Serial: Do not read IIR in serial8250_start_tx when UART_BUG_TXEN

Do not read IIR in serial8250_start_tx when UART_BUG_TXEN

Reading the IIR clears some oustanding interrupts so it is not safe.
Instead, simply transmit immediately if the buffer is empty without
regard to IIR.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 3 insertions, 5 deletions

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index b75e63e9d1ee..c3e37c8e7e26 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1339,14 +1339,12 @@ static void serial8250_start_tx(struct uart_port *port)
                 serial_out(up, UART_IER, up->ier);
 
                 if (up->bugs & UART_BUG_TXEN) {
-                        unsigned char lsr, iir;
+                        unsigned char lsr;
                         lsr = serial_in(up, UART_LSR);
                         up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
-                        iir = serial_in(up, UART_IIR) & 0x0f;
                         if ((up->port.type == PORT_RM9000) ?
-                                (lsr & UART_LSR_THRE &&
-                                (iir == UART_IIR_NO_INT || iir == UART_IIR_THRI)) :
-                                (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT))
+                                (lsr & UART_LSR_THRE) :
+                                (lsr & UART_LSR_TEMT))
                                 transmit_chars(up);
                 }
         }