diff --git a/sys/dev/cy/cy.c b/sys/dev/cy/cy.c
index bd8d7a49930b..7852317cf456 100644
--- a/sys/dev/cy/cy.c
+++ b/sys/dev/cy/cy.c
@@ -27,7 +27,7 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
- *	$Id: cy.c,v 1.12 1995/07/29 04:05:53 bde Exp $
+ *	$Id: cy.c,v 1.13 1995/07/29 08:33:06 bde Exp $
  */
 
 #include "cy.h"
@@ -1598,31 +1598,6 @@ repeat:
 		}
 		if (incc <= 0 || !(tp->t_state & TS_ISOPEN))
 			continue;
-		/*
-		 * XXX only do this when we bypass ttyinput.
-		 */
-		if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
-		    && (com->state & CS_RTS_IFLOW || tp->t_iflag & IXOFF)
-		    && !(tp->t_state & TS_TBLOCK)
-		    /*
-		     * XXX - need flow control for all line disciplines.
-		     * Only have it in standard one now.
-		     */
-		    && linesw[tp->t_line].l_rint == ttyinput) {
-			int	putc_status = 0;
-
-			if ((tp->t_iflag & IXOFF
-			     && tp->t_cc[VSTOP] != _POSIX_VDISABLE
-			     && (putc_status = putc(tp->t_cc[VSTOP],
-						    &tp->t_outq)) == 0)
-			    || com->state & CS_RTS_IFLOW) {
-				tp->t_state |= TS_TBLOCK;
-				ttstart(tp);
-				if (putc_status != 0)
-					/* Try again later. */
-					tp->t_state &= ~TS_TBLOCK;
-			}
-		}
 		/*
 		 * Avoid the grotesquely inefficient lineswitch routine
 		 * (ttyinput) in "raw" mode.  It usually takes about 450
@@ -1631,6 +1606,11 @@ repeat:
 		 * call overhead).
 		 */
 		if (tp->t_state & TS_CAN_BYPASS_L_RINT) {
+			if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
+			    && (com->state & CS_RTS_IFLOW
+				|| tp->t_iflag & IXOFF)
+			    && !(tp->t_state & TS_TBLOCK))
+				ttyblock(tp);
 			tk_nin += incc;
 			tk_rawcc += incc;
 			tp->t_rawcc += incc;
diff --git a/sys/dev/cy/cy_isa.c b/sys/dev/cy/cy_isa.c
index bd8d7a49930b..7852317cf456 100644
--- a/sys/dev/cy/cy_isa.c
+++ b/sys/dev/cy/cy_isa.c
@@ -27,7 +27,7 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
- *	$Id: cy.c,v 1.12 1995/07/29 04:05:53 bde Exp $
+ *	$Id: cy.c,v 1.13 1995/07/29 08:33:06 bde Exp $
  */
 
 #include "cy.h"
@@ -1598,31 +1598,6 @@ repeat:
 		}
 		if (incc <= 0 || !(tp->t_state & TS_ISOPEN))
 			continue;
-		/*
-		 * XXX only do this when we bypass ttyinput.
-		 */
-		if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
-		    && (com->state & CS_RTS_IFLOW || tp->t_iflag & IXOFF)
-		    && !(tp->t_state & TS_TBLOCK)
-		    /*
-		     * XXX - need flow control for all line disciplines.
-		     * Only have it in standard one now.
-		     */
-		    && linesw[tp->t_line].l_rint == ttyinput) {
-			int	putc_status = 0;
-
-			if ((tp->t_iflag & IXOFF
-			     && tp->t_cc[VSTOP] != _POSIX_VDISABLE
-			     && (putc_status = putc(tp->t_cc[VSTOP],
-						    &tp->t_outq)) == 0)
-			    || com->state & CS_RTS_IFLOW) {
-				tp->t_state |= TS_TBLOCK;
-				ttstart(tp);
-				if (putc_status != 0)
-					/* Try again later. */
-					tp->t_state &= ~TS_TBLOCK;
-			}
-		}
 		/*
 		 * Avoid the grotesquely inefficient lineswitch routine
 		 * (ttyinput) in "raw" mode.  It usually takes about 450
@@ -1631,6 +1606,11 @@ repeat:
 		 * call overhead).
 		 */
 		if (tp->t_state & TS_CAN_BYPASS_L_RINT) {
+			if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
+			    && (com->state & CS_RTS_IFLOW
+				|| tp->t_iflag & IXOFF)
+			    && !(tp->t_state & TS_TBLOCK))
+				ttyblock(tp);
 			tk_nin += incc;
 			tk_rawcc += incc;
 			tp->t_rawcc += incc;
diff --git a/sys/dev/rc/rc.c b/sys/dev/rc/rc.c
index ec93d681f0c6..1e9f6667eced 100644
--- a/sys/dev/rc/rc.c
+++ b/sys/dev/rc/rc.c
@@ -664,24 +664,12 @@ repeat:
 				if (icnt <= 0 || !(tp->t_state & TS_ISOPEN))
 					goto done1;
 
