Add config knob for mixer channel name (ALSA/Linux only)

In some complex ALSA setups, using PCM or Master is not the right thing
to do, so allow a custom mixer element name to be specified in the config
file for this use case.

Request and initial patch by Kurt Vanderlinden <kurt.vanderlinden@skynet.be>.

forked-daapd.conf

@@ -45,6 +45,9 @@ audio {
 	nickname = "Computer"
 	# Audio device name for local audio output
 #	card = "default"
+	# Mixer channel to use for volume control - ALSA/Linux only
+	# If not set, PCM will be used if available, otherwise Master.
+#	mixer = ""
 }
 
 # Airport Express device

src/conffile.c

@@ -76,6 +76,7 @@ static cfg_opt_t sec_audio[] =
 #elif defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
     CFG_STR("card", "/dev/dsp", CFGF_NONE),
 #endif
+    CFG_STR("mixer", NULL, CFGF_NONE),
     CFG_END()
   };
 

src/laudio_alsa.c

@@ -57,6 +57,7 @@ static struct pcm_packet *pcm_pkt_head;
 static struct pcm_packet *pcm_pkt_tail;
 
 static char *card_name;
+static char *mixer_name;
 static snd_pcm_t *hdl;
 static snd_mixer_t *mixer_hdl;
 static snd_mixer_elem_t *vol_elem;
@@ -412,6 +413,7 @@ mixer_open(void)
   snd_mixer_elem_t *elem;
   snd_mixer_elem_t *master;
   snd_mixer_elem_t *pcm;
+  snd_mixer_elem_t *custom;
   snd_mixer_selem_id_t *sid;
   int ret;
 
@@ -453,17 +455,34 @@ mixer_open(void)
 
   pcm = NULL;
   master = NULL;
+  custom = NULL;
   for (elem = snd_mixer_first_elem(mixer_hdl); elem; elem = snd_mixer_elem_next(elem))
     {
       snd_mixer_selem_get_id(elem, sid);
 
-      if (strcmp(snd_mixer_selem_id_get_name(sid), "PCM") == 0)
+      if (mixer_name && (strcmp(snd_mixer_selem_id_get_name(sid), mixer_name) == 0))
+	{
+	  custom = elem;
+	  break;
+	}
+      else if (strcmp(snd_mixer_selem_id_get_name(sid), "PCM") == 0)
         pcm = elem;
       else if (strcmp(snd_mixer_selem_id_get_name(sid), "Master") == 0)
 	master = elem;
     }
 
-  if (pcm)
+  if (mixer_name)
+    {
+      if (custom)
+	vol_elem = custom;
+      else
+	{
+	  DPRINTF(E_LOG, L_LAUDIO, "Failed to open configured mixer element '%s'\n", mixer_name);
+
+	  goto out_detach;
+	}
+    }
+  else if (pcm)
     vol_elem = pcm;
   else if (master)
     vol_elem = master;
@@ -652,6 +671,7 @@ laudio_init(laudio_status_cb cb)
   status_cb = cb;
 
   card_name = cfg_getstr(cfg_getsec(cfg, "audio"), "card");
+  mixer_name = cfg_getstr(cfg_getsec(cfg, "audio"), "mixer");
 
   hdl = NULL;
   mixer_hdl = NULL;