alsa-util: Add functions for accessing mixer elements through mixer class

Instead of using the hctl interface, we can find controls belonging
to other iface types than "mixer". We do this by introducing a new
mixer class "SND_MIXER_ELEM_PULSEAUDIO" and create snd_mixer_elem's
for all PCM and CARD iface types (as Jacks are of the CARD type and
ELD controls are of the PCM type).

Signed-off-by: David Henningsson <david.henningsson@canonical.com>

src/modules/alsa/alsa-util.c

@@ -1457,6 +1457,25 @@ bool pa_alsa_may_tsched(bool want) {
     return true;
 }
 
+#define SND_MIXER_ELEM_PULSEAUDIO (SND_MIXER_ELEM_LAST + 10)
+
+snd_mixer_elem_t *pa_alsa_mixer_find(snd_mixer_t *mixer, const char *name, unsigned int device) {
+    snd_mixer_elem_t *elem;
+
+    for (elem = snd_mixer_first_elem(mixer); elem; elem = snd_mixer_elem_next(elem)) {
+        snd_hctl_elem_t *helem;
+        if (snd_mixer_elem_get_type(elem) != SND_MIXER_ELEM_PULSEAUDIO)
+            continue;
+        helem = snd_mixer_elem_get_private(elem);
+        if (!pa_streq(snd_hctl_elem_get_name(helem), name))
+            continue;
+        if (snd_hctl_elem_get_device(helem) != device)
+            continue;
+        return elem;
+    }
+    return NULL;
+}
+
 snd_hctl_elem_t* pa_alsa_find_jack(snd_hctl_t *hctl, const char* jack_name) {
     snd_ctl_elem_id_t *id;
 
@@ -1481,8 +1500,53 @@ snd_hctl_elem_t* pa_alsa_find_eld_ctl(snd_hctl_t *hctl, int device) {
     return snd_hctl_find_elem(hctl, id);
 }
 
+static int mixer_class_compare(const snd_mixer_elem_t *c1, const snd_mixer_elem_t *c2)
+{
+    /* Dummy compare function */
+    return c1 == c2 ? 0 : (c1 > c2 ? 1 : -1);
+}
+
+static int mixer_class_event(snd_mixer_class_t *class, unsigned int mask,
+			snd_hctl_elem_t *helem, snd_mixer_elem_t *melem)
+{
+    int err;
+    const char *name = snd_hctl_elem_get_name(helem);
+    if (mask & SND_CTL_EVENT_MASK_ADD) {
+        snd_ctl_elem_iface_t iface = snd_hctl_elem_get_interface(helem);
+        if (iface == SND_CTL_ELEM_IFACE_CARD || iface == SND_CTL_ELEM_IFACE_PCM) {
+            snd_mixer_elem_t *new_melem;
+
+            /* Put the hctl pointer as our private data - it will be useful for callbacks */
+            if ((err = snd_mixer_elem_new(&new_melem, SND_MIXER_ELEM_PULSEAUDIO, 0, helem, NULL)) < 0) {
+                pa_log_warn("snd_mixer_elem_new failed: %s", pa_alsa_strerror(err));
+                return 0;
+            }
+
+            if ((err = snd_mixer_elem_attach(new_melem, helem)) < 0) {
+                pa_log_warn("snd_mixer_elem_attach failed: %s", pa_alsa_strerror(err));
+		snd_mixer_elem_free(melem);
+                return 0;
+            }
+
+            if ((err = snd_mixer_elem_add(new_melem, class)) < 0) {
+                pa_log_warn("snd_mixer_elem_add failed: %s", pa_alsa_strerror(err));
+                return 0;
+            }
+        }
+    }
+    else if (mask & SND_CTL_EVENT_MASK_VALUE) {
+        snd_mixer_elem_value(melem); /* Calls the element callback */
+        return 0;
+    }
+    else
+        pa_log_info("Got an unknown mixer class event for %s: mask 0x%x\n", name, mask);
+
+    return 0;
+}
+
 static int prepare_mixer(snd_mixer_t *mixer, const char *dev, snd_hctl_t **hctl) {
     int err;
+    snd_mixer_class_t *class;
 
     pa_assert(mixer);
     pa_assert(dev);
@@ -1499,6 +1563,19 @@ static int prepare_mixer(snd_mixer_t *mixer, const char *dev, snd_hctl_t **hctl)
         return -1;
     }
 
+    if (snd_mixer_class_malloc(&class)) {
+        pa_log_info("Failed to allocate mixer class for %s", dev);
+        return -1;
+    }
+    snd_mixer_class_set_event(class, mixer_class_event);
+    snd_mixer_class_set_compare(class, mixer_class_compare);
+    if ((err = snd_mixer_class_register(class, mixer)) < 0) {
+        pa_log_info("Unable register mixer class for %s: %s", dev, pa_alsa_strerror(err));
+        snd_mixer_class_free(class);
+        return -1;
+    }
+    /* From here on, the mixer class is deallocated by alsa on snd_mixer_close/free. */
+
     if ((err = snd_mixer_selem_register(mixer, NULL, NULL)) < 0) {
         pa_log_warn("Unable to register mixer: %s", pa_alsa_strerror(err));
         return -1;

src/modules/alsa/alsa-util.h

@@ -145,6 +145,8 @@ bool pa_alsa_may_tsched(bool want);
 snd_hctl_elem_t* pa_alsa_find_jack(snd_hctl_t *hctl, const char* jack_name);
 snd_hctl_elem_t* pa_alsa_find_eld_ctl(snd_hctl_t *hctl, int device);
 
+snd_mixer_elem_t *pa_alsa_mixer_find(snd_mixer_t *mixer, const char *name, unsigned int device);
+
 snd_mixer_t *pa_alsa_open_mixer(int alsa_card_index, char **ctl_device, snd_hctl_t **hctl);
 
 typedef struct pa_hdmi_eld pa_hdmi_eld;