Index: head/share/man/man9/timeout.9
===================================================================
--- head/share/man/man9/timeout.9
+++ head/share/man/man9/timeout.9
@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 27, 2016
+.Dd December 10, 2019
 .Dt TIMEOUT 9
 .Os
 .Sh NAME
@@ -62,8 +62,10 @@
 .Nd execute a function after a specified length of time
 .Sh SYNOPSIS
 .In sys/types.h
+.In sys/callout.h
 .In sys/systm.h
 .Bd -literal
+typedef void callout_func_t (void *);
 typedef void timeout_t (void *);
 .Ed
 .Ft int
@@ -71,7 +73,7 @@
 .Ft void
 .Fn callout_deactivate "struct callout *c"
 .Ft int
-.Fn callout_async_drain "struct callout *c" "timeout_t *drain"
+.Fn callout_async_drain "struct callout *c" "callout_func_t *drain"
 .Ft int
 .Fn callout_drain "struct callout *c"
 .Ft void
@@ -90,19 +92,24 @@
 .Ft int
 .Fn callout_pending "struct callout *c"
 .Ft int
-.Fn callout_reset "struct callout *c" "int ticks" "timeout_t *func" "void *arg"
+.Fo callout_reset
+.Fa "struct callout *c"
+.Fa "int ticks"
+.Fa "callout_func_t *func"
+.Fa "void *arg"
+.Fc
 .Ft int
 .Fo callout_reset_curcpu
 .Fa "struct callout *c"
 .Fa "int ticks"
-.Fa "timeout_t *func"
+.Fa "callout_func_t *func"
 .Fa "void *arg"
 .Fc
 .Ft int
 .Fo callout_reset_on
 .Fa "struct callout *c"
 .Fa "int ticks"
-.Fa "timeout_t *func"
+.Fa "callout_func_t *func"
 .Fa "void *arg"
 .Fa "int cpu"
 .Fc
@@ -111,7 +118,7 @@
 .Fa "struct callout *c"
 .Fa "sbintime_t sbt"
 .Fa "sbintime_t pr"
-.Fa "timeout_t *func"
+.Fa "callout_func_t *func"
 .Fa "void *arg"
 .Fa "int flags"
 .Fc
@@ -120,7 +127,7 @@
 .Fa "struct callout *c"
 .Fa "sbintime_t sbt"
 .Fa "sbintime_t pr"
-.Fa "timeout_t *func"
+.Fa "callout_func_t *func"
 .Fa "void *arg"
 .Fa "int flags"
 .Fc
@@ -129,7 +136,7 @@
 .Fa "struct callout *c"
 .Fa "sbintime_t sbt"
 .Fa "sbintime_t pr"
-.Fa "timeout_t *func"
+.Fa "callout_func_t *func"
 .Fa "void *arg"
 .Fa "int cpu"
 .Fa "int flags"
Index: head/sys/kern/kern_timeout.c
===================================================================
--- head/sys/kern/kern_timeout.c
+++ head/sys/kern/kern_timeout.c
@@ -145,11 +145,11 @@
  */
 struct cc_exec {
 	struct callout		*cc_curr;
-	void			(*cc_drain)(void *);
+	callout_func_t		*cc_drain;
 	void			*cc_last_func;
 	void			*cc_last_arg;
 #ifdef SMP
-	void			(*ce_migration_func)(void *);
+	callout_func_t		*ce_migration_func;
 	void			*ce_migration_arg;
 	sbintime_t		ce_migration_time;
 	sbintime_t		ce_migration_prec;
@@ -656,7 +656,7 @@
     int direct)
 {
 	struct rm_priotracker tracker;
-	void (*c_func)(void *);
+	callout_func_t *c_func, *drain;
 	void *c_arg;
 	struct lock_class *class;
 	struct lock_object *c_lock;
@@ -664,7 +664,7 @@
 	int c_iflags;
 #ifdef SMP
 	struct callout_cpu *new_cc;
-	void (*new_func)(void *);
+	callout_func_t *new_func;
 	void *new_arg;
 	int flags, new_cpu;
 	sbintime_t new_prec, new_time;
@@ -673,7 +673,7 @@
 	sbintime_t sbt1, sbt2;
 	struct timespec ts2;
 	static sbintime_t maxdt = 2 * SBT_1MS;	/* 2 msec */
-	static timeout_t *lastfunc;
+	static callout_func_t *lastfunc;
 #endif
 
 	KASSERT((c->c_iflags & CALLOUT_PENDING) == CALLOUT_PENDING,
@@ -768,8 +768,6 @@
 	KASSERT(cc_exec_curr(cc, direct) == c, ("mishandled cc_curr"));
 	cc_exec_curr(cc, direct) = NULL;
 	if (cc_exec_drain(cc, direct)) {
-		void (*drain)(void *);
-		
 		drain = cc_exec_drain(cc, direct);
 		cc_exec_drain(cc, direct) = NULL;
 		CC_UNLOCK(cc);
@@ -1031,7 +1029,7 @@
  */
 int
 callout_reset_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t prec,
-    void (*ftn)(void *), void *arg, int cpu, int flags)
+    callout_func_t *ftn, void *arg, int cpu, int flags)
 {
 	sbintime_t to_sbt, precision;
 	struct callout_cpu *cc;
@@ -1190,7 +1188,7 @@
 }
 
 int
-_callout_stop_safe(struct callout *c, int flags, void (*drain)(void *))
+_callout_stop_safe(struct callout *c, int flags, callout_func_t *drain)
 {
 	struct callout_cpu *cc, *old_cc;
 	struct lock_class *class;
Index: head/sys/sys/_callout.h
===================================================================
--- head/sys/sys/_callout.h
+++ head/sys/sys/_callout.h
@@ -48,6 +48,8 @@
 SLIST_HEAD(callout_slist, callout);
 TAILQ_HEAD(callout_tailq, callout);
 
+typedef void callout_func_t(void *);
+
 struct callout {
 	union {
 		LIST_ENTRY(callout) le;
@@ -57,7 +59,7 @@
 	sbintime_t c_time;			/* ticks to the event */
 	sbintime_t c_precision;			/* delta allowed wrt opt */
 	void	*c_arg;				/* function argument */
-	void	(*c_func)(void *);		/* function to call */
+	callout_func_t *c_func;			/* function to call */
 	struct lock_object *c_lock;		/* lock to handle */
 	short	c_flags;			/* User State */
 	short	c_iflags;			/* Internal State */