serial: Test for no tx data on tx restart

Commit 717f3bbab3,
'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>

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);

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);

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

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");
 }
 

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);
 	

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);