data, struct audio_format *audio_format)