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>

1 files changed, 2 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.