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

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Date:   Wed, 18 Oct 2023 20:34:23 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Jagath Jog J <jagathjog1996@...il.com>
Cc:     Matti Vaittinen <mazziesaccount@...il.com>,
        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
Subject: Re: [PATCH v5 2/3] iio: accel: Support Kionix/ROHM KX022A
 accelerometer

On Wed, 18 Oct 2023 01:37:12 +0530
Jagath Jog J <jagathjog1996@...il.com> wrote:

> 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



> 
> > +
> > +static int kx022a_read_avail(struct iio_dev *indio_dev,
> > +                            struct iio_chan_spec const *chan,
> > +                            const int **vals, int *type, int *length,
> > +                            long mask)
> > +{
> > +       switch (mask) {
> > +       case IIO_CHAN_INFO_SAMP_FREQ:
> > +               *vals = (const int *)kx022a_accel_samp_freq_table;
> > +               *length = ARRAY_SIZE(kx022a_accel_samp_freq_table) *
> > +                         ARRAY_SIZE(kx022a_accel_samp_freq_table[0]);
> > +               *type = IIO_VAL_INT_PLUS_MICRO;
> > +               return IIO_AVAIL_LIST;
> > +       case IIO_CHAN_INFO_SCALE:
> > +               *vals = (const int *)kx022a_scale_table;
> > +               *length = ARRAY_SIZE(kx022a_scale_table) *
> > +                         ARRAY_SIZE(kx022a_scale_table[0]);
> > +               *type = IIO_VAL_INT_PLUS_MICRO;
> > +               return IIO_AVAIL_LIST;
> > +       default:
> > +               return -EINVAL;
> > +       }
> > +}  
> 
> Regards
> Jagath