iio: adc: hx711: fix bug in sampling of data

Fix bug in sampling function hx711_cycle() when interrupt occures while
PD_SCK is high. If PD_SCK is high for at least 60 us power down mode of
the sensor is entered which in turn leads to a wrong measurement.

Switch off interrupts during a PD_SCK high period and move query of DOUT
to the latest point of time which is at the end of PD_SCK low period.

This bug exists in the driver since it's initial addition. The more
interrupts on the system the higher is the probability that it happens.

Fixes: c3b2fdd0ea7e ("iio: adc: hx711: Add IIO driver for AVIA HX711")
Signed-off-by: Andreas Klinger <[email protected]>
Cc: <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>

diff --git a/drivers/iio/adc/hx711.c b/drivers/iio/adc/hx711.c
index 88c7fe15003b7179d8c4e565fcd7bb3415289baa..62e6c8badd22a8d645c9edb1e0d10784157a5b61 100644 (file)
--- a/drivers/iio/adc/hx711.c
+++ b/drivers/iio/adc/hx711.c
@@ -100,14 +100,14 @@ struct hx711_data {
 
 static int hx711_cycle(struct hx711_data *hx711_data)
 {
-       int val;
+       unsigned long flags;
 
        /*
         * if preempted for more then 60us while PD_SCK is high:
         * hx711 is going in reset
         * ==> measuring is false
         */
-       preempt_disable();
+       local_irq_save(flags);
        gpiod_set_value(hx711_data->gpiod_pd_sck, 1);
 
        /*
@@ -117,7 +117,6 @@ static int hx711_cycle(struct hx711_data *hx711_data)
         */
        ndelay(hx711_data->data_ready_delay_ns);
 
-       val = gpiod_get_value(hx711_data->gpiod_dout);
        /*
         * here we are not waiting for 0.2 us as suggested by the datasheet,
         * because the oscilloscope showed in a test scenario
@@ -125,7 +124,7 @@ static int hx711_cycle(struct hx711_data *hx711_data)
         * and 0.56 us for PD_SCK low on TI Sitara with 800 MHz
         */
        gpiod_set_value(hx711_data->gpiod_pd_sck, 0);
-       preempt_enable();
+       local_irq_restore(flags);
 
        /*
         * make it a square wave for addressing cases with capacitance on
@@ -133,7 +132,8 @@ static int hx711_cycle(struct hx711_data *hx711_data)
         */
        ndelay(hx711_data->data_ready_delay_ns);
 
-       return val;
+       /* sample as late as possible */
+       return gpiod_get_value(hx711_data->gpiod_dout);
 }
 
 static int hx711_read(struct hx711_data *hx711_data)