tty: Implement lookahead to process XON/XOFF timely

When tty is not read from, XON/XOFF may get stuck into an
intermediate buffer. As those characters are there to do software
flow-control, it is not very useful. In the case where neither end
reads from ttys, the receiving ends might not be able receive the
XOFF characters and just keep sending more data to the opposite
direction. This problem is almost guaranteed to occur with DMA
which sends data in large chunks.

If TTY is slow to process characters, that is, eats less than given
amount in receive_buf, invoke lookahead for the rest of the chars
to process potential XON/XOFF characters.

We need to keep track of how many characters have been processed by the
lookahead to avoid processing the flow control char again on the normal
path. Bookkeeping occurs parallel on two layers (tty_buffer and n_tty)
to avoid passing the lookahead_count through the whole call chain.

When a flow-control char is processed, two things must occur:
  a) it must not be treated as normal char
  b) if not yet processed, flow-control actions need to be taken
The return value of n_tty_receive_char_flow_ctrl() tells caller a), and
b) is kept internal to n_tty_receive_char_flow_ctrl().

If characters were previous looked ahead, __receive_buf() makes two
calls to the appropriate n_tty_receive_buf_* function. First call is
made with lookahead_done=true for the characters that were subject to
lookahead earlier and then with lookahead=false for the new characters.
Either of the calls might be skipped when it has no characters to
handle.

Reported-by: Gilles Buloz <gilles.buloz@kontron.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606153652.63554-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 75 insertions, 16 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 640c9e871044..8d80384df874 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -118,6 +118,9 @@ struct n_tty_data {
 	size_t read_tail;
 	size_t line_start;
 
+	/* # of chars looked ahead (to find software flow control chars) */
+	size_t lookahead_count;
+
 	/* protected by output lock */
 	unsigned int column;
 	unsigned int canon_column;
@@ -333,6 +336,8 @@ static void reset_buffer_flags(struct n_tty_data *ldata)
 	ldata->erasing = 0;
 	bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
 	ldata->push = 0;
+
+	ldata->lookahead_count = 0;
 }
 
 static void n_tty_packet_mode_flush(struct tty_struct *tty)
@@ -1225,12 +1230,30 @@ static bool n_tty_is_char_flow_ctrl(struct tty_struct *tty, unsigned char c)
 	return c == START_CHAR(tty) || c == STOP_CHAR(tty);
 }
 
-/* Returns true if c is consumed as flow-control character */
-static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, unsigned char c)
+/**
+ * n_tty_receive_char_flow_ctrl - receive flow control chars
+ * @tty: terminal device
+ * @c: character
+ * @lookahead_done: lookahead has processed this character already
+ *
+ * Receive and process flow control character actions.
+ *
+ * In case lookahead for flow control chars already handled the character in
+ * advance to the normal receive, the actions are skipped during normal
+ * receive.
+ *
+ * Returns true if @c is consumed as flow-control character, the character
+ * must not be treated as normal character.
+ */
+static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, unsigned char c,
+					 bool lookahead_done)
 {
 	if (!n_tty_is_char_flow_ctrl(tty, c))
 		return false;
 
+	if (lookahead_done)
+		return true;
+
 	if (c == START_CHAR(tty)) {
 		start_tty(tty);
 		process_echoes(tty);
@@ -1242,11 +1265,12 @@ static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, unsigned char c
 	return true;
 }
 
-static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
+static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c,
+				       bool lookahead_done)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	if (I_IXON(tty) && n_tty_receive_char_flow_ctrl(tty, c))
+	if (I_IXON(tty) && n_tty_receive_char_flow_ctrl(tty, c, lookahead_done))
 		return;
 
 	if (L_ISIG(tty)) {
@@ -1401,7 +1425,8 @@ static void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
 	put_tty_queue(c, ldata);
 }
 
-static void n_tty_receive_char_closing(struct tty_struct *tty, unsigned char c)
+static void n_tty_receive_char_closing(struct tty_struct *tty, unsigned char c,
+				       bool lookahead_done)
 {
 	if (I_ISTRIP(tty))
 		c &= 0x7f;
@@ -1409,9 +1434,12 @@ static void n_tty_receive_char_closing(struct tty_struct *tty, unsigned char c)
 		c = tolower(c);
 
 	if (I_IXON(tty)) {
-		if (c == STOP_CHAR(tty))
-			stop_tty(tty);
-		else if (c == START_CHAR(tty) ||
+		if (c == STOP_CHAR(tty)) {
+			if (!lookahead_done)
+				stop_tty(tty);
+		} else if (c == START_CHAR(tty) && lookahead_done) {
+			return;
+		} else if (c == START_CHAR(tty) ||
 			 (tty->flow.stopped && !tty->flow.tco_stopped && I_IXANY(tty) &&
 			  c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
 			  c != SUSP_CHAR(tty))) {
@@ -1457,6 +1485,27 @@ n_tty_receive_char_lnext(struct tty_struct *tty, unsigned char c, char flag)
 		n_tty_receive_char_flagged(tty, c, flag);
 }
 
