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Subject[PATCH v2 1/3] 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 <ak@it-klinger.de>
---
drivers/iio/adc/hx711.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/adc/hx711.c b/drivers/iio/adc/hx711.c
index 88c7fe15003b..0678964dbd21 100644
--- a/drivers/iio/adc/hx711.c
+++ b/drivers/iio/adc/hx711.c
@@ -101,13 +101,14 @@ 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 +118,6 @@ 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 +125,7 @@ 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 +133,8 @@ 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)
--
2.11.0
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