Protect against broken hardware. In this particular case, protect against

H/W not de-asserting the interrupt at all. On x86, and because of the
following conditions, this results in a hard hang with interrupts disabled:
1.  The uart(4) driver uses a spin lock to protect against concurrent
    access to the H/W. Spin locks disable and restore interrupts.
2.  Restoring the interrupt on x86 always writes the flags register. Even
    if we're restoring the interrupt from disabled to disabled.
3.  The x86 CPU has a short window in which interrupts are enabled when the
    flags register is written.
4.  The uart(4) driver registers a fast interrupt by default.

To catch this case, we first try to clear any pending H/W interrupts and in
particular, before setting up the interrupt. This makes sure the interrupt
is masked on the PIC. The interrupt handler now has a limit set on the
number of iterations it'll go through to clear interrupt conditions. If the
limit is hit, the handler will return FILTER_SCHEDULE_THREAD. The attach
function will check for this return code and avoid setting up the interrupt
and foce polling in that case.

Obtained from:	Juniper Networks, Inc.

1 files changed, 60 insertions, 48 deletions

diff --git a/sys/dev/uart/uart_core.c b/sys/dev/uart/uart_core.c
index b6bed03..c2f53b8 100644
--- a/sys/dev/uart/uart_core.c
+++ b/sys/dev/uart/uart_core.c
@@ -248,10 +248,14 @@ static int
 uart_intr(void *arg)
 {
 	struct uart_softc *sc = arg;
-	int flag = 0, ipend;
+	int cnt, ipend;
 
-	while (!sc->sc_leaving && (ipend = UART_IPEND(sc)) != 0) {
-		flag = 1;
+	if (sc->sc_leaving)
+		return (FILTER_STRAY);
+
+	cnt = 0;
+	while (cnt < 20 && (ipend = UART_IPEND(sc)) != 0) {
+		cnt++;
 		if (ipend & SER_INT_OVERRUN)
 			uart_intr_overrun(sc);
 		if (ipend & SER_INT_BREAK)
@@ -269,7 +273,8 @@ uart_intr(void *arg)
 		    (timeout_t *)uart_intr, sc);
 	}
 
-	return((flag)?FILTER_HANDLED:FILTER_STRAY);
+	return ((cnt == 0) ? FILTER_STRAY :
+	    ((cnt == 20) ? FILTER_SCHEDULE_THREAD : FILTER_HANDLED));
 }
 
 serdev_intr_t *
@@ -390,7 +395,7 @@ uart_bus_attach(device_t dev)
 {
 	struct uart_softc *sc, *sc0;
 	const char *sep;
-	int error;
+	int error, filt;
 
 	/*
 	 * The sc_class field defines the type of UART we're going to work
@@ -430,33 +435,6 @@ uart_bus_attach(device_t dev)
 	sc->sc_bas.bsh = rman_get_bushandle(sc->sc_rres);
 	sc->sc_bas.bst = rman_get_bustag(sc->sc_rres);
 
-	sc->sc_irid = 0;
-	sc->sc_ires = bus_alloc_resource_any(dev, SYS_RES_IRQ, &sc->sc_irid,
-	    RF_ACTIVE | RF_SHAREABLE);
-	if (sc->sc_ires != NULL) {
-		error = bus_setup_intr(dev,
-		    sc->sc_ires, INTR_TYPE_TTY, 
-		    uart_intr, NULL, sc, &sc->sc_icookie);		    
-		if (error)
-			error = bus_setup_intr(dev,
-			    sc->sc_ires, INTR_TYPE_TTY | INTR_MPSAFE,
-			    NULL, (driver_intr_t *)uart_intr, sc, &sc->sc_icookie);
-		else
-			sc->sc_fastintr = 1;
-
-		if (error) {
-			device_printf(dev, "could not activate interrupt\n");
-			bus_release_resource(dev, SYS_RES_IRQ, sc->sc_irid,
-			    sc->sc_ires);
-			sc->sc_ires = NULL;
-		}
-	}
-	if (sc->sc_ires == NULL) {
-		/* No interrupt resource. Force polled mode. */
-		sc->sc_polled = 1;
-		callout_init(&sc->sc_timer, 1);
-	}
-
 	/*
 	 * Ensure there is room for at least three full FIFOs of data in the
 	 * receive buffer (handles the case of low-level drivers with huge
@@ -487,20 +465,6 @@ uart_bus_attach(device_t dev)
 		printf("\n");
 	}
 
