ALSA: introduce the snd-aloop module for the PCM loopback

The snd-aloop module allows redirecting of the PCM playback in the
kernel back to the user space using the standard ALSA PCM capture API.

The module also allows time synchronization with another timing source
and notifications of playback stream parameter changes.
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

sound/drivers/Kconfig

@@ -74,6 +74,25 @@ config SND_DUMMY
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-dummy.
 
+config SND_ALOOP
+        tristate "Generic loopback driver (PCM)"
+        select SND_PCM
+        help
+          Say 'Y' or 'M' to include support for the PCM loopback device.
+	  This module returns played samples back to the user space using
+	  the standard ALSA PCM device. The devices are routed 0->1 and
+          1->0, where first number is the playback PCM device and second
+	  number is the capture device. Module creates two PCM devices and
+	  configured number of substreams (see the pcm_substreams module
+          parameter).
+
+	  The looback device allow time sychronization with an external
+	  timing source using the time shift universal control (+-20%
+	  of system time).
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called snd-aloop.
+
 config SND_VIRMIDI
 	tristate "Virtual MIDI soundcard"
 	depends on SND_SEQUENCER

sound/drivers/Makefile

@@ -4,6 +4,7 @@
 #
 
 snd-dummy-objs := dummy.o
+snd-aloop-objs := aloop.o
 snd-mtpav-objs := mtpav.o
 snd-mts64-objs := mts64.o
 snd-portman2x4-objs := portman2x4.o
@@ -13,6 +14,7 @@ snd-ml403-ac97cr-objs := ml403-ac97cr.o pcm-indirect2.o
 
 # Toplevel Module Dependency
 obj-$(CONFIG_SND_DUMMY) += snd-dummy.o
+obj-$(CONFIG_SND_ALOOP) += snd-aloop.o
 obj-$(CONFIG_SND_VIRMIDI) += snd-virmidi.o
 obj-$(CONFIG_SND_SERIAL_U16550) += snd-serial-u16550.o
 obj-$(CONFIG_SND_MTPAV) += snd-mtpav.o