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

[Matti Vaittinen]

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

Hi Jagath - and thanks!

> > Hi Matti,
> >
> > On Mon, Oct 24, 2022 at 6:10 PM Matti Vaittinen
> > <mazziesaccount@xxxxxxxxx> 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

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