sc16is7xx: Allow sharing the IRQ line

When the interrupt line is shared with other devices, the IRQ must be
level-triggered, as only one device can trigger a falling edge. To support
this, try to acquire the IRQ with IRQF_TRIGGER_LOW|IRQF_SHARED first.

Interrupt controllers that lack support for level-triggers will return an
error, in which case the driver will now retry the acqusition with
IRQF_TRIGGER_FALLING, which was also the default before.

Signed-off-by: Daniel Mack <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/tty/serial/sc16is7xx.c

@@ -1293,7 +1293,19 @@ static int sc16is7xx_probe(struct device *dev,
 		sc16is7xx_power(&s->p[i].port, 0);
 	}
 
-	/* Setup interrupt */
+	/*
+	 * Setup interrupt. We first try to acquire the IRQ line as level IRQ.
+	 * If that succeeds, we can allow sharing the interrupt as well.
+	 * In case the interrupt controller doesn't support that, we fall
+	 * back to a non-shared falling-edge trigger.
+	 */
+	ret = devm_request_threaded_irq(dev, irq, NULL, sc16is7xx_irq,
+					IRQF_TRIGGER_LOW | IRQF_SHARED |
+					IRQF_ONESHOT,
+					dev_name(dev), s);
+	if (!ret)
+		return 0;
+
 	ret = devm_request_threaded_irq(dev, irq, NULL, sc16is7xx_irq,
 					IRQF_TRIGGER_FALLING | IRQF_ONESHOT,
 					dev_name(dev), s);