linux-kernel - Re: [PATCH 1/3] iio: adc: hx711: optimize sampling of data

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Date:   Mon, 9 Sep 2019 14:35:46 +0200
From:   Andreas Klinger <ak@...klinger.de>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     knaack.h@....de, lars@...afoo.de, pmeerw@...erw.net,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] iio: adc: hx711: optimize sampling of data

Hi Jonathan,

only patch 1 fixes a bug which is occurring on systems. In my test it happened
every one of about 10 - 15.000 measurements. But this depends on the interrupt
load of the system. With a high interrupt load there might be much more wrong
measurements.

Patch 2 is a performance optimization which prevents unneded reads. There is no
bug which is fixed by it.

I'll send out the patch set with more detailed commit messages.

Thanks,

Andreas


Jonathan Cameron <jic23@...nel.org> schrieb am So, 08. Sep 14:49:
> On Sat, 7 Sep 2019 12:18:00 +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.
> > 
> > Move query of DOUT at the latest point of time which is at the end of
> > PD_SCK low period.
> > 
> > Signed-off-by: Andreas Klinger <ak@...klinger.de>
> 
> Hi Andreas,
> 
> One thing I'm not clear on from these is how much a 'fix' they
> are.  That just effects whether we mark them for stable / push them
> out as quickly as possible or not.  So has this been seen in 
> normal operation?
> 
> + please add fixes tags to the two fixes.
> 
> For patch 3, it's in the very low importance category so it may
> well get forgotten if these two go through the fixes tree
> (up to you to remind me!)
> 
> 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)
> 

-- 
Andreas Klinger
Grabenreith 27
84508 Burgkirchen
+49 8623 919966
ak@...klinger.de
www.it-klinger.de
www.grabenreith.de