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Date:   Wed, 26 Jun 2019 19:21:34 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Andreas Klinger <ak@...klinger.de>
Cc:     linux-iio@...r.kernel.org, knaack.h@....de, lars@...afoo.de,
        pmeerw@...erw.net, zbynek.kocur@....cvut.cz,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: srf04: fix wrong limitation in distance measuring

On Sun, 23 Jun 2019 14:29:10 +0200
Andreas Klinger <ak@...klinger.de> wrote:

> The measured time value in the driver is limited to the maximum distance
> which can be read by the sensor. This limitation was wrong and is fixed
> by this patch.
> 
> It also takes into account that we are supporting a variety of sensors
> today and that the recently added sensors have a higher maximum
> distance range.
> 
> Suggested-by: Zbyněk Kocur <zbynek.kocur@....cvut.cz>
> Signed-off-by: Andreas Klinger <ak@...klinger.de>
Ideally I'm looking for Zbyněk to confirm that this addresses the
original question.

Thanks,

Jonathan

> ---
>  drivers/iio/proximity/srf04.c | 29 +++++++++++++++--------------
>  1 file changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/iio/proximity/srf04.c b/drivers/iio/proximity/srf04.c
> index 8b50d56b0a03..01eb8cc63076 100644
> --- a/drivers/iio/proximity/srf04.c
> +++ b/drivers/iio/proximity/srf04.c
> @@ -110,7 +110,7 @@ static int srf04_read(struct srf04_data *data)
>  	udelay(data->cfg->trigger_pulse_us);
>  	gpiod_set_value(data->gpiod_trig, 0);
>  
> -	/* it cannot take more than 20 ms */
> +	/* it should not take more than 20 ms until echo is rising */
>  	ret = wait_for_completion_killable_timeout(&data->rising, HZ/50);
>  	if (ret < 0) {
>  		mutex_unlock(&data->lock);
> @@ -120,7 +120,8 @@ static int srf04_read(struct srf04_data *data)
>  		return -ETIMEDOUT;
>  	}
>  
> -	ret = wait_for_completion_killable_timeout(&data->falling, HZ/50);
> +	/* it cannot take more than 50 ms until echo is falling */
> +	ret = wait_for_completion_killable_timeout(&data->falling, HZ/20);
>  	if (ret < 0) {
>  		mutex_unlock(&data->lock);
>  		return ret;
> @@ -135,19 +136,19 @@ static int srf04_read(struct srf04_data *data)
>  
>  	dt_ns = ktime_to_ns(ktime_dt);
>  	/*
> -	 * measuring more than 3 meters is beyond the capabilities of
> -	 * the sensor
> +	 * measuring more than 6,45 meters is beyond the capabilities of
> +	 * the supported sensors
>  	 * ==> filter out invalid results for not measuring echos of
>  	 *     another us sensor
>  	 *
>  	 * formula:
> -	 *         distance       3 m
> -	 * time = ---------- = --------- = 9404389 ns
> -	 *          speed       319 m/s
> +	 *         distance     6,45 * 2 m
> +	 * time = ---------- = ------------ = 40438871 ns
> +	 *          speed         319 m/s
>  	 *
>  	 * using a minimum speed at -20 °C of 319 m/s
>  	 */
> -	if (dt_ns > 9404389)
> +	if (dt_ns > 40438871)
>  		return -EIO;
>  
>  	time_ns = dt_ns;
> @@ -159,20 +160,20 @@ static int srf04_read(struct srf04_data *data)
>  	 *   with Temp in °C
>  	 *   and speed in m/s
>  	 *
> -	 * use 343 m/s as ultrasonic speed at 20 °C here in absence of the
> +	 * use 343,5 m/s as ultrasonic speed at 20 °C here in absence of the
>  	 * temperature
>  	 *
>  	 * therefore:
> -	 *             time     343
> -	 * distance = ------ * -----
> -	 *             10^6       2
> +	 *             time     343,5     time * 106
> +	 * distance = ------ * ------- = ------------
> +	 *             10^6         2         617176
>  	 *   with time in ns
>  	 *   and distance in mm (one way)
>  	 *
> -	 * because we limit to 3 meters the multiplication with 343 just
> +	 * because we limit to 6,45 meters the multiplication with 106 just
>  	 * fits into 32 bit
>  	 */
> -	distance_mm = time_ns * 343 / 2000000;
> +	distance_mm = time_ns * 106 / 617176;
>  
>  	return distance_mm;
>  }

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