Re: BUG: iio: mpu3050: Wrong temperature scale

From: Linus Walleij <linus.walleij@linaro.org>
To: Dmitry Osipenko <digetx@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Svyatoslav Ryhel <clamor95@gmail.com>,
	Nathan Royer <nroyer@invensense.com>,
	Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Subject: Re: BUG: iio: mpu3050: Wrong temperature scale
Date: Mon, 19 Apr 2021 12:07:48 +0200	[thread overview]
Message-ID: <CACRpkdbEue3OLpU0L_SDAsxpLTY7aqRP5sOZ90pF=o-Yb0ot4Q@mail.gmail.com> (raw)
In-Reply-To: <483ac17b-705a-38c3-54ee-7f0089262c03@gmail.com>

On Mon, Apr 19, 2021 at 8:06 AM Dmitry Osipenko <digetx@gmail.com> wrote:

> The driver uses
> (x+23000)/280 formula for the conversion of raw temperature value, which
> gives 82C for x=0, thus apparently formula is wrong because x=50000
> should give us ~25C.
>
> I tried to search for the datasheet with the formula, but couldn't find it.

There is no public datasheet. I have never seen a non-public datasheet
either.

As the initial submission of the driver says:

"This driver is based on information from the rough input driver
 in drivers/input/misc/mpu3050.c and the scratch misc driver
 posted by Nathan Royer in 2011. Some years have passed but this
 is finally a fully-fledged driver for this gyroscope. It was
 developed and tested on the Qualcomm APQ8060 Dragonboard."

Nathans submission:
https://lore.kernel.org/lkml/1309486707-1658-1-git-send-email-nroyer@invensense.com/
(you find the threads at the bottom)

This submission came from inside Invensense so it is the closest
authoritative source we have.

> Linus, will you be able to check whether the formula used by the driver
> is correct? Thanks in advance.

Sadly the code is the documentation when it comes to Invensense stuff,
I am CC:ing Nathans Invensense address in the vain hope he is still
working there and could help, also CC to Jean-Baptiste who was
there last year and maybe can help out.

I don't anymore remember exactly how I found this equation,
but it wasn't from any datasheet. I vaguely remember browsing
through some Android userspace sensor code.

What I tend to do is dig around in old mobile
phone Android trees, and there you sometimes find this information
in different GPL code drops. I bet I got it from browsing some of
those.

Here is an example (Tegra):
https://android.googlesource.com/kernel/tegra/+/dba2740d025c8e7e7e3c61d84a4f964d2c1c0ac9/drivers/misc/inv_mpu

Worst case what one *can* do is to calibrate the scale, like put
the device in a controlled environment of some two reasonably
far apart temperatures and measure, assuming it is at least
linear. Some professionals use controlled environment
chambers for this. But I hope there is a better way.

Yours,
Linus Walleij