serial: xuartps: Rewrite the interrupt handling logic

The existing interrupt handling logic has following issues.
- Upon a parity error with default configuration, the control
  never comes out of the ISR thereby hanging Linux.
- The error handling logic around framing and parity error are buggy.
  There are chances that the errors will never be captured.
This patch ensures that the status registers are cleared on all cases so
that a hang situation never arises.

Signed-off-by: Anirudha Sarangi <anirudh@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
[stelford@cadence.com: cherry picked from
https://github.com/Xilinx/linux-xlnx commit
ac297e20d399850d7a8e373b6eccf2e183c15165 with manual conflict resolution]
Signed-off-by: Scott Telford <stelford@cadence.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -198,58 +198,43 @@ struct cdns_platform_data {
 #define to_cdns_uart(_nb) container_of(_nb, struct cdns_uart, \
 		clk_rate_change_nb);
 
-static void cdns_uart_handle_rx(struct uart_port *port, unsigned int isrstatus)
+/**
+ * cdns_uart_handle_rx - Handle the received bytes along with Rx errors.
+ * @dev_id: Id of the UART port
+ * @isrstatus: The interrupt status register value as read
+ * Return: None
+ */
+static void cdns_uart_handle_rx(void *dev_id, unsigned int isrstatus)
 {
+	struct uart_port *port = (struct uart_port *)dev_id;
 	struct cdns_uart *cdns_uart = port->private_data;
-	bool is_rxbs_support;
+	unsigned int data;
 	unsigned int rxbs_status = 0;
 	unsigned int status_mask;
+	unsigned int framerrprocessed = 0;
+	char status = TTY_NORMAL;
+	bool is_rxbs_support;
 
 	is_rxbs_support = cdns_uart->quirks & CDNS_UART_RXBS_SUPPORT;
 
-	/*
-	 * There is no hardware break detection, so we interpret framing
-	 * error with all-zeros data as a break sequence. Most of the time,
-	 * there's another non-zero byte at the end of the sequence.
-	 */
-	if (!is_rxbs_support && (isrstatus & CDNS_UART_IXR_FRAMING)) {
-		while (!(readl(port->membase + CDNS_UART_SR) &
-					CDNS_UART_SR_RXEMPTY)) {
-			if (!readl(port->membase + CDNS_UART_FIFO)) {
-				port->read_status_mask |= CDNS_UART_IXR_BRK;
-				isrstatus &= ~CDNS_UART_IXR_FRAMING;
-			}
-		}
-		writel(CDNS_UART_IXR_FRAMING, port->membase + CDNS_UART_ISR);
-	}
-
-	/* drop byte with parity error if IGNPAR specified */
-	if (isrstatus & port->ignore_status_mask & CDNS_UART_IXR_PARITY)
-		isrstatus &= ~(CDNS_UART_IXR_RXTRIG | CDNS_UART_IXR_TOUT);
-
-	isrstatus &= port->read_status_mask;
-	isrstatus &= ~port->ignore_status_mask;
-	status_mask = port->read_status_mask;
-	status_mask &= ~port->ignore_status_mask;
-
-	if (!(isrstatus & (CDNS_UART_IXR_TOUT | CDNS_UART_IXR_RXTRIG)))
-		return;
-
-	while (!(readl(port->membase + CDNS_UART_SR) & CDNS_UART_SR_RXEMPTY)) {
-		u32 data;
-		char status = TTY_NORMAL;
-
+	while ((readl(port->membase + CDNS_UART_SR) &
+		CDNS_UART_SR_RXEMPTY) != CDNS_UART_SR_RXEMPTY) {
 		if (is_rxbs_support)
 			rxbs_status = readl(port->membase + CDNS_UART_RXBS);
-
 		data = readl(port->membase + CDNS_UART_FIFO);
-
-		/* Non-NULL byte after BREAK is garbage (99%) */
-		if (data && (port->read_status_mask & CDNS_UART_IXR_BRK)) {
-			port->read_status_mask &= ~CDNS_UART_IXR_BRK;
-			port->icount.brk++;
-			if (uart_handle_break(port))
+		port->icount.rx++;
+		/*
+		 * There is no hardware break detection in Zynq, so we interpret
+		 * framing error with all-zeros data as a break sequence.
+		 * Most of the time, there's another non-zero byte at the
+		 * end of the sequence.
+		 */
+		if (!is_rxbs_support && (isrstatus & CDNS_UART_IXR_FRAMING)) {
+			if (!data) {
+				port->read_status_mask |= CDNS_UART_IXR_BRK;
+				framerrprocessed = 1;
 				continue;
+			}
 		}
 		if (is_rxbs_support && (rxbs_status & CDNS_UART_RXBS_BRK)) {
 			port->icount.brk++;
@@ -258,74 +243,101 @@ static void cdns_uart_handle_rx(struct uart_port *port, unsigned int isrstatus)
 				continue;
 		}
 
-		if (uart_handle_sysrq_char(port, data))
-			continue;
+		isrstatus &= port->read_status_mask;
+		isrstatus &= ~port->ignore_status_mask;
+		status_mask = port->read_status_mask;
+		status_mask &= ~port->ignore_status_mask;
+
+		if ((isrstatus & CDNS_UART_IXR_TOUT) ||
+		    (isrstatus & CDNS_UART_IXR_RXTRIG)) {
+			if (data &&
+			    (port->read_status_mask & CDNS_UART_IXR_BRK)) {
+				port->read_status_mask &= ~CDNS_UART_IXR_BRK;
+				port->icount.brk++;
+				if (uart_handle_break(port))
+					continue;
+			}
 
-		port->icount.rx++;
+			if (uart_handle_sysrq_char(port, data))
+				continue;
 
-		if (is_rxbs_support) {
-			if ((rxbs_status & CDNS_UART_RXBS_PARITY)
-				&& (status_mask & CDNS_UART_IXR_PARITY)) {
-				port->icount.parity++;
-				status = TTY_PARITY;
+			if (is_rxbs_support) {
+				if ((rxbs_status & CDNS_UART_RXBS_PARITY)
+				    && (status_mask & CDNS_UART_IXR_PARITY)) {
+					port->icount.parity++;
+					status = TTY_PARITY;
+				}
+				if ((rxbs_status & CDNS_UART_RXBS_FRAMING)
+				    && (status_mask & CDNS_UART_IXR_PARITY)) {
+					port->icount.frame++;
+					status = TTY_FRAME;
+				}
+			} else {
+				if (isrstatus & CDNS_UART_IXR_PARITY) {
+					port->icount.parity++;
+					status = TTY_PARITY;
+				}
+				if ((isrstatus & CDNS_UART_IXR_FRAMING) &&
+				    !framerrprocessed) {
+					port->icount.frame++;
+					status = TTY_FRAME;
+				}
 			}
-			if ((rxbs_status & CDNS_UART_RXBS_FRAMING)
-				&& (status_mask & CDNS_UART_IXR_PARITY)) {
-				port->icount.frame++;
-				status = TTY_FRAME;
-			}
-		} else {
-			if (isrstatus & CDNS_UART_IXR_PARITY) {
-				port->icount.parity++;
-				status = TTY_PARITY;
-			}
-			if (isrstatus & CDNS_UART_IXR_FRAMING) {
-				port->icount.frame++;
-				status = TTY_FRAME;
+			if (isrstatus & CDNS_UART_IXR_OVERRUN) {
+				port->icount.overrun++;
+				tty_insert_flip_char(&port->state->port, 0,
+						     TTY_OVERRUN);
 			}
+			tty_insert_flip_char(&port->state->port, data, status);
 		}
-		if (isrstatus & CDNS_UART_IXR_OVERRUN)
-			port->icount.overrun++;
-
-		uart_insert_char(port, isrstatus, CDNS_UART_IXR_OVERRUN,
-				 data, status);
 	}
+	spin_unlock(&port->lock);
 	tty_flip_buffer_push(&port->state->port);
+	spin_lock(&port->lock);
 }
 