+/* Caller must ensure count > 0 */
+static void n_tty_lookahead_flow_ctrl(struct tty_struct *tty, const unsigned char *cp,
+				      const unsigned char *fp, unsigned int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	unsigned char flag = TTY_NORMAL;
+
+	ldata->lookahead_count += count;
+
+	if (!I_IXON(tty))
+		return;
+
+	while (count--) {
+		if (fp)
+			flag = *fp++;
+		if (likely(flag == TTY_NORMAL))
+			n_tty_receive_char_flow_ctrl(tty, *cp, false);
+		cp++;
+	}
+}
+
 static void
 n_tty_receive_buf_real_raw(struct tty_struct *tty, const unsigned char *cp,
 			   const char *fp, int count)
@@ -1496,7 +1545,7 @@ n_tty_receive_buf_raw(struct tty_struct *tty, const unsigned char *cp,
 
 static void
 n_tty_receive_buf_closing(struct tty_struct *tty, const unsigned char *cp,
-			  const char *fp, int count)
+			  const char *fp, int count, bool lookahead_done)
 {
 	char flag = TTY_NORMAL;
 
@@ -1504,12 +1553,12 @@ n_tty_receive_buf_closing(struct tty_struct *tty, const unsigned char *cp,
 		if (fp)
 			flag = *fp++;
 		if (likely(flag == TTY_NORMAL))
-			n_tty_receive_char_closing(tty, *cp++);
+			n_tty_receive_char_closing(tty, *cp++, lookahead_done);
 	}
 }
 
 static void n_tty_receive_buf_standard(struct tty_struct *tty,
-		const unsigned char *cp, const char *fp, int count)
+		const unsigned char *cp, const char *fp, int count, bool lookahead_done)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 	char flag = TTY_NORMAL;
@@ -1540,7 +1589,7 @@ static void n_tty_receive_buf_standard(struct tty_struct *tty,
 		}
 
 		if (test_bit(c, ldata->char_map))
-			n_tty_receive_char_special(tty, c);
+			n_tty_receive_char_special(tty, c, lookahead_done);
 		else
 			n_tty_receive_char(tty, c);
 	}
@@ -1551,21 +1600,30 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
 {
 	struct n_tty_data *ldata = tty->disc_data;
 	bool preops = I_ISTRIP(tty) || (I_IUCLC(tty) && L_IEXTEN(tty));
+	size_t la_count = min_t(size_t, ldata->lookahead_count, count);
 
 	if (ldata->real_raw)
 		n_tty_receive_buf_real_raw(tty, cp, fp, count);
 	else if (ldata->raw || (L_EXTPROC(tty) && !preops))
 		n_tty_receive_buf_raw(tty, cp, fp, count);
-	else if (tty->closing && !L_EXTPROC(tty))
-		n_tty_receive_buf_closing(tty, cp, fp, count);
-	else {
-		n_tty_receive_buf_standard(tty, cp, fp, count);
+	else if (tty->closing && !L_EXTPROC(tty)) {
+		if (la_count > 0)
+			n_tty_receive_buf_closing(tty, cp, fp, la_count, true);
+		if (count > la_count)
+			n_tty_receive_buf_closing(tty, cp, fp, count - la_count, false);
+	} else {
+		if (la_count > 0)
+			n_tty_receive_buf_standard(tty, cp, fp, la_count, true);
+		if (count > la_count)
+			n_tty_receive_buf_standard(tty, cp, fp, count - la_count, false);
 
 		flush_echoes(tty);
 		if (tty->ops->flush_chars)
 			tty->ops->flush_chars(tty);
 	}
 
+	ldata->lookahead_count -= la_count;
+
 	if (ldata->icanon && !L_EXTPROC(tty))
 		return;
 
@@ -2446,6 +2504,7 @@ static struct tty_ldisc_ops n_tty_ops = {
 	.receive_buf     = n_tty_receive_buf,
 	.write_wakeup    = n_tty_write_wakeup,
 	.receive_buf2	 = n_tty_receive_buf2,
+	.lookahead_buf	 = n_tty_lookahead_flow_ctrl,
 };
 
 /**