-				if (   linesw[tp->t_line].l_rint == ttyinput
-				    && ((rc->rc_flags & RC_RTSFLOW) || (tp->t_iflag & IXOFF))
-				    && !(tp->t_state & TS_TBLOCK)
-				    && (tp->t_rawq.c_cc + icnt) > RB_I_HIGH_WATER) {
-					int queue_full = 0;
-
-					if ((tp->t_iflag & IXOFF) &&
-					    tp->t_cc[VSTOP] != _POSIX_VDISABLE &&
-					    (queue_full = putc(tp->t_cc[VSTOP], &tp->t_outq)) == 0 ||
-					    (rc->rc_flags & RC_RTSFLOW)) {
-						tp->t_state |= TS_TBLOCK;
-						ttstart(tp);
-						if (queue_full) /* try again */
-							tp->t_state &= ~TS_TBLOCK;
-					}
-				}
 				if (   (tp->t_state & TS_CAN_BYPASS_L_RINT)
 				    && !(tp->t_state & TS_LOCAL)) {
+					if ((tp->t_rawq.c_cc + icnt) >= RB_I_HIGH_WATER
+					    && ((rc->rc_flags & RC_RTSFLOW) || (tp->t_iflag & IXOFF))
+					    && !(tp->t_state & TS_TBLOCK))
+						ttyblock(tp);
 					tk_nin += icnt;
 					tk_rawcc += icnt;
 					tp->t_rawcc += icnt;
diff --git a/sys/dev/sio/sio.c b/sys/dev/sio/sio.c
index a428eeb9411c..97f9dd786f7b 100644
--- a/sys/dev/sio/sio.c
+++ b/sys/dev/sio/sio.c
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	from: @(#)com.c	7.5 (Berkeley) 5/16/91
- *	$Id: sio.c,v 1.106 1995/07/29 04:05:57 bde Exp $
+ *	$Id: sio.c,v 1.107 1995/07/29 08:33:13 bde Exp $
  */
 
