Re: n_tty.c driver patch (semantic and performance correction) (all recent versions)

--- drivers/char/n_tty.c.orig	Tue Jan 29 18:26:54 2002
+++ drivers/char/n_tty.c	Thu Jan 31 02:28:51 2002

@@ -18,11 +18,35 @@
  * This file may be redistributed under the terms of the GNU General Public
  * License.
  *
+ * 2002/01/29	Fixed & Extended VMIN handling.
+ *		Patch By:  Robert White <rwhite@pobox.com>
+ *		Problem:  Where VTIME == 0 and VMIN > nr durring read,
+ *		   read would block for characters it couldn't possibly return.
+ *		   Reading variable sized blocks required multiple syscalls
+ *		   (termio set followed by read) or reliance on timeout.
+ *		   or multiple reads of fragments. For large reads n_tty
+ *		   was not capible of returning more than 255 bytes (bad
+ *		   for performance of any serial protocols. c.f. Z-MODEM etc)
+ *		Fix: if VMIN > nr, use nr to set minimum instead. (adjust down)
+ *		Extension: if VMIN == 255, always set minimum to nr (up OR down)
+ *		(Fix and Extension apply to each read call, VMIN itself is not adjusted)
+ *		Consider: (w/8 or 1500 chars from device to be delivered respectively)
+ *		VMIN=30, VTIME=0, read(fd,buf,8);
+ *		  Old: read never returns, New: returns immed on 8th char
+ *		VMIN=30, VTIME=200, read(fd,buf,8);
+ *		  Old: return in 20 seconds, New: returns immed on 8th char
+ *		VMIN=255, VTIME=0, read(fd,buf,1500); 
+ *		  Old: 5 reads of ~255 then deadlock, New: 1 read to complete
+ *		VMIN=255, VTIME>0, read(fd,buf,1500);
+ *		  Old: 5 reads then timout for remainder, New: 1 read
+ *			or more depending timeout occurences)
+ *
  * Reduced memory usage for older ARM systems  - Russell King
  *
  * 2000/01/20   Fixed SMP locking on put_tty_queue using bits of 
  *		the patch by Andrew J. Kroll <ag784@freenet.buffalo.edu>
  *		who actually finally proved there really was a race.
+ *		
  */
 
 #include <linux/types.h>

@@ -974,6 +998,11 @@
 	if (!tty->icanon) {
 		time = (HZ / 10) * TIME_CHAR(tty);
 		minimum = MIN_CHAR(tty);
+		/* Added rwhite@pobox.com Jan 29, 2002 */
+		if ((minimum == 255) || (minimum > nr))	{
+			minimum = nr;
+		}
+		/* End Addition */
 		if (minimum) {
 			if (time)
 				tty->minimum_to_wake = 1;

@@ -1021,6 +1050,16 @@
 		if (((minimum - (b - buf)) < tty->minimum_to_wake) &&
 		    ((minimum - (b - buf)) >= 1))
 			tty->minimum_to_wake = (minimum - (b - buf));
+
+		/* Added rwhite@pobox.com Jan 29, 2002 */
+		// minimum and therefore minimum_to_wake could be much larger
+		// than the actual buffer here, so...
+		if (tty->minimum_to_wake >= TTY_FLIPBUF_SIZE)	{
+			// Flow Control would deadlock at  (N_TTY_BUF_SIZE - 

TTY_THRESHOLD_THROTTLE)
+			// using TTY_FLIPBUF_SIZE-1 is safe and likely linear/streaming.
+			tty->minimum_to_wake = TTY_FLIPBUF_SIZE - 1;
+		}
+		/* End Addition */