[PATCH v2 1/3] iio: adc: hx711: fix bug in sampling of data

[PATCH v2 1/3] iio: adc: hx711: fix bug in sampling of data

[Andreas Klinger]

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

Re: [PATCH v2 1/3] iio: adc: hx711: fix bug in sampling of data

[Jonathan Cameron]

On Mon, 9 Sep 2019 14:37:21 +0200
Andreas Klinger <ak@it-klinger.de> wrote:

> 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>

Applied to the fixes-togreg branch of iio.git and marked for stable.

Thanks,

Jonathan

> ---
>  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)

Re: [PATCH v2 1/3] iio: adc: hx711: fix bug in sampling of data

[Jonathan Cameron]

On Sun, 15 Sep 2019 13:53:26 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> On Mon, 9 Sep 2019 14:37:21 +0200
> Andreas Klinger <ak@it-klinger.de> wrote:
> 
> > 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>  
> 
> Applied to the fixes-togreg branch of iio.git and marked for stable.
> 
> Thanks,
I should have waited for my local tests to finish.  This result in
val being unused, so I've removed it.

Thanks,

Jonathan

> 
> Jonathan
> 
> > ---
> >  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)  
>