 #include "sio.h"
@@ -1547,31 +1547,6 @@ repeat:
 		}
 		if (incc <= 0 || !(tp->t_state & TS_ISOPEN))
 			continue;
-		/*
-		 * XXX only do this when we bypass ttyinput.
-		 */
-		if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
-		    && (com->state & CS_RTS_IFLOW || tp->t_iflag & IXOFF)
-		    && !(tp->t_state & TS_TBLOCK)
-		    /*
-		     * XXX - need flow control for all line disciplines.
-		     * Only have it in standard one now.
-		     */
-		    && linesw[tp->t_line].l_rint == ttyinput) {
-			int	putc_status = 0;
-
-			if ((tp->t_iflag & IXOFF
-			     && tp->t_cc[VSTOP] != _POSIX_VDISABLE
-			     && (putc_status = putc(tp->t_cc[VSTOP],
-						    &tp->t_outq)) == 0)
-			    || com->state & CS_RTS_IFLOW) {
-				tp->t_state |= TS_TBLOCK;
-				ttstart(tp);
-				if (putc_status != 0)
-					/* Try again later. */
-					tp->t_state &= ~TS_TBLOCK;
-			}
-		}
 		/*
 		 * Avoid the grotesquely inefficient lineswitch routine
 		 * (ttyinput) in "raw" mode.  It usually takes about 450
@@ -1580,6 +1555,11 @@ repeat:
 		 * call overhead).
 		 */
 		if (tp->t_state & TS_CAN_BYPASS_L_RINT) {
+			if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
+			    && (com->state & CS_RTS_IFLOW
+				|| tp->t_iflag & IXOFF)
+			    && !(tp->t_state & TS_TBLOCK))
+				ttyblock(tp);
 			tk_nin += incc;
 			tk_rawcc += incc;
 			tp->t_rawcc += incc;
diff --git a/sys/i386/isa/cy.c b/sys/i386/isa/cy.c
index bd8d7a49930b..7852317cf456 100644
--- a/sys/i386/isa/cy.c
+++ b/sys/i386/isa/cy.c
@@ -27,7 +27,7 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
- *	$Id: cy.c,v 1.12 1995/07/29 04:05:53 bde Exp $
+ *	$Id: cy.c,v 1.13 1995/07/29 08:33:06 bde Exp $
  */
 
 #include "cy.h"
@@ -1598,31 +1598,6 @@ repeat:
 		}
 		if (incc <= 0 || !(tp->t_state & TS_ISOPEN))
 			continue;
-		/*
-		 * XXX only do this when we bypass ttyinput.
-		 */
-		if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
-		    && (com->state & CS_RTS_IFLOW || tp->t_iflag & IXOFF)
-		    && !(tp->t_state & TS_TBLOCK)
-		    /*
-		     * XXX - need flow control for all line disciplines.
-		     * Only have it in standard one now.
-		     */
-		    && linesw[tp->t_line].l_rint == ttyinput) {
-			int	putc_status = 0;
-
-			if ((tp->t_iflag & IXOFF
-			     && tp->t_cc[VSTOP] != _POSIX_VDISABLE
-			     && (putc_status = putc(tp->t_cc[VSTOP],
-						    &tp->t_outq)) == 0)
-			    || com->state & CS_RTS_IFLOW) {
-				tp->t_state |= TS_TBLOCK;
-				ttstart(tp);
-				if (putc_status != 0)
-					/* Try again later. */
-					tp->t_state &= ~TS_TBLOCK;
-			}
-		}
 		/*
 		 * Avoid the grotesquely inefficient lineswitch routine
 		 * (ttyinput) in "raw" mode.  It usually takes about 450
@@ -1631,6 +1606,11 @@ repeat:
 		 * call overhead).
 		 */
 		if (tp->t_state & TS_CAN_BYPASS_L_RINT) {
+			if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
+			    && (com->state & CS_RTS_IFLOW
+				|| tp->t_iflag & IXOFF)
+			    && !(tp->t_state & TS_TBLOCK))
+				ttyblock(tp);
 			tk_nin += incc;
 			tk_rawcc += incc;
 			tp->t_rawcc += incc;
diff --git a/sys/i386/isa/rc.c b/sys/i386/isa/rc.c
index ec93d681f0c6..1e9f6667eced 100644
--- a/sys/i386/isa/rc.c
+++ b/sys/i386/isa/rc.c
@@ -664,24 +664,12 @@ repeat:
 				if (icnt <= 0 || !(tp->t_state & TS_ISOPEN))
 					goto done1;
 
