serial: Test for no tx data on tx restart

Commit 717f3bbab3c7628736ef738fdbf3d9a28578c26c,
'serial_core: Fix conditional start_tx on ring buffer not empty'
exposes an incorrect assumption in several drivers' start_tx methods;
the tx ring buffer can, in fact, be empty when restarting tx while
performing flow control.

Affected drivers:
sunsab.c
ip22zilog.c
pmac_zilog.c
sunzilog.c
m32r_sio.c
imx.c

Other in-tree serial drivers either are not affected or already
test for empty tx ring buffer before transmitting.

Test for empty tx ring buffer in start_tx() method, after transmitting
x_char (if applicable).

Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Seth Bollinger <sethb@digi.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Cc: stable <stable@vger.kernel.org> # 3.15
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

6 files changed, 18 insertions, 3 deletions

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index e2f93874989b..56bd9aa56151 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -567,6 +567,9 @@ static void imx_start_tx(struct uart_port *port)
         struct imx_port *sport = (struct imx_port *)port;
         unsigned long temp;
 
+        if (uart_circ_empty(&port.state->xmit))
+                return;
+
         if (USE_IRDA(sport)) {
                 /* half duplex in IrDA mode; have to disable receive mode */
                 temp = readl(sport->port.membase + UCR4);
diff --git a/drivers/tty/serial/ip22zilog.c b/drivers/tty/serial/ip22zilog.c
index 1efd4c36ba0c..99b7b8697861 100644
--- a/drivers/tty/serial/ip22zilog.c
+++ b/drivers/tty/serial/ip22zilog.c
@@ -603,6 +603,8 @@ static void ip22zilog_start_tx(struct uart_port *port)
         } else {
                 struct circ_buf *xmit = &port->state->xmit;
 
+                if (uart_circ_empty(xmit))
+                        return;
                 writeb(xmit->buf[xmit->tail], &channel->data);
                 ZSDELAY();
                 ZS_WSYNC(channel);
diff --git a/drivers/tty/serial/m32r_sio.c b/drivers/tty/serial/m32r_sio.c
index 68f2c53e0b54..5702828fb62e 100644
--- a/drivers/tty/serial/m32r_sio.c
+++ b/drivers/tty/serial/m32r_sio.c
@@ -266,9 +266,11 @@ static void m32r_sio_start_tx(struct uart_port *port)
         if (!(up->ier & UART_IER_THRI)) {
                 up->ier |= UART_IER_THRI;
                 serial_out(up, UART_IER, up->ier);
-                serial_out(up, UART_TX, xmit->buf[xmit->tail]);
-                xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-                up->port.icount.tx++;
+                if (!uart_circ_empty(xmit)) {
+                        serial_out(up, UART_TX, xmit->buf[xmit->tail]);
+                        xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
+                        up->port.icount.tx++;
+                }
         }
         while((serial_in(up, UART_LSR) & UART_EMPTY) != UART_EMPTY);
 #else
diff --git a/drivers/tty/serial/pmac_zilog.c b/drivers/tty/serial/pmac_zilog.c
index 8193635103ee..f7ad5b903055 100644
--- a/drivers/tty/serial/pmac_zilog.c
+++ b/drivers/tty/serial/pmac_zilog.c
@@ -653,6 +653,8 @@ static void pmz_start_tx(struct uart_port *port)
         } else {
                 struct circ_buf *xmit = &port->state->xmit;
 
+                if (uart_circ_empty(xmit))
+                        goto out;
                 write_zsdata(uap, xmit->buf[xmit->tail]);
                 zssync(uap);
                 xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
@@ -661,6 +663,7 @@ static void pmz_start_tx(struct uart_port *port)
                 if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
                         uart_write_wakeup(&uap->port);
         }
+ out:
         pmz_debug("pmz: start_tx() done.\n");
 }
 
diff --git a/drivers/tty/serial/sunsab.c b/drivers/tty/serial/sunsab.c
index 80a58eca785b..2f57df9a71d9 100644
--- a/drivers/tty/serial/sunsab.c
+++ b/drivers/tty/serial/sunsab.c
@@ -427,6 +427,9 @@ static void sunsab_start_tx(struct uart_port *port)
         struct circ_buf *xmit = &up->port.state->xmit;
         int i;
 
+        if (uart_circ_empty(xmit))
+                return;
+
         up->interrupt_mask1 &= ~(SAB82532_IMR1_ALLS|SAB82532_IMR1_XPR);
         writeb(up->interrupt_mask1, &up->regs->w.imr1);
         
diff --git a/drivers/tty/serial/sunzilog.c b/drivers/tty/serial/sunzilog.c
index a85db8b87156..02df3940b95e 100644
--- a/drivers/tty/serial/sunzilog.c
+++ b/drivers/tty/serial/sunzilog.c
@@ -703,6 +703,8 @@ static void sunzilog_start_tx(struct uart_port *port)
         } else {
                 struct circ_buf *xmit = &port->state->xmit;
 
+                if (uart_circ_empty(xmit))
+                        return;
                 writeb(xmit->buf[xmit->tail], &channel->data);
                 ZSDELAY();
                 ZS_WSYNC(channel);