tty: cumulate and document tty_struct::flow* members

Group the flow flags under a single struct called flow. The new struct
contains 'stopped' and 'tco_stopped' bools which used to be bits in a
bitfield. The struct also contains the lock protecting them to
potentially share the same cache line.

Note that commit c545b66c6922b (tty: Serialize tcflow() with other tty
flow control changes) added a padding to the original bitfield. It was
for the bitfield to occupy a whole 64b word to avoid interferring stores
on Alpha (cannot we evaporate this arch with weird implications to C
code yet?). But it doesn't work as expected as the padding
(tty_struct::unused) is aligned to a 8B boundary too and occupies some
bytes from the next word.

So make it reliable by:
1) setting __aligned of the struct -- that aligns the start, and
2) making 'unsigned long unused[0]' as the last member of the struct --
   pads the end.

This is also the perfect time to start the documentation of tty_struct
where all this lives. So we start by documenting what these bools
actually serve for. And why we do all the alignment dances. Only the few
up-to-date information from the Theodore's comment made it into this new
Kerneldoc comment.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/20210505091928.22010-13-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index c32318da5190..3566bb577eb0 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1289,7 +1289,7 @@ static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
 		}
 	}
 
-	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
+	if (tty->flow.stopped && !tty->flow.tco_stopped && I_IXON(tty) && I_IXANY(tty)) {
 		start_tty(tty);
 		process_echoes(tty);
 	}
@@ -1398,7 +1398,7 @@ static void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
+	if (tty->flow.stopped && !tty->flow.tco_stopped && I_IXON(tty) && I_IXANY(tty)) {
 		start_tty(tty);
 		process_echoes(tty);
 	}
@@ -1427,7 +1427,7 @@ static void n_tty_receive_char_closing(struct tty_struct *tty, unsigned char c)
 		if (c == STOP_CHAR(tty))
 			stop_tty(tty);
 		else if (c == START_CHAR(tty) ||
-			 (tty->stopped && !tty->flow_stopped && I_IXANY(tty) &&
+			 (tty->flow.stopped && !tty->flow.tco_stopped && I_IXANY(tty) &&
 			  c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
 			  c != SUSP_CHAR(tty))) {
 			start_tty(tty);
@@ -1797,7 +1797,7 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 	 * Fix tty hang when I_IXON(tty) is cleared, but the tty
 	 * been stopped by STOP_CHAR(tty) before it.
 	 */
-	if (!I_IXON(tty) && old && (old->c_iflag & IXON) && !tty->flow_stopped) {
+	if (!I_IXON(tty) && old && (old->c_iflag & IXON) && !tty->flow.tco_stopped) {
 		start_tty(tty);
 		process_echoes(tty);
 	}