-				if (   linesw[tp->t_line].l_rint == ttyinput
-				    && ((rc->rc_flags & RC_RTSFLOW) || (tp->t_iflag & IXOFF))
-				    && !(tp->t_state & TS_TBLOCK)
-				    && (tp->t_rawq.c_cc + icnt) > RB_I_HIGH_WATER) {
-					int queue_full = 0;
-
-					if ((tp->t_iflag & IXOFF) &&
-					    tp->t_cc[VSTOP] != _POSIX_VDISABLE &&
-					    (queue_full = putc(tp->t_cc[VSTOP], &tp->t_outq)) == 0 ||
-					    (rc->rc_flags & RC_RTSFLOW)) {
-						tp->t_state |= TS_TBLOCK;
-						ttstart(tp);
-						if (queue_full) /* try again */
-							tp->t_state &= ~TS_TBLOCK;
-					}
-				}
 				if (   (tp->t_state & TS_CAN_BYPASS_L_RINT)
 				    && !(tp->t_state & TS_LOCAL)) {
+					if ((tp->t_rawq.c_cc + icnt) >= RB_I_HIGH_WATER
+					    && ((rc->rc_flags & RC_RTSFLOW) || (tp->t_iflag & IXOFF))
+					    && !(tp->t_state & TS_TBLOCK))
+						ttyblock(tp);
 					tk_nin += icnt;
 					tk_rawcc += icnt;
 					tp->t_rawcc += icnt;
diff --git a/sys/i386/isa/sio.c b/sys/i386/isa/sio.c
index a428eeb9411c..97f9dd786f7b 100644
--- a/sys/i386/isa/sio.c
+++ b/sys/i386/isa/sio.c
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	from: @(#)com.c	7.5 (Berkeley) 5/16/91
- *	$Id: sio.c,v 1.106 1995/07/29 04:05:57 bde Exp $
+ *	$Id: sio.c,v 1.107 1995/07/29 08:33:13 bde Exp $
  */
 
 #include "sio.h"
@@ -1547,31 +1547,6 @@ repeat:
 		}
 		if (incc <= 0 || !(tp->t_state & TS_ISOPEN))
 			continue;
-		/*
-		 * XXX only do this when we bypass ttyinput.
-		 */
-		if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
-		    && (com->state & CS_RTS_IFLOW || tp->t_iflag & IXOFF)
-		    && !(tp->t_state & TS_TBLOCK)
-		    /*
-		     * XXX - need flow control for all line disciplines.
-		     * Only have it in standard one now.
-		     */
-		    && linesw[tp->t_line].l_rint == ttyinput) {
-			int	putc_status = 0;
-
-			if ((tp->t_iflag & IXOFF
-			     && tp->t_cc[VSTOP] != _POSIX_VDISABLE
-			     && (putc_status = putc(tp->t_cc[VSTOP],
-						    &tp->t_outq)) == 0)
-			    || com->state & CS_RTS_IFLOW) {
-				tp->t_state |= TS_TBLOCK;
-				ttstart(tp);
-				if (putc_status != 0)
-					/* Try again later. */
-					tp->t_state &= ~TS_TBLOCK;
-			}
-		}
 		/*
 		 * Avoid the grotesquely inefficient lineswitch routine
 		 * (ttyinput) in "raw" mode.  It usually takes about 450
@@ -1580,6 +1555,11 @@ repeat:
 		 * call overhead).
 		 */
 		if (tp->t_state & TS_CAN_BYPASS_L_RINT) {
+			if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
+			    && (com->state & CS_RTS_IFLOW
+				|| tp->t_iflag & IXOFF)
+			    && !(tp->t_state & TS_TBLOCK))
+				ttyblock(tp);
 			tk_nin += incc;
 			tk_rawcc += incc;
 			tp->t_rawcc += incc;
diff --git a/sys/isa/sio.c b/sys/isa/sio.c
index a428eeb9411c..97f9dd786f7b 100644
--- a/sys/isa/sio.c
+++ b/sys/isa/sio.c
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	from: @(#)com.c	7.5 (Berkeley) 5/16/91
- *	$Id: sio.c,v 1.106 1995/07/29 04:05:57 bde Exp $
+ *	$Id: sio.c,v 1.107 1995/07/29 08:33:13 bde Exp $
  */
 
