linux-kernel - Re: [PATCH RFC] Input: cyapa: use time based retry loop

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Message-ID: <20170113201430.GA3063@osadl.at>
Date:   Fri, 13 Jan 2017 20:14:30 +0000
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Henrik Rydberg <rydberg@...math.org>,
        Dudley Du <dudl@...ress.com>, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] Input: cyapa: use time based retry loop

On Fri, Jan 13, 2017 at 12:07:45PM -0800, Dmitry Torokhov wrote:
> On Fri, Jan 13, 2017 at 08:57:36PM +0100, Nicholas Mc Guire wrote:
> > Using counter based retry loops for peripherals results in the delay
> > being significantly overrun during high-load situations where delay
> > functions tend to be vary imprecise and overrun there timeouts. So
> > condition the termination on the actual condition of 2s for the 
> > re-calibration to have been successful.
> > 
> > As this is a very long delay there is no advantage in using 
> > high-resolution timers thus switching this to msleep().
> > 
> > Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> > ---
> > Problem detected with coccinelle script (usleep_range for long delay)
> > 
> > The basic model for the fix up here is to use
> > 
> >   timeout = jiffies + (retry_time)
> >   do {
> >       check()
> >       mleep()
> >   } while (time_is_after_jiffies(timeout));
> > 
> > rather than using a counter and have very long retry loops under load
> > conditions. In the specific case it is sleep()/check() though to wait
> > on the previous write to complete. The initial intent was to move the
> > usleep_range() to msleep() as the delays are very long - if this
> > refactoring of the timeout loop is desirable or not is not clear (both
> > time and count based models seem to be common).
> > 
> > Patch was compile tested with: x86_64_defconfig + CONFIG_MOUSE_CYAPA=m
> > 
> > Patch is against 4.10-rc3 (localversion-next is next-20170113)
> > 
> >  drivers/input/mouse/cyapa_gen3.c | 27 ++++++++++++---------------
> >  1 file changed, 12 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/input/mouse/cyapa_gen3.c b/drivers/input/mouse/cyapa_gen3.c
> > index f9600753..0c544de 100644
> > --- a/drivers/input/mouse/cyapa_gen3.c
> > +++ b/drivers/input/mouse/cyapa_gen3.c
> > @@ -789,7 +789,7 @@ static ssize_t cyapa_gen3_do_calibrate(struct device *dev,
> >  				     const char *buf, size_t count)
> >  {
> >  	struct cyapa *cyapa = dev_get_drvdata(dev);
> > -	int tries;
> > +	unsigned long timeout;
> >  	int ret;
> >  
> >  	ret = cyapa_read_byte(cyapa, CYAPA_CMD_DEV_STATUS);
> > @@ -812,31 +812,28 @@ static ssize_t cyapa_gen3_do_calibrate(struct device *dev,
> >  		goto out;
> >  	}
> >  
> > -	tries = 20;  /* max recalibration timeout 2s. */
> > +	/* max recalibration timeout 2s. */
> > +	timeout = jiffies + 20 * HZ;
> 
> Doesn't this make timeout 20 sec instead of 2?

yes - my bad - that should have been 2

> 
> >  	do {
> >  		/*
> >  		 * For this recalibration, the max time will not exceed 2s.
> >  		 * The average time is approximately 500 - 700 ms, and we
> >  		 * will check the status every 100 - 200ms.
> >  		 */
> > -		usleep_range(100000, 200000);
> > -
> > +		msleep(100);
> >  		ret = cyapa_read_byte(cyapa, CYAPA_CMD_DEV_STATUS);
> >  		if (ret < 0) {
> > -			dev_err(dev, "Error reading dev status: %d\n",
> > -				ret);
> > +			dev_err(dev, "Error reading dev status: %d\n", ret);
> >  			goto out;
> >  		}
> > -		if ((ret & CYAPA_DEV_NORMAL) == CYAPA_DEV_NORMAL)
> > -			break;
> > -	} while (--tries);
> > +		if ((ret & CYAPA_DEV_NORMAL) == CYAPA_DEV_NORMAL) {
> > +			dev_dbg(dev, "Calibration successful.\n");
> > +			goto out;
> > +		}
> > +	} while (time_is_after_jiffies(timeout));
> >  
> > -	if (tries == 0) {
> > -		dev_err(dev, "Failed to calibrate. Timeout.\n");
> > -		ret = -ETIMEDOUT;
> > -		goto out;
> > -	}
> > -	dev_dbg(dev, "Calibration successful.\n");
> > +	dev_err(dev, "Failed to calibrate. Timeout.\n");
> > +	ret = -ETIMEDOUT;
> >  
> >  out:
> >  	return ret < 0 ? ret : count;
> > -- 
> > 2.1.4
> > 
> 
> -- 
> Dmitry