Re: [PATCH 2/4] iio: hi8435: avoid garbage event at first enable

[Vladimir Barinov]

Hi Nikita,

On 19.05.2017 17:48, Nikita Yushchenko wrote:
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>
---
  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 cb8e6342eddf..45a92e3e8f2b 100644
--- a/drivers/iio/adc/hi8435.c
+++ b/drivers/iio/adc/hi8435.c
@@ -141,10 +141,21 @@ 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);
  

This will race with hi8435_iio_push_event for priv->event_scan_mask.

Since you adding raw access then it is reasonable to initialize 
priv->event_prev_val in hi8435_read_raw.
Then use the following sequence for your application:
1. read raw value
2. enable events

Regards,
Vladimir