serial: Fix send_xchar() handlers

START_CHAR() & STOP_CHAR() can be disabled if set to '\0'
(__DISABLED_CHAR).  UART drivers which define a send_xchar()
handler must not transmit __DISABLED_CHAR.

Document requirement.

Affected drivers:
sunsab
sunhv

cc: David S. Miller <davem@davemloft.net>
cc: <sparclinux@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/serial/driver b/Documentation/serial/driver
index 3bba1aeb799c..ba64e4b892e9 100644
--- a/Documentation/serial/driver
+++ b/Documentation/serial/driver
@@ -140,6 +140,8 @@ hardware.
         will append the character to the circular buffer and then call
         start_tx() / stop_tx() to flush the data out.
 
+        Do not transmit if ch == '\0' (__DISABLED_CHAR).
+
         Locking: none.
         Interrupts: caller dependent.
 
diff --git a/drivers/tty/serial/sunhv.c b/drivers/tty/serial/sunhv.c
index 20521db2189f..25d43ce8b318 100644
--- a/drivers/tty/serial/sunhv.c
+++ b/drivers/tty/serial/sunhv.c
@@ -268,6 +268,9 @@ static void sunhv_send_xchar(struct uart_port *port, char ch)
         unsigned long flags;
         int limit = 10000;
 
+        if (ch == __DISABLED_CHAR)
+                return;
+
         spin_lock_irqsave(&port->lock, flags);
 
         while (limit-- > 0) {
diff --git a/drivers/tty/serial/sunsab.c b/drivers/tty/serial/sunsab.c
index b9598b227a45..0af75f8b98a4 100644
--- a/drivers/tty/serial/sunsab.c
+++ b/drivers/tty/serial/sunsab.c
@@ -468,6 +468,9 @@ static void sunsab_send_xchar(struct uart_port *port, char ch)
         struct uart_sunsab_port *up = (struct uart_sunsab_port *) port;
         unsigned long flags;
 
+        if (ch == __DISABLED_CHAR)
+                return;
+
         spin_lock_irqsave(&up->port.lock, flags);
 
         sunsab_tec_wait(up);