[PATCH AUTOSEL for 4.9 084/281] iio: hi8435: avoid garbage event at first enable

[Sasha Levin]

From: Nikita Yushchenko <nikita.yo...@cogentembedded.com>

[ 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.yo...@cogentembedded.com>
Signed-off-by: Jonathan Cameron <ji...@kernel.org>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
---
 drivers/iio/adc/hi8435.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/hi8435.c b/drivers/iio/adc/hi8435.c
index 678e8c7ea763..52b2dcf1d942 100644
--- a/drivers/iio/adc/hi8435.c
+++ b/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;
 }
-- 
2.14.1