iio: adc: at91: disable adc channel interrupt in timeout case

Having a brief look at at91_adc_read_raw() it is obvious that in the case
of a timeout the setting of AT91_ADC_CHDR and AT91_ADC_IDR registers is
omitted. If 2 different channels are queried we can end up with a
situation where two interrupts are enabled, but only one interrupt is
cleared in the interrupt handler. Resulting in a interrupt loop and a
system hang.
Signed-off-by: Georg Ottinger <g.ottinger@abatec.at>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/at91_adc.c

@@ -704,23 +704,29 @@ static int at91_adc_read_raw(struct iio_dev *idev,
 		ret = wait_event_interruptible_timeout(st->wq_data_avail,
 						       st->done,
 						       msecs_to_jiffies(1000));
-		if (ret == 0)
-			ret = -ETIMEDOUT;
-		if (ret < 0) {
-			mutex_unlock(&st->lock);
-			return ret;
-		}
-
-		*val = st->last_value;
 
+		/* Disable interrupts, regardless if adc conversion was
+		 * successful or not
+		 */
 		at91_adc_writel(st, AT91_ADC_CHDR,
 				AT91_ADC_CH(chan->channel));
 		at91_adc_writel(st, AT91_ADC_IDR, BIT(chan->channel));
 
-		st->last_value = 0;
-		st->done = false;
+		if (ret > 0) {
+			/* a valid conversion took place */
+			*val = st->last_value;
+			st->last_value = 0;
+			st->done = false;
+			ret = IIO_VAL_INT;
+		} else if (ret == 0) {
+			/* conversion timeout */
+			dev_err(&idev->dev, "ADC Channel %d timeout.\n",
+				chan->channel);
+			ret = -ETIMEDOUT;
+		}
+
 		mutex_unlock(&st->lock);
-		return IIO_VAL_INT;
+		return ret;
 
 	case IIO_CHAN_INFO_SCALE:
 		*val = st->vref_mv;