tty_set_ldisc() receive_room fix

Fix tty_set_ldisc in tty_io.c so that tty->receive_room is only cleared if
actually changing line disciplines.

Without this fix a problem occurs when requesting the line discipline to
change to the same line discipline.  In this case tty->receive_room is
cleared but ldisc->open() is not called to set tty->receive_room back to a
sane value.  The result is that tty->receive_room is stuck at 0 preventing
the tty flip buffer from passing receive data to the line discipline.

For example: a switch from N_TTY to N_TTY followed by a select() call for
read input results in data never being received because tty->receive_room
is stuck at zero.

A switch from N_TTY to N_TTY followed by a read() call works because the
read() call itself sets tty->receive_room correctly (but select does not).

Previously (< 2.6.18) this was not a problem because the tty flip buffer
pushed data to the line discipline without regard for tty->receive room.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 7710a6a77d97..bf5a00145c0b 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -934,13 +934,6 @@ restart:
                 return -EINVAL;
 
         /*
-         *      No more input please, we are switching. The new ldisc
-         *      will update this value in the ldisc open function
-         */
-
-        tty->receive_room = 0;
-
-        /*
          *      Problem: What do we do if this blocks ?
          */
 
@@ -951,6 +944,13 @@ restart:
                 return 0;
         }
 
+        /*
+         *      No more input please, we are switching. The new ldisc
+         *      will update this value in the ldisc open function
+         */
+
+        tty->receive_room = 0;
+
         o_ldisc = tty->ldisc;
         o_tty = tty->link;