linux-kernel - Re: [PATCH] iio: mxs-lradc: Fix temperature offset

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Message-ID: <20151007004447.GC3485@piout.net>
Date:	Wed, 7 Oct 2015 02:44:47 +0200
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Marek Vasut <marex@...x.de>
Cc:	Stefan Wahren <stefan.wahren@...e.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	Janusz Użycki <j.uzycki@...romaelectronics.com>,
	Jonathan Cameron <jic23@...nel.org>
Subject: Re: [PATCH] iio: mxs-lradc: Fix temperature offset

Hi,

On 07/10/2015 at 01:19:09 +0200, Marek Vasut wrote :
> >>/* The calculated value from the ADC is in Kelvin, we
> >>* want Celsius for hwmon so the offset is
> >>- * -272.15 * scale
> >>+ * -273.15 * scale
> >>*/
> >>- *val = -1075;
> >>- *val2 = 691699;
> >>+ *val = -1079;
> >>+ *val2 = 644268;
> >>
> >
> >i think it would be nice to break down the calculation of these magic values in
> >a second patch.
> 
> Indeed, I agree. I hope Alexandre can send another patch for that soon :-)
> 

On my side, I'm not sure this is worth more than a comment. If
necessary, I can write that the datasheet specifies that the scale is
4/1.012 here also.

Else you end up with something like that (naive implementation):

#define SCALE (4 * 1000 / 1012)

val = -273150000 * SCALE / 1000000;
val2 = 273150000 * SCALE + 1000000 * val;

I don't feel this is any clearer because we can't use floating points in
the kernel anyway.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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