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, 2 insertions, 2 deletions

diff --git a/drivers/tty/moxa.c b/drivers/tty/moxa.c
index 4d4f15b5cd29..847ad3dac107 100644
--- a/drivers/tty/moxa.c
+++ b/drivers/tty/moxa.c
@@ -1221,7 +1221,7 @@ static int moxa_write_room(struct tty_struct *tty)
 {
 	struct moxa_port *ch;
 
-	if (tty->stopped)
+	if (tty->flow.stopped)
 		return 0;
 	ch = tty->driver_data;
 	if (ch == NULL)
@@ -1374,7 +1374,7 @@ static int moxa_poll_port(struct moxa_port *p, unsigned int handle,
 			clear_bit(EMPTYWAIT, &p->statusflags);
 			tty_wakeup(tty);
 		}
-		if (test_bit(LOWWAIT, &p->statusflags) && !tty->stopped &&
+		if (test_bit(LOWWAIT, &p->statusflags) && !tty->flow.stopped &&
 				MoxaPortTxQueue(p) <= WAKEUP_CHARS) {
 			clear_bit(LOWWAIT, &p->statusflags);
 			tty_wakeup(tty);