Document sysctl nodes that translate their values.

This documents the behavior of sysctl_msec_to_ticks and
SYSCTL_{ADD,}_SBINTIME_[UM]SEC.

Reviewed by:	cem
MFC after:	1 week
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D20596

share/man/man9/Makefile

@@ -2011,6 +2011,7 @@ MLINKS+=sysctl.9 SYSCTL_DECL.9 \
 	sysctl.9 SYSCTL_INT.9 \
 	sysctl.9 SYSCTL_INT_WITH_LABEL.9 \
 	sysctl.9 SYSCTL_LONG.9 \
+	sysctl.9 sysctl_msec_to_ticks.9 \
 	sysctl.9 SYSCTL_NODE.9 \
 	sysctl.9 SYSCTL_NODE_WITH_LABEL.9 \
 	sysctl.9 SYSCTL_OPAQUE.9 \

share/man/man9/sysctl.9

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 25, 2019
+.Dd June 11, 2019
 .Dt SYSCTL 9
 .Os
 .Sh NAME
@@ -70,6 +70,7 @@
 .Nm SYSCTL_INT ,
 .Nm SYSCTL_INT_WITH_LABEL ,
 .Nm SYSCTL_LONG ,
+.Nm sysctl_msec_to_ticks ,
 .Nm SYSCTL_NODE ,
 .Nm SYSCTL_NODE_WITH_LABEL ,
 .Nm SYSCTL_OPAQUE ,
@@ -442,6 +443,8 @@
 .Fn SYSCTL_INT parent number name ctlflags ptr val descr
 .Fn SYSCTL_INT_WITH_LABEL parent number name ctlflags ptr val descr label
 .Fn SYSCTL_LONG parent number name ctlflags ptr val descr
+.Ft int
+.Fn sysctl_msec_to_ticks SYSCTL_HANDLER_ARGS
 .Fn SYSCTL_NODE parent number name ctlflags handler descr
 .Fn SYSCTL_NODE_WITH_LABEL parent number name ctlflags handler descr label
 .Fn SYSCTL_OPAQUE parent number name ctlflags ptr len format descr
@@ -466,6 +469,7 @@
 .Fn SYSCTL_UQUAD parent number name ctlflags ptr val descr
 .Fn SYSCTL_UMA_MAX parent number name ctlflags ptr descr
 .Fn SYSCTL_UMA_CUR parent number name ctlflags ptr descr
+
 .Sh DESCRIPTION
 The
 .Nm SYSCTL
@@ -596,6 +600,55 @@ Labels should only be applied to siblings that are structurally similar
 and encode the same type of value,
 as aggregation is of no use otherwise.
 .El
+.Sh NODE VALUE TYPES
+Most of the macros and functions used to create sysctl nodes export a
+read-only constant or in-kernel variable whose type matches the type
+of the node's value.
+For example,
+.Fn SYSCTL_INT
+reports the raw value of an associated variable of type
+.Vt int .
+However, nodes may also export a value that is a translatation of an internal
+representation.
+.Pp
+The
+.Fn sysctl_msec_to_ticks
+handler can be used with
+.Fn SYSCTL_PROC
+or
+.Fn SYSCTL_ADD_PROC
+to export a millisecond time interval.
+When using this handler,
+the
+.Fa arg2
+parameter points to an in-kernel variable of type
+.Vt int
+which stores a tick count suitable for use with functions like
+.Xr tsleep 9 .
+The
+.Fn sysctl_msec_to_ticks
+function converts this value to milliseconds when reporting the node's value.
+Similarly,
+.Fn sysctl_msec_to_ticks
+accepts new values in milliseconds and stores an equivalent value in ticks to
+.Fa *arg2 .
+Note that new code should use kernel variables of type
+.Vt sbintime_t
+instead of tick counts.
+.Pp
+The
+.Fn SYSCTL_ADD_SBINTIME_MSEC
+and
+.Fn SYSCTL_ADD_SBINTIME_USEC
+functions and
+.Fn SYSCTL_SBINTIME_MSEC
+and
+.Fn SYSCTL_SBINTIME_USEC
+macros all create nodes which export an in-kernel variable of type
+.Vt sbintime_t .
+These nodes do not export the raw value of the associated variable.
+Instead, they export a 64-bit integer containing a count of either
+milliseconds (the MSEC variants) or microseconds (the USEC variants).
 .Sh CREATING ROOT NODES
 Sysctl MIBs or OIDs are created in a hierarchical tree.
 The nodes at the bottom of the tree are called root nodes, and have no