 #include "sio.h"
@@ -1547,31 +1547,6 @@ repeat:
 		}
 		if (incc <= 0 || !(tp->t_state & TS_ISOPEN))
 			continue;
-		/*
-		 * XXX only do this when we bypass ttyinput.
-		 */
-		if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
-		    && (com->state & CS_RTS_IFLOW || tp->t_iflag & IXOFF)
-		    && !(tp->t_state & TS_TBLOCK)
-		    /*
-		     * XXX - need flow control for all line disciplines.
-		     * Only have it in standard one now.
-		     */
-		    && linesw[tp->t_line].l_rint == ttyinput) {
-			int	putc_status = 0;
-
-			if ((tp->t_iflag & IXOFF
-			     && tp->t_cc[VSTOP] != _POSIX_VDISABLE
-			     && (putc_status = putc(tp->t_cc[VSTOP],
-						    &tp->t_outq)) == 0)
-			    || com->state & CS_RTS_IFLOW) {
-				tp->t_state |= TS_TBLOCK;
-				ttstart(tp);
-				if (putc_status != 0)
-					/* Try again later. */
-					tp->t_state &= ~TS_TBLOCK;
-			}
-		}
 		/*
 		 * Avoid the grotesquely inefficient lineswitch routine
 		 * (ttyinput) in "raw" mode.  It usually takes about 450
@@ -1580,6 +1555,11 @@ repeat:
 		 * call overhead).
 		 */
 		if (tp->t_state & TS_CAN_BYPASS_L_RINT) {
+			if (tp->t_rawq.c_cc + incc >= RB_I_HIGH_WATER
+			    && (com->state & CS_RTS_IFLOW
+				|| tp->t_iflag & IXOFF)
+			    && !(tp->t_state & TS_TBLOCK))
+				ttyblock(tp);
 			tk_nin += incc;
 			tk_rawcc += incc;
 			tp->t_rawcc += incc;
diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index 66947ae85ad5..940a75bda866 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -36,7 +36,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tty.c	8.8 (Berkeley) 1/21/94
- * $Id: tty.c,v 1.59 1995/07/30 12:39:16 bde Exp $
+ * $Id: tty.c,v 1.60 1995/07/30 13:52:56 bde Exp $
  */
 
 /*-
@@ -95,9 +95,9 @@
 
 static int	proc_compare __P((struct proc *p1, struct proc *p2));
 static int	ttnread __P((struct tty *));
-static void	ttyblock __P((struct tty *tp));
 static void	ttyecho __P((int, struct tty *tp));
 static void	ttyrubo __P((struct tty *, int));
+static void	ttyunblock __P((struct tty *tp));
 
 /*
  * Table with character classes and parity. The 8th bit indicates parity,
@@ -177,6 +177,14 @@ char const char_type[] = {
 #define	CLR(t, f)	(t) &= ~(f)
 #define	ISSET(t, f)	((t) & (f))
 
+/*
+ * Input control starts when we would not be able to fit the maximum
+ * contents of the ping-pong buffers and finishes when we would be able
+ * to fit that much plus 1/8 more.
+ */
+#define	I_HIGH_WATER	(TTYHOG - 2 * 256)	/* XXX */
+#define	I_LOW_WATER	((TTYHOG - 2 * 256) * 7 / 8)	/* XXX */
+
 #undef MAX_INPUT		/* XXX wrong in <sys/syslimits.h> */
 #define	MAX_INPUT	TTYHOG
 
@@ -285,9 +293,21 @@ ttyinput(c, tp)
 	}
 	++tk_nin;
 
