serial: imx: ensure that RX irqs are off if RX is off

Make sure that UCR1.RXDMAEN and UCR1.ATDMAEN (for the DMA case) and
UCR1.RRDYEN (for the PIO case) are off iff UCR1.RXEN is disabled. This
ensures that the fifo isn't read with RX disabled which results in an
exception.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -403,6 +403,28 @@ static void imx_port_rts_auto(struct imx_port *sport, u32 *ucr2)
 	*ucr2 |= UCR2_CTSC;
 }
 
+/* called with port.lock taken and irqs off */
+static void imx_start_rx(struct uart_port *port)
+{
+	struct imx_port *sport = (struct imx_port *)port;
+	unsigned int ucr1, ucr2;
+
+	ucr1 = imx_uart_readl(sport, UCR1);
+	ucr2 = imx_uart_readl(sport, UCR2);
+
+	ucr2 |= UCR2_RXEN;
+
+	if (sport->dma_is_enabled) {
+		ucr1 |= UCR1_RXDMAEN | UCR1_ATDMAEN;
+	} else {
+		ucr1 |= UCR1_RRDYEN;
+	}
+
+	/* Write UCR2 first as it includes RXEN */
+	imx_uart_writel(sport, ucr2, UCR2);
+	imx_uart_writel(sport, ucr1, UCR1);
+}
+
 /* called with port.lock taken and irqs off */
 static void imx_stop_tx(struct uart_port *port)
 {
@@ -427,9 +449,10 @@ static void imx_stop_tx(struct uart_port *port)
 			imx_port_rts_active(sport, &ucr2);
 		else
 			imx_port_rts_inactive(sport, &ucr2);
-		ucr2 |= UCR2_RXEN;
 		imx_uart_writel(sport, ucr2, UCR2);
 
+		imx_start_rx(port);
+
 		ucr4 = imx_uart_readl(sport, UCR4);
 		ucr4 &= ~UCR4_TCEN;
 		imx_uart_writel(sport, ucr4, UCR4);
@@ -442,12 +465,18 @@ static void imx_stop_rx(struct uart_port *port)
 	struct imx_port *sport = (struct imx_port *)port;
 	u32 ucr1, ucr2;
 
+	ucr1 = imx_uart_readl(sport, UCR1);
 	ucr2 = imx_uart_readl(sport, UCR2);
-	imx_uart_writel(sport, ucr2 & ~UCR2_RXEN, UCR2);
 
-	/* disable the `Receiver Ready Interrrupt` */
-	ucr1 = imx_uart_readl(sport, UCR1);
-	imx_uart_writel(sport, ucr1 & ~UCR1_RRDYEN, UCR1);
+	if (sport->dma_is_enabled) {
+		ucr1 &= ~(UCR1_RXDMAEN | UCR1_ATDMAEN);
+	} else {
+		ucr1 &= ~UCR1_RRDYEN;
+	}
+	imx_uart_writel(sport, ucr1, UCR1);
+
+	ucr2 &= ~UCR2_RXEN;
+	imx_uart_writel(sport, ucr2, UCR2);
 }
 
 /* called with port.lock taken and irqs off */
@@ -619,10 +648,11 @@ static void imx_start_tx(struct uart_port *port)
 			imx_port_rts_active(sport, &ucr2);
 		else
 			imx_port_rts_inactive(sport, &ucr2);
-		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
-			ucr2 &= ~UCR2_RXEN;
 		imx_uart_writel(sport, ucr2, UCR2);
 
+		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
+			imx_stop_rx(port);
+
 		/* enable transmitter and shifter empty irq */
 		ucr4 = imx_uart_readl(sport, UCR4);
 		ucr4 |= UCR4_TCEN;
@@ -1313,12 +1343,7 @@ static int imx_startup(struct uart_port *port)
 	imx_uart_writel(sport, USR1_RTSD | USR1_DTRD, USR1);
 	imx_uart_writel(sport, USR2_ORE, USR2);
 
