loop.4: Document the loop-control device

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man4/loop.4

@@ -1,4 +1,5 @@
 .\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
+.\" and Copyright 2015 Michael Kerrisk <mtk.manpages@gmail.com>
 .\"
 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
 .\" This is free documentation; you can redistribute it and/or
@@ -24,7 +25,7 @@
 .\"
 .TH LOOP 4 "2005-12-05" "Linux" "Linux Programmer's Manual"
 .SH NAME
-loop \- driver for loop devices
+loop, loop-control \- loop devices
 .SH SYNOPSIS
 #include <linux/loop.h>
 .SH DESCRIPTION
@@ -166,8 +167,44 @@ struct loop_info64 {
 };
 .in
 .fi
+.SS /dev/loop-control
+Since Linux 3.1,
+.\" commit 770fe30a46a12b6fb6b63fbe1737654d28e84844
+the kernel provides the
+.I /dev/loop-control
+device, which permits an application to dynamically find a free device,
+and to add and remove loop devices from the system.
+To perform these operations, one first opens
+.IR /dev/loop-control
+and then employs one of the following
+.BR ioctl (2)
+operations:
+.TP
+.B LOOP_CTL_GET_FREE
+Allocate or find a free loop device for use.
+On success, the device number is returned as the result of the call.
+This operation takes no argument.
+.TP
+.B LOOP_CTL_ADD
+Add the new loop device whose device number is specified
+as a long integer in the third
+.BR ioctl (2)
+argument.
+On success, the device index is returned as the result of the call.
+If the device is already allocated, the call fails with the error
+.BR EEXIST .
+.TP
+.B LOOP_CTL_REMOVE
+Remove the loop device whose device number is specified
+as a long integer in the third
+.BR ioctl (2)
+argument.
+On success, the device number is returned as the result of the call.
+If the device is in use, the call fails with the error
+.BR EBUSY .
 .SH FILES
-/dev/loop[0-7]: The loop block special device files
+.IR /dev/loop* :
+the loop block special device files
 .SH "SEE ALSO"
 .BR losetup (8),
 .BR mount (8)