alsa-pcm: add support for api.alsa.dll-bandwidth-max

In USB Audio Class 2 (UAC2) setups, pitch control is handled by
feedback endpoints. The host adjusts its data rate accordingly.

When pitch control is active (pitch_elem), applying the default
delay-locked loop (DLL) bandwidth can lead to instability and
oscillations around the target rate.

This patch adds a new parameter, api.alsa.dll-bandwidth-max, to
configure the maximum DLL bandwidth. It introduces a new field
in the ALSA state to store this value.

By default, it uses SPA_DLL_BW_MAX, but when pitch control is in
use, setting it to a lower value (e.g. 0.02) helps ensure better
stability, based on empirical testing.

doc/dox/config/pipewire-props.7.md

@@ -778,6 +778,12 @@ Setting this to 0 makes htimestamp never get disabled.
 Disable timer-based scheduling, and use IRQ for scheduling instead.
 The "Pro Audio" profile will usually enable this setting, if it is expected it works on the hardware.
 
+@PAR@ node-prop  api.alsa.dll-bandwidth-max    # double
+Sets the maximum bandwidth of the DLL (delay-locked loop) filter used to smooth out rate adjustments.
+The default value may be too responsive in some scenarios.
+For example, with UAC2 pitch control, the host reacts more slowly compared to local resampling,
+so using a lower bandwidth helps avoid oscillations or instability.
+
 @PAR@ node-prop  api.alsa.auto-link = false    # boolean
 Link follower PCM devices to the driver PCM device when using IRQ-based scheduling.
 The "Pro Audio" profile will usually enable this setting, if it is expected it works on the hardware.

spa/plugins/alsa/alsa-pcm.c

@@ -20,6 +20,7 @@
 
 static struct spa_list cards = SPA_LIST_INIT(&cards);
 static struct spa_list states = SPA_LIST_INIT(&states);
+#define SPA_ALSA_DLL_BW_MIN 0.001
 
 static struct card *find_card(uint32_t index)
 {
@@ -198,6 +199,9 @@ static int alsa_set_param(struct state *state, const char *k, const char *s)
 		state->disable_batch = spa_atob(s);
 	} else if (spa_streq(k, "api.alsa.disable-tsched")) {
 		state->disable_tsched = spa_atob(s);
+	} else if (spa_streq(k, "api.alsa.dll-bandwidth-max")) {
+		state->dll_bw_max = SPA_CLAMPD(spa_strtod(s, NULL),
+					       SPA_ALSA_DLL_BW_MIN, SPA_DLL_BW_MAX);
 	} else if (spa_streq(k, "api.alsa.use-chmap")) {
 		state->props.use_chmap = spa_atob(s);
 	} else if (spa_streq(k, "api.alsa.multi-rate")) {
@@ -2799,7 +2803,7 @@ static int update_time(struct state *state, uint64_t current_time, snd_pcm_sfram
 	int32_t diff;
 
 	if (SPA_UNLIKELY(state->dll.bw == 0.0)) {
-		spa_dll_set_bw(&state->dll, SPA_DLL_BW_MAX, state->threshold, state->rate);
+		spa_dll_set_bw(&state->dll, state->dll_bw_max, state->threshold, state->rate);
 		state->next_time = current_time;
 		state->base_time = current_time;
 	}
@@ -2862,7 +2866,7 @@ static int update_time(struct state *state, uint64_t current_time, snd_pcm_sfram
 				err, state->max_error, state->max_resync, state->err_avg, state->err_var, bw);
 
 		spa_dll_set_bw(&state->dll,
-				SPA_CLAMPD(bw, 0.001, SPA_DLL_BW_MAX),
+				SPA_CLAMPD(bw, SPA_ALSA_DLL_BW_MIN, state->dll_bw_max),
 				state->threshold, state->rate);
 	}
 

spa/plugins/alsa/alsa-pcm.h

@@ -244,6 +244,7 @@ struct state {
 	uint64_t underrun;
 
 	struct spa_dll dll;
+	double dll_bw_max;
 	double max_error;
 	double max_resync;
 	double err_avg, err_var, err_wdw;