linux-kernel - Re: [PATCH v5 2/3] iio: accel: Support Kionix/ROHM KX022A accelerometer

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Date:   Thu, 19 Oct 2023 08:53:31 +0300
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>,
        Jagath Jog J <jagathjog1996@...il.com>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dmitry Rokosov <DDRokosov@...rdevices.ru>,
        Cosmin Tanislav <demonsingur@...il.com>,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mehdi Djait <mehdi.djait.k@...il.com>
Subject: Re: [PATCH v5 2/3] iio: accel: Support Kionix/ROHM KX022A
 accelerometer

On 10/18/23 22:34, Jonathan Cameron wrote:
> On Wed, 18 Oct 2023 01:37:12 +0530
> Jagath Jog J <jagathjog1996@...il.com> wrote:

Hi Jagath - and thanks!

>> Hi Matti,
>>
>> On Mon, Oct 24, 2022 at 6:10 PM Matti Vaittinen
>> <mazziesaccount@...il.com> wrote:
>>>
>>> KX022A is a 3-axis accelerometer from ROHM/Kionix. The sensor features
>>> include variable ODRs, I2C and SPI control, FIFO/LIFO with watermark IRQ,
>>> tap/motion detection, wake-up & back-to-sleep events, four acceleration
>>> ranges (2, 4, 8 and 16g), and probably some other cool features.
>>
>> This is a nice driver, and I found it very helpful as a reference.
>> One question regarding scale please see below.
>>
>>> + * range is typically +-2G/4G/8G/16G, distributed over the amount of bits.
>>> + * The scale table can be calculated using
>>> + *     (range / 2^bits) * g = (range / 2^bits) * 9.80665 m/s^2
>>> + *     => KX022A uses 16 bit (HiRes mode - assume the low 8 bits are zeroed
>>> + *     in low-power mode(?) )
>>> + *     => +/-2G  => 4 / 2^16 * 9,80665 * 10^6 (to scale to micro)
>>> + *     => +/-2G  - 598.550415
>>> + *        +/-4G  - 1197.10083
>>> + *        +/-8G  - 2394.20166
>>> + *        +/-16G - 4788.40332
>>> + */
>>> +static const int kx022a_scale_table[][2] = {
>>> +       { 598, 550415 },
>>> +       { 1197, 100830 },
>>> +       { 2394, 201660 },
>>> +       { 4788, 403320 },
>>> +};
>>
>> Given that the integer part is non-zero, and
>> IIO_VAL_INT_PLUS_MICRO is returned for read_scale,
>> As raw value will never be fractional how does this
>> correspond to a reading of 9.8 m/s² for the Z-axis?
> 
> Definitely suspicious as should be in m/s^2 for an acceleration and
> it should be
> 
> 9.8*16/2^bits
> 
> So I think these are out by a factor of 10^6
I think you are right. Looks like I misinterpreted the meaning of 
IIO_VAL_INT_PLUS_MICRO when I took my first tour in the IIO with this 
driver. The comment above the scale table does support that assumption 
... 10^6 would match such a brainfart. (This is my first thought. I will 
take better look at this later today and see if I can come up with a fix 
if no-one else has sent a patch already).

I CC'd Mehdi who has also been working on this driver.

Regarding the KX022A - I am not aware of upstream users of this IC 
(yet). May be you're the first lucky one :) Hence, I am tempted to just 
fixing the driver - but it's Jonathan who will take the splatters when 
**** hits the fan - so it's his call to decide whether we can still fix 
this. _If_ there are users who have adapted to this buggy scale (users I 
am not aware of) then fix will break their apps. Mehdi, do you know any 
users of this upstream driver?

I will ping the HQ guy who has contacts to those who might be using the 
driver in a downstream repository and ask him to inform potential users.

It'd be very nice to get this fixed.

Sorry and thanks!

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~