+	/*
+	 * Block further input iff:
+	 * current input > threshold AND input is available to user program
+	 * AND input flow control is enabled and not yet invoked.
+	 * The 3 is slop for PARMRK.
+	 */
+	iflag = tp->t_iflag;
+	if (tp->t_rawq.c_cc + tp->t_canq.c_cc > I_HIGH_WATER - 3 &&
+	    (!ISSET(lflag, ICANON) || tp->t_canq.c_cc != 0) &&
+	    (ISSET(tp->t_cflag, CRTS_IFLOW) || ISSET(iflag, IXOFF)) &&
+	    !ISSET(tp->t_state, TS_TBLOCK))
+		ttyblock(tp);
+
 	/* Handle exceptional conditions (break, parity, framing). */
 	cc = tp->t_cc;
-	iflag = tp->t_iflag;
 	err = (ISSET(c, TTY_ERRORMASK));
 	if (err) {
 		CLR(c, TTY_ERRORMASK);
@@ -317,11 +337,7 @@ parmrk:
 				c = 0;
 		}
 	}
-	/*
-	 * In tandem mode, check high water mark.
-	 */
-	if (ISSET(iflag, IXOFF) || ISSET(tp->t_cflag, CRTS_IFLOW))
-		ttyblock(tp);
+
 	if (!ISSET(tp->t_state, TS_TYPEN) && ISSET(iflag, ISTRIP))
 		CLR(c, 0x80);
 	if (!ISSET(lflag, EXTPROC)) {
@@ -1109,6 +1125,7 @@ ttyflush(tp, rw)
 	register int s;
 
 	s = spltty();
+again:
 	if (rw & FWRITE)
 		CLR(tp->t_state, TS_TTSTOP);
 #ifdef sun4c						/* XXX */
@@ -1123,25 +1140,33 @@ ttyflush(tp, rw)
 		tp->t_rocol = 0;
 		CLR(tp->t_state, TS_LOCAL);
 		ttwakeup(tp);
+		if (ISSET(tp->t_state, TS_TBLOCK)) {
+			ttyunblock(tp);
+			if (ISSET(tp->t_iflag, IXOFF)) {
+				/*
+				 * XXX wait a bit in the hope that the stop
+				 * character (if any) will go out.  Waiting
+				 * isn't good since it allows races.  This
+				 * will be fixed when the stop character is
+				 * put in a special queue.  Don't bother with
+				 * the checks in ttywait() since the timeout
+				 * will save us.
+				 */
+				SET(tp->t_state, TS_SO_OCOMPLETE);
+				ttysleep(tp, TSA_OCOMPLETE(tp), TTOPRI,
+					 "ttyfls", hz / 10);
+				/*
+				 * Don't try sending the stop character again.
+				 */
+				CLR(tp->t_state, TS_TBLOCK);
+				goto again;
+			}
+		}
 	}
 	if (rw & FWRITE) {
 		FLUSHQ(&tp->t_outq);
 		ttwwakeup(tp);
 	}
-	if ((rw & FREAD) &&
-	    ISSET(tp->t_state, TS_TBLOCK) && tp->t_rawq.c_cc < TTYHOG/5) {
-		int queue_full = 0;
-
-		if (ISSET(tp->t_iflag, IXOFF) &&
-		    tp->t_cc[VSTART] != _POSIX_VDISABLE &&
-		    (queue_full = putc(tp->t_cc[VSTART], &tp->t_outq)) == 0 ||
-		    ISSET(tp->t_cflag, CRTS_IFLOW)) {
-			CLR(tp->t_state, TS_TBLOCK);
-			ttstart(tp);
-			if (queue_full) /* try again */
-				SET(tp->t_state, TS_TBLOCK);
-		}
-	}
 	splx(s);
 }
 
@@ -1168,34 +1193,37 @@ ttychars(tp)
 }
 
 /*
- * Send stop character on input overflow.
+ * Handle input high water.  Send stop character for the IXOFF case.  Turn
+ * on our input flow control bit and propagate the changes to the driver.
+ * XXX the stop character should be put in a special high priority queue.
+ */
+void
+ttyblock(tp)
+	struct tty *tp;
+{
+
+	SET(tp->t_state, TS_TBLOCK);
+	if (ISSET(tp->t_iflag, IXOFF) && tp->t_cc[VSTOP] != _POSIX_VDISABLE &&
+	    putc(tp->t_cc[VSTOP], &tp->t_outq) != 0)
+		CLR(tp->t_state, TS_TBLOCK);	/* try again later */
+	ttstart(tp);
+}
+
+/*
+ * Handle input low water.  Send start character for the IXOFF case.  Turn
+ * off our input flow control bit and propagate the changes to the driver.
+ * XXX the start character should be put in a special high priority queue.
  */
 static void
-ttyblock(tp)
-	register struct tty *tp;
+ttyunblock(tp)
+	struct tty *tp;
 {
-	register int total;
 
