[PATCH 4.9 083/310] iio: hi8435: avoid garbage event at first enable

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nikita Yushchenko <nikita.yoush@xxxxxxxxxxxxxxxxxx>


[ Upstream commit ee19ac340c5fdfd89c6348be4563453c61ab54a9 ]

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jonathan Cameron <jic23@xxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/iio/adc/hi8435.c |   15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

--- a/drivers/iio/adc/hi8435.c
+++ b/drivers/iio/adc/hi8435.c
@@ -121,10 +121,21 @@ static int hi8435_write_event_config(str
 				     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;
 }