settings: add link.min-buffers option

Add link.min-buffers option to set the minimum amount of buffers to
create for links. Set this by default to 2 because we really need two
now in case the sink xruns and does async mixing.

Make this an option because in low-memory cases and when xruns are not
expected, we can set this to 1.

doc/dox/config/pipewire.conf.5.md

@@ -217,6 +217,10 @@ Default video rate denominator
 @PAR@ pipewire.conf  library.name.system = support/libspa-support
 The name of the shared library to use for the system functions for the main thread.
 
+@PAR@ pipewire.conf  link.min-buffers = 2
+The minimum number of buffers to negotiate between nodes. Using 1 buffer will consume
+less memory but might cause glitches when using async nodes.
+
 @PAR@ pipewire.conf  link.max-buffers = 64
 The maximum number of buffers to negotiate between nodes. Note that version < 3 clients
 can only support 16 buffers. More buffers is almost always worse than less, latency

src/daemon/minimal.conf.in

@@ -13,6 +13,7 @@ context.properties = {
     #library.name.system                   = support/libspa-support
     #context.data-loop.library.name.system = support/libspa-support
     #support.dbus                          = true
+    #link.min-buffers                      = 2
     #link.max-buffers                      = 64
     link.max-buffers                       = 16                       # version < 3 clients can't handle more
     #mem.warn-mlock                        = false

src/daemon/pipewire-vulkan.conf.in

@@ -9,8 +9,6 @@ context.properties = {
     #library.name.system                   = support/libspa-support
     #context.data-loop.library.name.system = support/libspa-support
     #support.dbus                          = true
-    #link.max-buffers                      = 64
-    #link.max-buffers                       = 16                       # version < 3 clients can't handle more
     #mem.warn-mlock                        = false
     #mem.allow-mlock                       = true
     #mem.mlock-all                         = false

src/daemon/pipewire.conf.in

@@ -13,6 +13,7 @@ context.properties = {
     #library.name.system                   = support/libspa-support
     #context.data-loop.library.name.system = support/libspa-support
     #support.dbus                          = true
+    #link.min-buffers                      = 2
     #link.max-buffers                      = 64
     link.max-buffers                       = 16                       # version < 3 clients can't handle more
     #mem.warn-mlock                        = false

src/pipewire/buffers.c

@@ -98,8 +98,8 @@ static int alloc_buffers(struct pw_mempool *pool,
 		data = NULL;
 	}
 
-	pw_log_debug("%p: layout buffers skel:%p data:%p buffers:%p",
+	pw_log_debug("%p: layout buffers skel:%p data:%p n_buffers:%d buffers:%p",
-			allocation, skel, data, buffers);
+			allocation, skel, data, n_buffers, buffers);
 	spa_buffer_alloc_layout_array(&info, n_buffers, buffers, skel, data);
 
 	allocation->mem = m;
@@ -295,9 +295,6 @@ int pw_buffers_negotiate(struct pw_context *context, uint32_t flags,
 		align = SPA_MAX(align, qalign);
 		types = qtypes;
 
-		if (SPA_FLAG_IS_SET(flags, PW_BUFFERS_FLAG_ASYNC))
-			max_buffers = SPA_MAX(2u, max_buffers);
-
 		pw_log_debug("%p: %d %d %d %d %d %d -> %d %zd %zd %d %zd %d", result,
 				qblocks, qminsize, qstride, qmax_buffers, qalign, qtypes,
 				blocks, minsize, stride, max_buffers, align, types);
@@ -306,9 +303,13 @@ int pw_buffers_negotiate(struct pw_context *context, uint32_t flags,
 	if (i == n_params) {
 		pw_log_warn("%p: no buffers param", result);
 		minsize = context->settings.clock_quantum_limit;
-		max_buffers = 2;
+		max_buffers = 2u;
 	}
 
+	if (SPA_FLAG_IS_SET(flags, PW_BUFFERS_FLAG_ASYNC))
+		max_buffers = SPA_MAX(2u, max_buffers);
+	max_buffers = SPA_MAX(context->settings.link_min_buffers, max_buffers);
+
 	if (SPA_FLAG_IS_SET(flags, PW_BUFFERS_FLAG_SHARED_MEM)) {
 		if (types != SPA_ID_INVALID)
 			SPA_FLAG_CLEAR(types, 1<<SPA_DATA_MemPtr);

src/pipewire/private.h

@@ -46,6 +46,7 @@ struct settings {
 	uint32_t clock_quantum_floor;		/* quantum floor (lower bound) */
 	struct spa_rectangle video_size;
 	struct spa_fraction video_rate;
+	uint32_t link_min_buffers;
 	uint32_t link_max_buffers;
 	unsigned int mem_warn_mlock:1;
 	unsigned int mem_allow_mlock:1;

src/pipewire/settings.c

@@ -32,6 +32,7 @@
 #define DEFAULT_VIDEO_HEIGHT			480
 #define DEFAULT_VIDEO_RATE_NUM			25u
 #define DEFAULT_VIDEO_RATE_DENOM		1u
+#define DEFAULT_LINK_MIN_BUFFERS		2u
 #define DEFAULT_LINK_MAX_BUFFERS		64u
 #define DEFAULT_MEM_WARN_MLOCK			false
 #define DEFAULT_MEM_ALLOW_MLOCK			true
@@ -222,6 +223,7 @@ void pw_settings_init(struct pw_context *this)
 	d->log_level = get_default_int(p, "log.level", pw_log_level);
 	d->clock_power_of_two_quantum = get_default_bool(p, "clock.power-of-two-quantum",
 			DEFAULT_CLOCK_POWER_OF_TWO_QUANTUM);
+	d->link_min_buffers = get_default_int(p, "link.min-buffers", DEFAULT_LINK_MIN_BUFFERS);
 	d->link_max_buffers = get_default_int(p, "link.max-buffers", DEFAULT_LINK_MAX_BUFFERS);
 	d->mem_warn_mlock = get_default_bool(p, "mem.warn-mlock", DEFAULT_MEM_WARN_MLOCK);
 	d->mem_allow_mlock = get_default_bool(p, "mem.allow-mlock", DEFAULT_MEM_ALLOW_MLOCK);
@@ -229,6 +231,9 @@ void pw_settings_init(struct pw_context *this)
 	d->check_quantum = get_default_bool(p, "settings.check-quantum", DEFAULT_CHECK_QUANTUM);
 	d->check_rate = get_default_bool(p, "settings.check-rate", DEFAULT_CHECK_RATE);
 
+	d->link_min_buffers = SPA_MAX(d->link_min_buffers, 1u);
+	d->link_max_buffers = SPA_MAX(d->link_max_buffers, d->link_min_buffers);
+
 	d->clock_quantum_limit = SPA_CLAMP(d->clock_quantum_limit,
 			CLOCK_QUANTUM_FLOOR, CLOCK_QUANTUM_LIMIT);
 	d->clock_quantum_floor = SPA_CLAMP(d->clock_quantum_floor,