tty: serial: 8250: omap: synchronize rx_running

The rx_running flag should show if DMA is currently active. However
there is a window between when the flag is set/cleared and when
the DMA is started/stopped. Because the flag is queried from both
hard and soft irq contexts, the driver can make incorrect
decisions and do things like start a DMA transfer using a buffer
that is already setup to be used for a DMA transfer.

This patch adds a spinlock to synchronize the rx_running flag and
close the above mentioned window.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -98,6 +98,7 @@ struct omap8250_priv {
 	struct pm_qos_request pm_qos_request;
 	struct work_struct qos_work;
 	struct uart_8250_dma omap8250_dma;
+	spinlock_t rx_dma_lock;
 };
 
 static u32 uart_read(struct uart_8250_port *up, u32 reg)
@@ -669,14 +670,21 @@ static int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir);
 
 static void __dma_rx_do_complete(struct uart_8250_port *p, bool error)
 {
+	struct omap8250_priv	*priv = p->port.private_data;
 	struct uart_8250_dma    *dma = p->dma;
 	struct tty_port         *tty_port = &p->port.state->port;
 	struct dma_tx_state     state;
 	int                     count;
+	unsigned long		flags;
 
 	dma_sync_single_for_cpu(dma->rxchan->device->dev, dma->rx_addr,
 				dma->rx_size, DMA_FROM_DEVICE);
 
+	spin_lock_irqsave(&priv->rx_dma_lock, flags);
+
+	if (!dma->rx_running)
+		goto unlock;
+
 	dma->rx_running = 0;
 	dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
 	dmaengine_terminate_all(dma->rxchan);
@@ -685,6 +693,9 @@ static void __dma_rx_do_complete(struct uart_8250_port *p, bool error)
 
 	tty_insert_flip_string(tty_port, dma->rx_buf, count);
 	p->port.icount.rx += count;
+unlock:
+	spin_unlock_irqrestore(&priv->rx_dma_lock, flags);
+
 	if (!error)
 		omap_8250_rx_dma(p, 0);
 
@@ -696,28 +707,45 @@ static void __dma_rx_complete(void *param)
 	__dma_rx_do_complete(param, false);
 }
 
+static void omap_8250_rx_dma_flush(struct uart_8250_port *p)
+{
+	struct omap8250_priv	*priv = p->port.private_data;
+	struct uart_8250_dma	*dma = p->dma;
+	unsigned long		flags;
+
+	spin_lock_irqsave(&priv->rx_dma_lock, flags);
+
+	if (!dma->rx_running) {
+		spin_unlock_irqrestore(&priv->rx_dma_lock, flags);
+		return;
+	}
+
+	dmaengine_pause(dma->rxchan);
+
+	spin_unlock_irqrestore(&priv->rx_dma_lock, flags);
+
+	__dma_rx_do_complete(p, true);
+}
+
 static int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
 {
+	struct omap8250_priv		*priv = p->port.private_data;
 	struct uart_8250_dma            *dma = p->dma;
+	int				err = 0;
 	struct dma_async_tx_descriptor  *desc;
+	unsigned long			flags;
 
 	switch (iir & 0x3f) {
 	case UART_IIR_RLSI:
 		/* 8250_core handles errors and break interrupts */
-		if (dma->rx_running) {
-			dmaengine_pause(dma->rxchan);
-			__dma_rx_do_complete(p, true);
-		}
+		omap_8250_rx_dma_flush(p);
 		return -EIO;
 	case UART_IIR_RX_TIMEOUT:
 		/*
 		 * If RCVR FIFO trigger level was not reached, complete the
 		 * transfer and let 8250_core copy the remaining data.
 		 */
-		if (dma->rx_running) {
-			dmaengine_pause(dma->rxchan);
-			__dma_rx_do_complete(p, true);
-		}
+		omap_8250_rx_dma_flush(p);
 		return -ETIMEDOUT;
 	case UART_IIR_RDI:
 		/*
@@ -729,24 +757,25 @@ static int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
 		 * the DMA won't do anything soon so we have to cancel the DMA
 		 * transfer and purge the FIFO manually.
 		 */
-		if (dma->rx_running) {
-			dmaengine_pause(dma->rxchan);
-			__dma_rx_do_complete(p, true);
-		}
+		omap_8250_rx_dma_flush(p);
 		return -ETIMEDOUT;
 
 	default:
 		break;
 	}
 
+	spin_lock_irqsave(&priv->rx_dma_lock, flags);
+
 	if (dma->rx_running)
-		return 0;
+		goto out;
 
 	desc = dmaengine_prep_slave_single(dma->rxchan, dma->rx_addr,
 					   dma->rx_size, DMA_DEV_TO_MEM,
 					   DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
-	if (!desc)
-		return -EBUSY;
+	if (!desc) {
+		err = -EBUSY;
+		goto out;
+	}
 
 	dma->rx_running = 1;
 	desc->callback = __dma_rx_complete;
@@ -758,7 +787,9 @@ static int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
 				   dma->rx_size, DMA_FROM_DEVICE);
 
 	dma_async_issue_pending(dma->rxchan);
-	return 0;
+out:
+	spin_unlock_irqrestore(&priv->rx_dma_lock, flags);
+	return err;
 }
 
 static int omap_8250_tx_dma(struct uart_8250_port *p);
@@ -1065,6 +1096,8 @@ static int omap8250_probe(struct platform_device *pdev)
 			   priv->latency);
 	INIT_WORK(&priv->qos_work, omap8250_uart_qos_work);
 
+	spin_lock_init(&priv->rx_dma_lock);
+
 	device_init_wakeup(&pdev->dev, true);
 	pm_runtime_use_autosuspend(&pdev->dev);
 	pm_runtime_set_autosuspend_delay(&pdev->dev, -1);