alsa: add api.alsa.headroom setting

Can be used to tweak the extra distance kept between the hardware
and software pointers when reading and writing.

See #289

spa/plugins/alsa/alsa-pcm-sink.c

@@ -795,6 +795,8 @@ impl_init(const struct spa_handle_factory *factory,
 			}
 		} else if (!strcmp(info->items[i].key, "api.alsa.period-size")) {
 			this->default_period_size = atoi(info->items[i].value);
+		} else if (!strcmp(info->items[i].key, "api.alsa.headroom")) {
+			this->default_headroom = atoi(info->items[i].value);
 		}
 	}
 	return 0;

spa/plugins/alsa/alsa-pcm-source.c

@@ -816,6 +816,8 @@ impl_init(const struct spa_handle_factory *factory,
 			}
 		} else if (!strcmp(info->items[i].key, "api.alsa.period-size")) {
 			this->default_period_size = atoi(info->items[i].value);
+		} else if (!strcmp(info->items[i].key, "api.alsa.headroom")) {
+			this->default_headroom = atoi(info->items[i].value);
 		}
 	}
 	return 0;

spa/plugins/alsa/alsa-pcm.c

@@ -605,8 +605,12 @@ int spa_alsa_set_format(struct state *state, struct spa_audio_info *fmt, uint32_
 	CHECK(snd_pcm_hw_params_get_buffer_size_max(params, &state->buffer_frames), "get_buffer_size_max");
 	CHECK(snd_pcm_hw_params_set_buffer_size_near(hndl, params, &state->buffer_frames), "set_buffer_size_near");
 
+	if (state->default_headroom == 0)
+		state->headroom = is_batch ? period_size : 0;
+	else
+		state->headroom = state->default_headroom;
+
 	state->period_frames = period_size;
-	state->headroom = is_batch ? period_size : 0;
 	periods = state->buffer_frames / state->period_frames;
 
 	spa_log_info(state->log, NAME" %s (%s): format:%s access:%s-%s rate:%d channels:%d "

spa/plugins/alsa/alsa-pcm.h

@@ -111,6 +111,7 @@ struct state {
 	struct spa_audio_info current_format;
 
 	uint32_t default_period_size;
+	uint32_t default_headroom;
 	uint32_t default_format;
 	unsigned int default_channels;
 	unsigned int default_rate;

src/daemon/media-session.d/alsa-monitor.conf

@@ -50,9 +50,12 @@ rules = [
                 #resample.quality = 		4
                 #channelmix.normalize =		false
                 #channelmix.mix-lfe = 		false
+                #audio.channels = 		2
                 #audio.format = 		"S16LE"
                 #audio.rate = 			44100
                 #audio.position = 		"FL,FR"
+                #api.alsa.period-size =         1024
+                #api.alsa.headroom =            0
             }
         }
     }