Fix tty_drain() and, thus, TIOCDRAIN of the current tty(4) incarnation

to actually wait until the TX FIFOs of UARTs have be drained before
returning. This is done by bringing the equivalent of the TS_BUSY flag
found in the previous implementation back in an ABI-preserving way.
Reported and tested by: Patrick Powell

Most likely, drivers for USB-serial-adapters likewise incorporating
TX FIFOs as well as other terminal devices that buffer output in some
form should also provide implementations of tsw_busy.

MFC after:	3 days

3 files changed, 36 insertions, 3 deletions

diff --git a/sys/dev/uart/uart_tty.c b/sys/dev/uart/uart_tty.c
index ada4927..6b526ae 100644
--- a/sys/dev/uart/uart_tty.c
+++ b/sys/dev/uart/uart_tty.c
@@ -355,6 +355,18 @@ uart_tty_free(void *arg)
 	 */
 }
 
+static bool
+uart_tty_busy(struct tty *tp)
+{
+	struct uart_softc *sc;
+ 
+	sc = tty_softc(tp);
+	if (sc == NULL || sc->sc_leaving)
+                return (FALSE);
+
+	return (sc->sc_txbusy);
+}
+
 static struct ttydevsw uart_tty_class = {
 	.tsw_flags	= TF_INITLOCK|TF_CALLOUT,
 	.tsw_open	= uart_tty_open,
@@ -365,6 +377,7 @@ static struct ttydevsw uart_tty_class = {
 	.tsw_param	= uart_tty_param,
 	.tsw_modem	= uart_tty_modem,
 	.tsw_free	= uart_tty_free,
+	.tsw_busy	= uart_tty_busy,
 };
 
 int
diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index 9695312..1dc6af2 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -132,11 +132,11 @@ tty_drain(struct tty *tp, int leaving)
 		/* buffer is inaccessible */
 		return (0);
 
-	while (ttyoutq_bytesused(&tp->t_outq) > 0) {
+	while (ttyoutq_bytesused(&tp->t_outq) > 0 || ttydevsw_busy(tp)) {
 		ttydevsw_outwakeup(tp);
 		/* Could be handled synchronously. */
 		bytesused = ttyoutq_bytesused(&tp->t_outq);
-		if (bytesused == 0)
+		if (bytesused == 0 && !ttydevsw_busy(tp))
 			return (0);
 
 		/* Wait for data to be drained. */
@@ -955,6 +955,13 @@ ttydevsw_deffree(void *softc)
 	panic("Terminal device freed without a free-handler");
 }
 
+static bool
+ttydevsw_defbusy(struct tty *tp __unused)
+{
+
+	return (FALSE);
+}
+
 /*
  * TTY allocation and deallocation. TTY devices can be deallocated when
  * the driver doesn't use it anymore, when the TTY isn't a session's
@@ -989,6 +996,7 @@ tty_alloc_mutex(struct ttydevsw *tsw, void *sc, struct mtx *mutex)
 	PATCH_FUNC(mmap);
 	PATCH_FUNC(pktnotify);
 	PATCH_FUNC(free);
+	PATCH_FUNC(busy);
 #undef PATCH_FUNC
 
 	tp = malloc(sizeof(struct tty), M_TTY, M_WAITOK|M_ZERO);
diff --git a/sys/sys/ttydevsw.h b/sys/sys/ttydevsw.h
index 748ae0b..ae70613 100644
--- a/sys/sys/ttydevsw.h
+++ b/sys/sys/ttydevsw.h
@@ -54,6 +54,7 @@ typedef int tsw_mmap_t(struct tty *tp, vm_ooffset_t offset,
     vm_paddr_t * paddr, int nprot, vm_memattr_t *memattr);
 typedef void tsw_pktnotify_t(struct tty *tp, char event);
 typedef void tsw_free_t(void *softc);
+typedef bool tsw_busy_t(struct tty *tp);
 
 struct ttydevsw {
 	unsigned int	tsw_flags;	/* Default TTY flags. */
@@ -74,7 +75,9 @@ struct ttydevsw {
 
 	tsw_free_t	*tsw_free;	/* Destructor. */
 
-	void		*tsw_spare[4];	/* For future use. */
+	tsw_busy_t	*tsw_busy;	/* Draining output. */
+
+	void		*tsw_spare[3];	/* For future use. */
 };
 
 static __inline int
@@ -181,4 +184,13 @@ ttydevsw_free(struct tty *tp)
 	tp->t_devsw->tsw_free(tty_softc(tp));
 }
 
+static __inline bool
+ttydevsw_busy(struct tty *tp)
+{
+
+	MPASS(tty_gone(tp));
+
+	return (tp->t_devsw->tsw_busy(tp));
+}
+
 #endif /* !_SYS_TTYDEVSW_H_ */