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

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Date:   Sun, 15 Sep 2019 13:55:43 +0100
From:   Jonathan Cameron <jic23@...23.retrosnub.co.uk>
To:     Andreas Klinger <ak@...klinger.de>
Cc:     knaack.h@....de, lars@...afoo.de, pmeerw@...erw.net,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] iio: adc: hx711: fix bug in sampling of data

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

> On Mon, 9 Sep 2019 14:37:21 +0200
> Andreas Klinger <ak@...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@...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)  
>