-	if (bootverbose && (sc->sc_fastintr || sc->sc_polled)) {
-		sep = "";
-		device_print_prettyname(dev);
-		if (sc->sc_fastintr) {
-			printf("%sfast interrupt", sep);
-			sep = ", ";
-		}
-		if (sc->sc_polled) {
-			printf("%spolled mode", sep);
-			sep = ", ";
-		}
-		printf("\n");
-	}
-
 	if (sc->sc_sysdev != NULL) {
 		if (sc->sc_sysdev->baudrate == 0) {
 			if (UART_IOCTL(sc, UART_IOCTL_BAUD,
@@ -529,6 +493,56 @@ uart_bus_attach(device_t dev)
 	sc->sc_pps.ppscap = PPS_CAPTUREBOTH;
 	pps_init(&sc->sc_pps);
 
+	sc->sc_leaving = 0;
+	filt = uart_intr(sc);
+
+	/*
+	 * Don't use interrupts if we couldn't clear any pending interrupt
+	 * conditions. We may have broken H/W and polling is probably the
+	 * safest thing to do.
+	 */
+	if (filt != FILTER_SCHEDULE_THREAD) {
+		sc->sc_irid = 0;
+		sc->sc_ires = bus_alloc_resource_any(dev, SYS_RES_IRQ,
+		    &sc->sc_irid, RF_ACTIVE | RF_SHAREABLE);
+	}
+	if (sc->sc_ires != NULL) {
+		error = bus_setup_intr(dev, sc->sc_ires, INTR_TYPE_TTY,
+		    uart_intr, NULL, sc, &sc->sc_icookie);
+		sc->sc_fastintr = (error == 0) ? 1 : 0;
+
+		if (!sc->sc_fastintr)
+			error = bus_setup_intr(dev, sc->sc_ires,
+			    INTR_TYPE_TTY | INTR_MPSAFE, NULL,
+			    (driver_intr_t *)uart_intr, sc, &sc->sc_icookie);
+
+		if (error) {
+			device_printf(dev, "could not activate interrupt\n");
+			bus_release_resource(dev, SYS_RES_IRQ, sc->sc_irid,
+			    sc->sc_ires);
+			sc->sc_ires = NULL;
+		}
+	}
+	if (sc->sc_ires == NULL) {
+		/* No interrupt resource. Force polled mode. */
+		sc->sc_polled = 1;
+		callout_init(&sc->sc_timer, 1);
+	}
+
+	if (bootverbose && (sc->sc_fastintr || sc->sc_polled)) {
+		sep = "";
+		device_print_prettyname(dev);
+		if (sc->sc_fastintr) {
+			printf("%sfast interrupt", sep);
+			sep = ", ";
+		}
+		if (sc->sc_polled) {
+			printf("%spolled mode", sep);
+			sep = ", ";
+		}
+		printf("\n");
+	}
+
 	error = (sc->sc_sysdev != NULL && sc->sc_sysdev->attach != NULL)
 	    ? (*sc->sc_sysdev->attach)(sc) : uart_tty_attach(sc);
 	if (error)
@@ -537,8 +551,6 @@ uart_bus_attach(device_t dev)
 	if (sc->sc_sysdev != NULL)
 		sc->sc_sysdev->hwmtx = sc->sc_hwmtx;
 
-	sc->sc_leaving = 0;
-	uart_intr(sc);
 	return (0);
 
  fail: