backup timer for UARTs that lose interrupts (take 3)

diff -r 6ed2da14626d drivers/serial/8250.c
@@ -1425,6 +1425,50 @@
 	mod_timer(&up->timer, jiffies + timeout);
 }
 
+static void serial8250_backup_timeout(unsigned long data)
+{
+	struct uart_8250_port *up = (struct uart_8250_port *)data;
+	unsigned int iir, ier = 0;
+	unsigned int timeout = up->port.timeout;
+
+	/*
+	 * Must disable interrupts or else we risk triggering an interrupt
+	 * and getting dead locked by the port.lock.
+	 */
+	if (is_real_interrupt(up->port.irq)) {
+		ier = serial_in(up, UART_IER);
+		serial_out(up, UART_IER, 0);
+	}
+
+	iir = serial_in(up, UART_IIR);
+
+	/*
+	 * This should be a safe test for anyone who doesn't trust the
+	 * IIR bits on their UART, but it's specifically designed for
+	 * the "Diva" UART used on the management processor on many HP
+	 * ia64 and parisc boxes.
+	 */
+	if ((iir & UART_IIR_NO_INT) && (up->ier & UART_IER_THRI) &&
+	    (!uart_circ_empty(&up->port.info->xmit) || up->port.x_char) &&
+	    (serial_in(up, UART_LSR) & UART_LSR_THRE)) {
+		iir &= ~(UART_IIR_ID | UART_IIR_NO_INT);
+		iir |= UART_IIR_THRI;
+	}
+
+	if (!(iir & UART_IIR_NO_INT)) {
+		spin_lock(&up->port.lock);
+		serial8250_handle_port(up, NULL);
+		spin_unlock(&up->port.lock);
+	}
+
+	if (is_real_interrupt(up->port.irq))
+		serial_out(up, UART_IER, ier);
+
+	timeout = timeout > 6 ? (timeout / 2 - 2) : 1;
+
+	mod_timer(&up->timer, jiffies + (timeout * 100));
+}
+
 static unsigned int serial8250_tx_empty(struct uart_port *port)
 {
 	struct uart_8250_port *up = (struct uart_8250_port *)port;

@@ -1491,6 +1535,36 @@
 		up->lcr &= ~UART_LCR_SBC;
 	serial_out(up, UART_LCR, up->lcr);
 	spin_unlock_irqrestore(&up->port.lock, flags);
+}
+
+#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE)
+
+/*
+ *	Wait for transmitter & holding register to empty
+ */
+static void wait_for_xmitr(struct uart_8250_port *up)
+{
+	unsigned int status, tmout = 10000;
+
+	/* Wait up to 10ms for the character(s) to be sent. */
+	do {
+		status = serial_in(up, UART_LSR);
+
+		if (status & UART_LSR_BI)
+			up->lsr_break_flag = UART_LSR_BI;
+
+		if (--tmout == 0)
+			break;
+		udelay(1);
+	} while ((status & BOTH_EMPTY) != BOTH_EMPTY);
+
+	/* Wait up to 1s for flow control if necessary */
+	if (up->port.flags & UPF_CONS_FLOW) {
+		tmout = 1000000;
+		while (--tmout &&
+		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
+			udelay(1);
+	}
 }
 
 #ifdef CONFIG_KGDB

@@ -1631,6 +1705,37 @@
 		up->bugs &= ~UART_BUG_TXEN;
 	}
 
+	if (is_real_interrupt(up->port.irq) && !timer_pending(&up->timer)) {
+		/*
+		 * Test for UARTs that do not reassert THRE when the
+		 * transmitter is idle and the interrupt has already
+		 * been cleared.  Real 16550s should always reassert
+		 * this interrupt whenever the transmitter is idle and
+		 * the interrupt is enabled.
+		 */
+		wait_for_xmitr(up);
+		serial_outp(up, UART_IER, UART_IER_THRI);
+		(void)serial_in(up, UART_IIR);
+		serial_outp(up, UART_IER, 0);
+		serial_outp(up, UART_IER, UART_IER_THRI);
+		iir = serial_in(up, UART_IIR);
+		serial_outp(up, UART_IER, 0);
+
+		/*
+		 * If the interrupt is not reasserted, setup a timer to
+		 * kick the UART on a regular basis.
+		 */
+		if (iir & UART_IIR_NO_INT) {
+			unsigned int timeout = up->port.timeout;
+
+			pr_debug("ttyS%d - using backup timer\n", port->line);
+			timeout = timeout > 6 ? (timeout / 2 - 2) : 1;
+			up->timer.function = serial8250_backup_timeout;
+			up->timer.data = (unsigned long)up;
+			mod_timer(&up->timer, jiffies + (timeout * 100));
+		}
+	}
+
 	spin_unlock_irqrestore(&up->port.lock, flags);
 
 	/*

@@ -1703,9 +1808,9 @@
 	 */
 	(void) serial_in(up, UART_RX);
 
-	if (!is_real_interrupt(up->port.irq))
-		del_timer_sync(&up->timer);
-	else
+	del_timer_sync(&up->timer);
+	up->timer.function = serial8250_timeout;
+	if (is_real_interrupt(up->port.irq))
 		serial_unlink_irq_chain(up);
 }
 

@@ -2212,36 +2317,6 @@
 
 #ifdef CONFIG_SERIAL_8250_CONSOLE
 
-#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE)
-
-/*
- *	Wait for transmitter & holding register to empty
- */
-static inline void wait_for_xmitr(struct uart_8250_port *up)
-{
-	unsigned int status, tmout = 10000;
-
-	/* Wait up to 10ms for the character(s) to be sent. */
-	do {
-		status = serial_in(up, UART_LSR);
-
-		if (status & UART_LSR_BI)
-			up->lsr_break_flag = UART_LSR_BI;
-
-		if (--tmout == 0)
-			break;
-		udelay(1);
-	} while ((status & BOTH_EMPTY) != BOTH_EMPTY);
-
-	/* Wait up to 1s for flow control if necessary */
-	if (up->port.flags & UPF_CONS_FLOW) {
-		tmout = 1000000;
-		while (--tmout &&
-		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
-			udelay(1);
-	}
-}
-
 /*
  *	Print a string to the serial port trying not to disturb
  *	any possible real use of the port...