replay_gain: optionally use hardware mixer to apply replay gain

Add an option for each audio output which enables the use of the
hardware mixer, instead of the software volume code.

This is hardware specific, and assumes linear volume control.  This is
not the case for hardware mixers which were tested, making this patch
somewhat useless, but we will use it to experiment with the settings,
to find a good solution.

NEWS

@@ -73,6 +73,7 @@ ver 0.16 (20??/??/??)
 * replay gain:
   - reimplemented as a filter plugin
   - fall back to track gain if album gain is unavailable
+  - optionally use hardware mixer to apply replay gain
 * log unused/unknown block parameters
 * removed the deprecated "error_file" option
 * save state when stopped

doc/mpd.conf.5

@@ -270,6 +270,12 @@ audio that is sent to the audio output device.  See documentation for the
 whatever audio format is passed to the audio output.
 Any of the three attributes may be an asterisk to specify that this
 attribute should not be enforced
+.TP
+.B replay_gain_handler <software, mixer or none>
+Specifies how replay gain is applied.  The default is "software",
+which uses an internal software volume control.  "mixer" uses the
+configured (hardware) mixer control.  "none" disables replay gain on
+this audio output.
 .SH OPTIONAL ALSA OUTPUT PARAMETERS
 .TP
 .B device <dev>

doc/user.xml

@@ -350,6 +350,19 @@ cd mpd-version</programlisting>
                 devices which support it, and none for the others.
               </entry>
             </row>
+            <row>
+              <entry>
+                <varname>replay_gain_handler</varname>
+                <parameter>software|mixer|none</parameter>
+              </entry>
+              <entry>
+                Specifies how replay gain is applied.  The default is
+                "software", which uses an internal software volume
+                control.  "mixer" uses the configured (hardware) mixer
+                control.  "none" disables replay gain on this audio
+                output.
+              </entry>
+            </row>
           </tbody>
         </tgroup>
       </informaltable>

src/filter/replay_gain_filter_plugin.c

@@ -27,7 +27,9 @@
 #include "pcm_volume.h"
 #include "replay_gain_info.h"
 #include "replay_gain_config.h"
+#include "mixer_control.h"
 
+#include <assert.h>
 #include <string.h>
 
 #undef G_LOG_DOMAIN
@@ -36,6 +38,18 @@
 struct replay_gain_filter {
 	struct filter filter;
 
+	/**
+	 * If set, then this hardware mixer is used for applying
+	 * replay gain, instead of the software volume library.
+	 */
+	struct mixer *mixer;
+
+	/**
+	 * The base volume level for scale=1.0, between 1 and 100
+	 * (including).
+	 */
+	unsigned base;
+
 	enum replay_gain_mode mode;
 
 	struct replay_gain_info info;
@@ -74,6 +88,21 @@ replay_gain_filter_update(struct replay_gain_filter *filter)
 		filter->volume = pcm_float_to_volume(scale);
 	} else
 		filter->volume = PCM_VOLUME_1;
+
+	if (filter->mixer != NULL) {
+		/* update the hardware mixer volume */
+
+		unsigned volume = (filter->volume * filter->base) / PCM_VOLUME_1;
+		if (volume > 100)
+			volume = 100;
+
+		GError *error = NULL;
+		if (!mixer_set_volume(filter->mixer, volume, &error)) {
+			g_warning("Failed to update hardware mixer: %s",
+				  error->message);
+			g_error_free(error);
+		}
+	}
 }
 
 static struct filter *
@@ -83,6 +112,7 @@ replay_gain_filter_init(G_GNUC_UNUSED const struct config_param *param,
 	struct replay_gain_filter *filter = g_new(struct replay_gain_filter, 1);
 
 	filter_init(&filter->filter, &replay_gain_filter_plugin);
+	filter->mixer = NULL;
 
 	filter->mode = replay_gain_mode;
 	replay_gain_info_init(&filter->info);
@@ -177,6 +207,21 @@ const struct filter_plugin replay_gain_filter_plugin = {
 	.filter = replay_gain_filter_filter,
 };
 
