Re: [PATCH] TTY: Fix loss of echoed characters

[Joe Peterson <joe@skyrush.com>]

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Joe Peterson wrote:
> Andrew, attached is a patch that fixes the loss of echoed characters in
> two common cases: 1) tty output buffer is full due to lots of output and
> 2) tty is stopped.

Attached is a follow-on patch.  It just does some code style cleanup and
minor code efficiency tweaks.  Please apply it after the main patch
(probably should be combined into one patch).

						Thanks, Joe

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Minor code efficiency and style cleanup.  No behavior changes.
Apply after tty-fix-loss-of-echoed-characters.patch.

Signed-off-by: Joe Peterson <joe@skyrush.com>
---

--- linux/drivers/char/n_tty.c.orig	2008-08-20 18:19:06.000000000 -0600
+++ linux/drivers/char/n_tty.c	2008-08-25 13:41:30.000000000 -0600
@@ -446,8 +446,6 @@ static ssize_t process_output_block(stru
 		}
 	}
 break_out:
-	//if (tty->ops->flush_chars)
-	//	tty->ops->flush_chars(tty);
 	i = tty->ops->write(tty, buf, i);
 
 	unlock_kernel();
@@ -455,11 +453,11 @@ break_out:
 }
 
 /**
- *	process_echoes	-	write pending echoed characters
+ *	process_echoes	-	write pending echo characters
  *	@tty: terminal device
  *
- *	Write echoed (and other ldisc-generated) characters to the
- *	tty that have been stored previously in the echo buffer.
+ *	Write previously buffered echo (and other ldisc-generated)
+ *	characters to the tty.
  *
  *	Characters generated by the ldisc (including echoes) need to
  *	be buffered because the driver's write buffer can fill during
@@ -488,11 +486,11 @@ break_out:
 
 static void process_echoes(struct tty_struct *tty)
 {
-	int	space, num, nr;
+	int	space, nr;
 	unsigned char c;
 	unsigned char *cp, *buf_end;
 
-	if (!(num = tty->echo_cnt))
+	if (!tty->echo_cnt)
 		return;
 
 	lock_kernel();
@@ -501,7 +499,7 @@ static void process_echoes(struct tty_st
 
 	buf_end = tty->echo_buf + N_TTY_BUF_SIZE;
 	cp = tty->echo_buf + tty->echo_pos;
-	nr = num;
+	nr = tty->echo_cnt;
 	while (nr > 0) {
 		c = *cp;
 		if (c == ECHO_OP_START) {
@@ -635,14 +633,15 @@ static void process_echoes(struct tty_st
 			cp -= N_TTY_BUF_SIZE;
 	}
 
-	tty->echo_cnt = nr;
-	if (tty->echo_cnt == 0) {
+	if (nr == 0) {
 		tty->echo_pos = 0;
+		tty->echo_cnt = 0;
 		tty->echo_overrun = 0;
 	} else {
-		int num_processed = (num - nr);
+		int num_processed = tty->echo_cnt - nr;
 		tty->echo_pos += num_processed;
-		tty->echo_pos &= (N_TTY_BUF_SIZE - 1);
+		tty->echo_pos &= N_TTY_BUF_SIZE - 1;
+		tty->echo_cnt = nr;
 		if (num_processed > 0)
 			tty->echo_overrun = 0;
 	}
@@ -663,42 +662,43 @@ static void process_echoes(struct tty_st
 
 static void add_echo_byte(unsigned char c, struct tty_struct *tty)
 {
-	int 	add_char_pos;
+	int	new_byte_pos;
 	unsigned long flags;
 
 	spin_lock_irqsave(&tty->echo_lock, flags);
 
-	add_char_pos = tty->echo_pos + tty->echo_cnt;
-	if (add_char_pos >= N_TTY_BUF_SIZE)
-		add_char_pos -= N_TTY_BUF_SIZE;
-
-	/* Detect overrun */
 	if (tty->echo_cnt == N_TTY_BUF_SIZE) {
+		/* Circular buffer is already at capacity */
+		new_byte_pos = tty->echo_pos;
+
 		/*
-		 * If the start position pointer needs to be advanced
-		 * due to running out of buffer space, be sure it is
-		 * done in such a way as to remove whole
-		 * operation byte groups.
+		 * Since the buffer start position needs to be advanced,
+		 * be sure to step by a whole operation byte group.
 		 */
-		if (*(tty->echo_buf +
-		      (tty->echo_pos & (N_TTY_BUF_SIZE - 1))) == ECHO_OP_START)
+		if (tty->echo_buf[tty->echo_pos] == ECHO_OP_START)
 		{
-			if (*(tty->echo_buf +
-			      ((tty->echo_pos + 1) & (N_TTY_BUF_SIZE - 1))) == ECHO_OP_TAB_ERASE) {
+			if (tty->echo_buf[(tty->echo_pos + 1) &
+					  (N_TTY_BUF_SIZE - 1)] ==
+						ECHO_OP_TAB_ERASE) {
 				tty->echo_pos += 4;
 				tty->echo_cnt -= 3;
 			} else {
 				tty->echo_pos += 2;
 				tty->echo_cnt -= 1;
 			}
-		} else
+		} else {
 			tty->echo_pos++;
-		tty->echo_pos &= (N_TTY_BUF_SIZE - 1);
+		}
+		tty->echo_pos &= N_TTY_BUF_SIZE - 1;
+
 		tty->echo_overrun = 1;
-	} else
+	} else {
+		new_byte_pos = tty->echo_pos + tty->echo_cnt;
+		new_byte_pos &= N_TTY_BUF_SIZE - 1;
 		tty->echo_cnt++;
+	}
 
-	tty->echo_buf[add_char_pos] = c;
+	tty->echo_buf[new_byte_pos] = c;
 
 	spin_unlock_irqrestore(&tty->echo_lock, flags);
 }
@@ -762,8 +762,9 @@ static void echo_char_raw(unsigned char 
 	if (c == ECHO_OP_START) {
 		add_echo_byte(ECHO_OP_START, tty);
 		add_echo_byte(ECHO_OP_START, tty);
-	} else
+	} else {
 		add_echo_byte(c, tty);
+	}
 }
 
 /**