mfd: Add fast path for WM831x touchscreen interrupts

The WM831x interrupt controller provides reporting of the touchscreen
related interrupts in the primary interrupt status register as a
performance optimisation - use this to avoid reading the secondary
status registers for those interrupts.

For code simplicity and to avoid iterating over all interrupts we open
code for the two affected interrupt sources.
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

drivers/mfd/wm831x-irq.c

@@ -449,6 +449,18 @@ static irqreturn_t wm831x_irq_thread(int irq, void *data)
 		goto out;
 	}
 
+	/* The touch interrupts are visible in the primary register as
+	 * an optimisation; open code this to avoid complicating the
+	 * main handling loop and so we can also skip iterating the
+	 * descriptors.
+	 */
+	if (primary & WM831X_TCHPD_INT)
+		handle_nested_irq(wm831x->irq_base + WM831X_IRQ_TCHPD);
+	if (primary & WM831X_TCHDATA_INT)
+		handle_nested_irq(wm831x->irq_base + WM831X_IRQ_TCHDATA);
+	if (primary & (WM831X_TCHDATA_EINT | WM831X_TCHPD_EINT))
+		goto out;
+
 	for (i = 0; i < ARRAY_SIZE(wm831x_irqs); i++) {
 		int offset = wm831x_irqs[i].reg - 1;