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, 3 insertions, 3 deletions

diff --git a/drivers/tty/synclink_gt.c b/drivers/tty/synclink_gt.c
index 5523cf7bd1c2..1555dccc28af 100644
--- a/drivers/tty/synclink_gt.c
+++ b/drivers/tty/synclink_gt.c
@@ -768,7 +768,7 @@ static int write(struct tty_struct *tty,
 	if (!info->tx_buf || (count > info->max_frame_size))
 		return -EIO;
 
-	if (!count || tty->stopped || tty->hw_stopped)
+	if (!count || tty->flow.stopped || tty->hw_stopped)
 		return 0;
 
 	spin_lock_irqsave(&info->lock, flags);
@@ -889,7 +889,7 @@ static void flush_chars(struct tty_struct *tty)
 		return;
 	DBGINFO(("%s flush_chars entry tx_count=%d\n", info->device_name, info->tx_count));
 
-	if (info->tx_count <= 0 || tty->stopped ||
+	if (info->tx_count <= 0 || tty->flow.stopped ||
 	    tty->hw_stopped || !info->tx_buf)
 		return;
 
@@ -2241,7 +2241,7 @@ static void isr_txeom(struct slgt_info *info, unsigned short status)
 		else
 #endif
 		{
-			if (info->port.tty && (info->port.tty->stopped || info->port.tty->hw_stopped)) {
+			if (info->port.tty && (info->port.tty->flow.stopped || info->port.tty->hw_stopped)) {
 				tx_stop(info);
 				return;
 			}