-	if (dma_is_inited)
-		imx_enable_dma(sport);
-
 	ucr1 = imx_uart_readl(sport, UCR1) & ~UCR1_RRDYEN;
-	if (!sport->dma_is_enabled)
-		ucr1 |= UCR1_RRDYEN;
 	ucr1 |= UCR1_UARTEN;
 	if (sport->have_rtscts)
 		ucr1 |= UCR1_RTSDEN;
@@ -1361,13 +1386,14 @@ static int imx_startup(struct uart_port *port)
 	 */
 	imx_enable_ms(&sport->port);
 
-	/*
-	 * Start RX DMA immediately instead of waiting for RX FIFO interrupts.
-	 * In our iMX53 the average delay for the first reception dropped from
-	 * approximately 35000 microseconds to 1000 microseconds.
-	 */
-	if (sport->dma_is_enabled)
+	if (dma_is_inited) {
+		imx_enable_dma(sport);
 		start_rx_dma(sport);
+	} else {
+		ucr1 = imx_uart_readl(sport, UCR1);
+		ucr1 |= UCR1_RRDYEN;
+		imx_uart_writel(sport, ucr1, UCR1);
+	}
 
 	spin_unlock_irqrestore(&sport->port.lock, flags);
 
@@ -1413,7 +1439,7 @@ static void imx_shutdown(struct uart_port *port)
 
 	spin_lock_irqsave(&sport->port.lock, flags);
 	ucr1 = imx_uart_readl(sport, UCR1);
-	ucr1 &= ~(UCR1_TXMPTYEN | UCR1_RRDYEN | UCR1_RTSDEN | UCR1_UARTEN);
+	ucr1 &= ~(UCR1_TXMPTYEN | UCR1_RRDYEN | UCR1_RTSDEN | UCR1_UARTEN | UCR1_RXDMAEN | UCR1_ATDMAEN);
 
 	imx_uart_writel(sport, ucr1, UCR1);
 	spin_unlock_irqrestore(&sport->port.lock, flags);
@@ -1707,17 +1733,30 @@ static int imx_poll_init(struct uart_port *port)
 
 	spin_lock_irqsave(&sport->port.lock, flags);
 
+	/*
+	 * Be careful about the order of enabling bits here. First enable the
+	 * receiver (UARTEN + RXEN) and only then the corresponding irqs.
+	 * This prevents that a character that already sits in the RX fifo is
+	 * triggering an irq but the try to fetch it from there results in an
+	 * exception because UARTEN or RXEN is still off.
+	 */
 	ucr1 = imx_uart_readl(sport, UCR1);
+	ucr2 = imx_uart_readl(sport, UCR2);
+
 	if (is_imx1_uart(sport))
 		ucr1 |= IMX1_UCR1_UARTCLKEN;
-	ucr1 |= UCR1_UARTEN | UCR1_RRDYEN;
-	ucr1 &= ~(UCR1_TXMPTYEN | UCR1_RTSDEN);
-	imx_uart_writel(sport, ucr1, UCR1);
 
-	ucr2 = imx_uart_readl(sport, UCR2);
+	ucr1 |= UCR1_UARTEN;
+	ucr1 &= ~(UCR1_TXMPTYEN | UCR1_RTSDEN | UCR1_RRDYEN);
+
 	ucr2 |= UCR2_RXEN;
+
+	imx_uart_writel(sport, ucr1, UCR1);
 	imx_uart_writel(sport, ucr2, UCR2);
 
+	/* now enable irqs */
+	imx_uart_writel(sport, ucr1 | UCR1_RRDYEN, UCR1);
+
 	spin_unlock_irqrestore(&sport->port.lock, flags);
 
 	return 0;
@@ -1779,11 +1818,8 @@ static int imx_rs485_config(struct uart_port *port,
 
 	/* Make sure Rx is enabled in case Tx is active with Rx disabled */
 	if (!(rs485conf->flags & SER_RS485_ENABLED) ||
-	    rs485conf->flags & SER_RS485_RX_DURING_TX) {
-		ucr2 = imx_uart_readl(sport, UCR2);
-		ucr2 |= UCR2_RXEN;
-		imx_uart_writel(sport, ucr2, UCR2);
-	}
+	    rs485conf->flags & SER_RS485_RX_DURING_TX)
+		imx_start_rx(port);
 
 	port->rs485 = *rs485conf;