spa: alsa: Read ctl events instead of doing a global diff

This does a couple of things: first, we implement revents demangling,
which seems to be required (although hw: devices work fine without it).

The second is to actually read the ctl events so we can tell when
elements we care about have changed, instead of reading everything and
trying to do a diff.

The latter is also required from a correctness perspective, as otherwise
the ctl might keep triggering wakeups while the fd is ready to be read.

spa/plugins/alsa/alsa-pcm.c

@@ -674,16 +674,53 @@ static void fill_card_name(struct state *state, const char *params, char *card_n
 
 static void bind_ctl_event(struct spa_source *source)
 {
-	// We don't know if a bound element changed or not, so let's find out
 	struct state *state = source->data;
+	snd_ctl_event_t *ev;
+	snd_ctl_elem_id_t *id, *bound_id;
 	snd_ctl_elem_value_t *old_value;
+	unsigned short revents;
+	int err;
+
+	// Do the same demangling of revents we do for PCM pollfds
+	for (int i = 0; i < state->ctl_n_fds; i++) {
+		state->ctl_pfds[i].revents = state->ctl_sources[i].rmask;
+		state->ctl_sources[i].rmask = 0;
+	}
+
+	err = snd_ctl_poll_descriptors_revents(state->ctl, state->ctl_pfds, state->ctl_n_fds, &revents);
+	if (SPA_UNLIKELY(err < 0)) {
+		spa_log_warn(state->log, "Could not read ctl revents: %s", snd_strerror(err));
+		return;
+	}
+
+	if (!revents) {
+		spa_log_trace(state->log, "Got a bind ctl wakeup but no actual event");
+		return;
+	}
+
+	snd_ctl_event_alloca(&ev);
+	snd_ctl_elem_id_alloca(&id);
+	snd_ctl_elem_id_alloca(&bound_id);
+	snd_ctl_elem_value_alloca(&old_value);
+
+	while ((err = snd_ctl_read(state->ctl, ev) > 0)) {
 		bool changed = false;
 
-	snd_ctl_elem_value_alloca(&old_value);
+		if (snd_ctl_event_get_type(ev) != SND_CTL_EVENT_ELEM)
+			continue;
+
+		snd_ctl_event_elem_get_id(ev, id);
 
 		for (unsigned int i = 0; i < state->num_bind_ctls; i++) {
 			int err;
 
+			// Check if we have the right element
+			snd_ctl_elem_value_get_id(state->bound_ctls[i].value, bound_id);
+			if (snd_ctl_elem_id_compare_set(id, bound_id) ||
+					snd_ctl_elem_id_compare_numid(id, bound_id)) {
+				continue;
+			}
+
 			snd_ctl_elem_value_copy(old_value, state->bound_ctls[i].value);
 
 			err = snd_ctl_elem_read(state->ctl, state->bound_ctls[i].value);
@@ -710,6 +747,10 @@ static void bind_ctl_event(struct spa_source *source)
 		}
 	}
 
+	if (err < 0 && err != -EAGAIN)
+		spa_log_warn(state->log, "Could not read ctl: %s", snd_strerror(err));
+}
+
 static void fetch_bind_ctls(struct state *state)
 {
 	snd_ctl_elem_list_t* element_list;
@@ -791,7 +832,6 @@ cleanup:
 
 static void bind_ctls_for_params(struct state *state)
 {
-	struct pollfd pfds[16];
 	int err;
 
 	if (state->num_bind_ctls == 0)
@@ -817,7 +857,7 @@ static void bind_ctls_for_params(struct state *state)
 		state->ctl_n_fds = SPA_N_ELEMENTS(state->ctl_sources);
 	}
 
-	if ((err = snd_ctl_poll_descriptors(state->ctl, pfds, state->ctl_n_fds)) < 0) {
+	if ((err = snd_ctl_poll_descriptors(state->ctl, state->ctl_pfds, state->ctl_n_fds)) < 0) {
 		spa_log_warn(state->log, "Could not get poll descriptors: %s", snd_strerror(err));
 		return;
 	}
@@ -827,7 +867,7 @@ static void bind_ctls_for_params(struct state *state)
 	for (int i = 0; i < state->ctl_n_fds; i++) {
 		state->ctl_sources[i].func = bind_ctl_event;
 		state->ctl_sources[i].data = state;
-		state->ctl_sources[i].fd = pfds[i].fd;
+		state->ctl_sources[i].fd = state->ctl_pfds[i].fd;
 		state->ctl_sources[i].mask = SPA_IO_IN;
 		state->ctl_sources[i].rmask = 0;
 		spa_loop_add_source(state->main_loop, &state->ctl_sources[i]);

spa/plugins/alsa/alsa-pcm.h

@@ -259,6 +259,7 @@ struct state {
 	/* ALSA ctls exposed as params */
 	unsigned int num_bind_ctls;
 	struct bound_ctl bound_ctls[16];
+	struct pollfd ctl_pfds[MAX_POLL];
 	struct spa_source ctl_sources[MAX_POLL];
 	int ctl_n_fds;