-static void cdns_uart_handle_tx(struct uart_port *port)
+/**
+ * cdns_uart_handle_tx - Handle the bytes to be Txed.
+ * @dev_id: Id of the UART port
+ * Return: None
+ */
+static void cdns_uart_handle_tx(void *dev_id)
 {
+	struct uart_port *port = (struct uart_port *)dev_id;
 	unsigned int numbytes;
 
 	if (uart_circ_empty(&port->state->xmit)) {
 		writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_IDR);
-		return;
-	}
-
-	numbytes = port->fifosize;
-	while (numbytes && !uart_circ_empty(&port->state->xmit) &&
-	       !(readl(port->membase + CDNS_UART_SR) & CDNS_UART_SR_TXFULL)) {
-		/*
-		 * Get the data from the UART circular buffer
-		 * and write it to the cdns_uart's TX_FIFO
-		 * register.
-		 */
-		writel(port->state->xmit.buf[port->state->xmit.tail],
-			port->membase + CDNS_UART_FIFO);
-		port->icount.tx++;
-
-		/*
-		 * Adjust the tail of the UART buffer and wrap
-		 * the buffer if it reaches limit.
-		 */
-		port->state->xmit.tail =
-			(port->state->xmit.tail + 1) & (UART_XMIT_SIZE - 1);
+	} else {
+		numbytes = port->fifosize;
+		while (numbytes && !uart_circ_empty(&port->state->xmit) &&
+		       !(readl(port->membase + CDNS_UART_SR) & CDNS_UART_SR_TXFULL)) {
+			/*
+			 * Get the data from the UART circular buffer
+			 * and write it to the cdns_uart's TX_FIFO
+			 * register.
+			 */
+			writel(
+				port->state->xmit.buf[port->state->xmit.
+				tail], port->membase + CDNS_UART_FIFO);
+
+			port->icount.tx++;
+
+			/*
+			 * Adjust the tail of the UART buffer and wrap
+			 * the buffer if it reaches limit.
+			 */
+			port->state->xmit.tail =
+				(port->state->xmit.tail + 1) &
+					(UART_XMIT_SIZE - 1);
+
+			numbytes--;
+		}
 
-		numbytes--;
+		if (uart_circ_chars_pending(
+				&port->state->xmit) < WAKEUP_CHARS)
+			uart_write_wakeup(port);
 	}
-
-	if (uart_circ_chars_pending(&port->state->xmit) < WAKEUP_CHARS)
-		uart_write_wakeup(port);
 }
 
 /**
@@ -338,27 +350,24 @@ static void cdns_uart_handle_tx(struct uart_port *port)
 static irqreturn_t cdns_uart_isr(int irq, void *dev_id)
 {
 	struct uart_port *port = (struct uart_port *)dev_id;
-	unsigned long flags;
 	unsigned int isrstatus;
 
-	spin_lock_irqsave(&port->lock, flags);
+	spin_lock(&port->lock);
 
 	/* Read the interrupt status register to determine which
-	 * interrupt(s) is/are active.
+	 * interrupt(s) is/are active and clear them.
 	 */
 	isrstatus = readl(port->membase + CDNS_UART_ISR);
-
-	if (isrstatus & CDNS_UART_RX_IRQS)
-		cdns_uart_handle_rx(port, isrstatus);
-
-	if ((isrstatus & CDNS_UART_IXR_TXEMPTY) == CDNS_UART_IXR_TXEMPTY)
-		cdns_uart_handle_tx(port);
-
 	writel(isrstatus, port->membase + CDNS_UART_ISR);
 
-	/* be sure to release the lock and tty before leaving */
-	spin_unlock_irqrestore(&port->lock, flags);
+	if (isrstatus & CDNS_UART_IXR_TXEMPTY) {
+		cdns_uart_handle_tx(dev_id);
+		isrstatus &= ~CDNS_UART_IXR_TXEMPTY;
+	}
+	if (isrstatus & CDNS_UART_IXR_MASK)
+		cdns_uart_handle_rx(dev_id, isrstatus);
 
+	spin_unlock(&port->lock);
 	return IRQ_HANDLED;
 }