+void
+replay_gain_filter_set_mixer(struct filter *_filter, struct mixer *mixer,
+			     unsigned base)
+{
+	struct replay_gain_filter *filter =
+		(struct replay_gain_filter *)_filter;
+
+	assert(mixer == NULL || (base > 0 && base <= 100));
+
+	filter->mixer = mixer;
+	filter->base = base;
+
+	replay_gain_filter_update(filter);
+}
+
 void
 replay_gain_filter_set_info(struct filter *_filter,
 			    const struct replay_gain_info *info)

src/filter/replay_gain_filter_plugin.h

@@ -23,6 +23,19 @@
 #include "replay_gain_info.h"
 
 struct filter;
+struct mixer;
+
+/**
+ * Enables or disables the hardware mixer for applying replay gain.
+ *
+ * @param mixer the hardware mixer, or NULL to fall back to software
+ * volume
+ * @param base the base volume level for scale=1.0, between 1 and 100
+ * (including).
+ */
+void
+replay_gain_filter_set_mixer(struct filter *_filter, struct mixer *mixer,
+			     unsigned base);
 
 /**
  * Sets a new #replay_gain_info at the beginning of a new song.

src/output_init.c

@@ -32,6 +32,7 @@
 #include "filter_config.h"
 #include "filter/chain_filter_plugin.h"
 #include "filter/autoconvert_filter_plugin.h"
+#include "filter/replay_gain_filter_plugin.h"
 
 #include <glib.h>
 
@@ -196,12 +197,19 @@ audio_output_init(struct audio_output *ao, const struct config_param *param,
 
 	/* create the replay_gain filter */
 
-	ao->replay_gain_filter = filter_new(&replay_gain_filter_plugin,
-					    param, NULL);
-	assert(ao->replay_gain_filter != NULL);
+	const char *replay_gain_handler =
+		config_get_block_string(param, "replay_gain_handler",
+					"software");
 
-	filter_chain_append(ao->filter, ao->replay_gain_filter);
-	ao->replay_gain_serial = 0;
+	if (strcmp(replay_gain_handler, "none") != 0) {
+		ao->replay_gain_filter = filter_new(&replay_gain_filter_plugin,
+						    param, NULL);
+		assert(ao->replay_gain_filter != NULL);
+
+		filter_chain_append(ao->filter, ao->replay_gain_filter);
+		ao->replay_gain_serial = 0;
+	} else
+		ao->replay_gain_filter = NULL;
 
 	/* create the normalization filter (if configured) */
 
@@ -247,6 +255,21 @@ audio_output_init(struct audio_output *ao, const struct config_param *param,
 		g_error_free(error);
 	}
 
+	/* use the hardware mixer for replay gain? */
+
+	if (strcmp(replay_gain_handler, "mixer") == 0) {
+		if (ao->mixer != NULL)
+			replay_gain_filter_set_mixer(ao->replay_gain_filter,
+						     ao->mixer, 100);
+		else
+			g_warning("No such mixer for output '%s'", ao->name);
+	} else if (strcmp(replay_gain_handler, "software") != 0 &&
+		   ao->replay_gain_filter != NULL) {
+		g_set_error(error_r, audio_output_quark(), 0,
+			    "Invalid \"replay_gain_handler\" value");
+		return false;
+	}
+
 	/* the "convert" filter must be the last one in the chain */
 
 	ao->convert_filter = filter_new(&convert_filter_plugin, NULL, NULL);

src/output_thread.c

@@ -264,7 +264,8 @@ ao_play_chunk(struct audio_output *ao, const struct music_chunk *chunk)
 
 	/* update replay gain */
 
-	if (chunk->replay_gain_serial != ao->replay_gain_serial) {
+	if (ao->replay_gain_filter != NULL &&
+	    chunk->replay_gain_serial != ao->replay_gain_serial) {
 		replay_gain_filter_set_info(ao->replay_gain_filter,
 					    chunk->replay_gain_serial != 0
 					    ? &chunk->replay_gain_info

test/run_filter.c

@@ -24,6 +24,7 @@
 #include "filter_plugin.h"
 #include "pcm_volume.h"
 #include "idle.h"
+#include "mixer_control.h"
 
 #include <glib.h>
 
@@ -37,6 +38,13 @@ idle_add(G_GNUC_UNUSED unsigned flags)
 {
 }
 
+bool
+mixer_set_volume(G_GNUC_UNUSED struct mixer *mixer,
+		 G_GNUC_UNUSED unsigned volume, G_GNUC_UNUSED GError **error_r)
+{
+	return true;
+}
+
 static void
 my_log_func(const gchar *log_domain, G_GNUC_UNUSED GLogLevelFlags log_level,
 	    const gchar *message, G_GNUC_UNUSED gpointer user_data)