[RFC 1/2] n_tty: fix dropped output characters

[Johan Hovold <jhovold@gmail.com>]

Fix characters being dropped by n_tty_write() due to a failure to
check the return value of tty_put_char() in do_output_char().

Characters are currently being dropped by write if a tty driver claims
to have write room available, but still fails to buffer any data (e.g.
if a driver without internal buffer is run-time suspended between
write_room and write).

Note that process_output_block() handles such a failed buffer attempt,
but that the consecutive process_output() of the first character will
drop it. This could lead to the whole buffer being silently dropped
as the remainder of the buffer is processed in a loop.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
---
 drivers/tty/n_tty.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index d15624c1b751..d22a2d8f1cb7 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -513,7 +513,9 @@ static int do_output_char(unsigned char c, struct tty_struct *tty, int space)
 		break;
 	}
 
-	tty_put_char(tty, c);
+	if (!tty_put_char(tty, c))
+		return -1;
+
 	return 1;
 }
 
-- 
1.8.3.2