iio:st_magn: Fix enable device after trigger

Currently, we enable the device before we enable the device trigger. At
high frequencies, this can cause interrupts that don't yet have a poll
function associated with them and are thus treated as spurious. At high
frequencies with level interrupts, this can even cause an interrupt storm
of repeated spurious interrupts (~100,000 on my Beagleboard with the
LSM9DS1 magnetometer). If these repeat too much, the interrupt will get
disabled and the device will stop functioning.

To prevent these problems, enable the device prior to enabling the device
trigger, and disable the divec prior to disabling the trigger. This means
there's no window of time during which the device creates interrupts but we
have no trigger to answer them.

Fixes: 90efe05 ("iio: st_sensors: harden interrupt handling")
Signed-off-by: Martin Kelly <martin@martingkelly.com>
Tested-by: Denis Ciocca <denis.ciocca@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/magnetometer/st_magn_buffer.c

@@ -30,11 +30,6 @@ int st_magn_trig_set_state(struct iio_trigger *trig, bool state)
 	return st_sensors_set_dataready_irq(indio_dev, state);
 }
 
-static int st_magn_buffer_preenable(struct iio_dev *indio_dev)
-{
-	return st_sensors_set_enable(indio_dev, true);
-}
-
 static int st_magn_buffer_postenable(struct iio_dev *indio_dev)
 {
 	int err;
@@ -50,7 +45,7 @@ static int st_magn_buffer_postenable(struct iio_dev *indio_dev)
 	if (err < 0)
 		goto st_magn_buffer_postenable_error;
 
-	return err;
+	return st_sensors_set_enable(indio_dev, true);
 
 st_magn_buffer_postenable_error:
 	kfree(mdata->buffer_data);
@@ -63,19 +58,18 @@ static int st_magn_buffer_predisable(struct iio_dev *indio_dev)
 	int err;
 	struct st_sensor_data *mdata = iio_priv(indio_dev);
 
-	err = iio_triggered_buffer_predisable(indio_dev);
+	err = st_sensors_set_enable(indio_dev, false);
 	if (err < 0)
 		goto st_magn_buffer_predisable_error;
 
-	err = st_sensors_set_enable(indio_dev, false);
+	err = iio_triggered_buffer_predisable(indio_dev);
 
 st_magn_buffer_predisable_error:
 	kfree(mdata->buffer_data);
 	return err;
 }
 
 static const struct iio_buffer_setup_ops st_magn_buffer_setup_ops = {
-	.preenable = &st_magn_buffer_preenable,
 	.postenable = &st_magn_buffer_postenable,
 	.predisable = &st_magn_buffer_predisable,
 };