iio: hi8435: avoid garbage event at first enable

BugLink: http://bugs.launchpad.net/bugs/1765010

[ Upstream commit ee19ac34 ]

Currently, driver generates events for channels if new reading differs
from previous one. This "previous value" is initialized to zero, which
results into event if value is constant-one.

Fix that by initializing "previous value" by reading at event enable
time.

This provides reliable sequence for userspace:
- enable event,
- AFTER THAT read current value,
- AFTER THAT each event will correspond to change.
Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/iio/adc/hi8435.c

@@ -121,10 +121,21 @@ static int hi8435_write_event_config(struct iio_dev *idev,
 				     enum iio_event_direction dir, int state)
 {
 	struct hi8435_priv *priv = iio_priv(idev);
+	int ret;
+	u32 tmp;
+
+	if (state) {
+		ret = hi8435_readl(priv, HI8435_SO31_0_REG, &tmp);
+		if (ret < 0)
+			return ret;
+		if (tmp & BIT(chan->channel))
+			priv->event_prev_val |= BIT(chan->channel);
+		else
+			priv->event_prev_val &= ~BIT(chan->channel);
 
-	priv->event_scan_mask &= ~BIT(chan->channel);
-	if (state)
 		priv->event_scan_mask |= BIT(chan->channel);
+	} else
+		priv->event_scan_mask &= ~BIT(chan->channel);
 
 	return 0;
 }