-	total = tp->t_rawq.c_cc + tp->t_canq.c_cc;
-	/*
-	 * Block further input iff: current input > threshold
-	 * AND input is available to user program.
-	 */
-	if (total >= TTYHOG / 2 &&
-	    !ISSET(tp->t_state, TS_TBLOCK) &&
-	    (!ISSET(tp->t_lflag, ICANON) || tp->t_canq.c_cc > 0)) {
-		int queue_full = 0;
-
-		if (ISSET(tp->t_iflag, IXOFF) &&
-		    tp->t_cc[VSTOP] != _POSIX_VDISABLE &&
-		    (queue_full = putc(tp->t_cc[VSTOP], &tp->t_outq)) == 0 ||
-		    ISSET(tp->t_cflag, CRTS_IFLOW)) {
-			SET(tp->t_state, TS_TBLOCK);
-			ttstart(tp);
-			if (queue_full) /* try again */
-				CLR(tp->t_state, TS_TBLOCK);
-		}
-	}
+	CLR(tp->t_state, TS_TBLOCK);
+	if (ISSET(tp->t_iflag, IXOFF) && tp->t_cc[VSTART] != _POSIX_VDISABLE &&
+	    putc(tp->t_cc[VSTART], &tp->t_outq) != 0)
+		SET(tp->t_state, TS_TBLOCK);	/* try again later */
+	ttstart(tp);
 }
 
 void
@@ -1590,26 +1618,18 @@ slowcase:
 			break;
 		first = 0;
 	}
+
+out:
 	/*
 	 * Look to unblock input now that (presumably)
 	 * the input queue has gone down.
 	 */
-out:
 	s = spltty();
-	if (ISSET(tp->t_state, TS_TBLOCK) && tp->t_rawq.c_cc < TTYHOG/5) {
-		int queue_full = 0;
-
-		if (ISSET(tp->t_iflag, IXOFF) &&
-		    cc[VSTART] != _POSIX_VDISABLE &&
-		    (queue_full = putc(cc[VSTART], &tp->t_outq)) == 0 ||
-		    ISSET(tp->t_cflag, CRTS_IFLOW)) {
-			CLR(tp->t_state, TS_TBLOCK);
-			ttstart(tp);
-			if (queue_full) /* try again */
-				SET(tp->t_state, TS_TBLOCK);
-		}
-	}
+	if (ISSET(tp->t_state, TS_TBLOCK) &&
+	    tp->t_rawq.c_cc + tp->t_canq.c_cc <= I_LOW_WATER)
+		ttyunblock(tp);
 	splx(s);
+
 	return (error);
 }
 
diff --git a/sys/sys/tty.h b/sys/sys/tty.h
index 121926f6665c..fb6c9776ee07 100644
--- a/sys/sys/tty.h
+++ b/sys/sys/tty.h
@@ -36,7 +36,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tty.h	8.6 (Berkeley) 1/21/94
- * $Id: tty.h,v 1.25 1995/07/29 13:40:13 bde Exp $
+ * $Id: tty.h,v 1.26 1995/07/30 12:39:42 bde Exp $
  */
 
 #ifndef _SYS_TTY_H_
@@ -241,6 +241,7 @@ int	 ttstart __P((struct tty *tp));
 void	 ttwakeup __P((struct tty *tp));
 int	 ttwrite __P((struct tty *tp, struct uio *uio, int flag));
 void	 ttwwakeup __P((struct tty *tp));
+void	 ttyblock __P((struct tty *tp));
 void	 ttychars __P((struct tty *tp));
 int	 ttycheckoutq __P((struct tty *tp, int wait));
 int	 ttyclose __P((struct tty *tp));