virtual_oss: Introduce virtual_oss_default_control_device rc variable

The -t option gives the user the ability to create a control device for
a given virtual_oss(8) instance, so that the instance's configuration
can be manipulated during runtime with virtual_oss_cmd(8). As is
expected, the control device's name is not known, since it is specified
by the user.

This patch introduces a virtual_oss_default_control_device rc variable,
which defaults to "vdsp.ctl". The goal of this is that third-party
programs and scripts can access the control device of the default
virtual_oss(8) configuration without guessing. This is especially useful
for sbin/devd/snd.conf which deals with hot-swapping sound devices using
virtual_oss(8).

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D55670

libexec/rc/rc.d/virtual_oss

@@ -25,6 +25,10 @@ required_modules="cuse"
 configs=
 pidpath="/var/run/${name}"
 default_unit=$(sysctl -n hw.snd.default_unit 2> /dev/null)
+
+# Default configuration's control device.
+: "${virtual_oss_default_control_device:="vdsp.ctl"}"
+
 virtual_oss_default_args="\
 	-S \
 	-C 2 \
@@ -35,7 +39,7 @@ virtual_oss_default_args="\
 	-i 8 \
 	-f /dev/dsp${default_unit} \
 	-d dsp \
-	-t vdsp.ctl"
+	-t ${virtual_oss_default_control_device}"
 
 # Set to NO by default. Set it to "YES" to enable virtual_oss.
 : "${virtual_oss_enable:="NO"}"

sbin/devd/snd.conf

@@ -7,12 +7,8 @@ notify 0 {
 
 	# Other audio servers or device switching commands can be used here
 	# instead of virtual_oss(8).
-	#
-	# FIXME: We are hardcoding /dev/vdsp.ctl here, simply because it is a
-	# common virtual_oss control device name. Until we find a proper way to
-	# define control devices here, /dev/vdsp.ctl can be changed to the
-	# control device of choice.
-	action			"/usr/sbin/virtual_oss_cmd /dev/vdsp.ctl -R /dev/$cdev";
+	action			"/usr/sbin/virtual_oss_cmd \
+		/dev/$(sysrc virtual_oss_default_control_device) -R /dev/$cdev";
 };
 
 notify 0 {
@@ -22,7 +18,8 @@ notify 0 {
 	match "cdev"		"dsp[0-9]+";
 
 	# See comment above.
-	action			"/usr/sbin/virtual_oss_cmd /dev/vdsp.ctl -P /dev/$cdev";
+	action			"/usr/sbin/virtual_oss_cmd \
+		/dev/$(sysrc virtual_oss_default_control_device) -P /dev/$cdev";
 };
 
 notify 0 {
@@ -32,5 +29,6 @@ notify 0 {
 
 	# No connected devices. Disable both recording and playback to avoid
 	# repeated virtual_oss error messages.
-	action			"/usr/sbin/virtual_oss_cmd /dev/vdsp.ctl -f /dev/null";
+	action			"/usr/sbin/virtual_oss_cmd \
+		/dev/$(sysrc virtual_oss_default_control_device) -f /dev/null";
 };