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Date:   Wed, 18 Oct 2023 20:34:23 +0100
From:   Jonathan Cameron <jic23@kernel.org>
To:     Jagath Jog J <jagathjog1996@gmail.com>
Cc:     Matti Vaittinen <mazziesaccount@gmail.com>,
        Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Dmitry Rokosov <DDRokosov@sberdevices.ru>,
        Cosmin Tanislav <demonsingur@gmail.com>,
        linux-iio@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 2/3] iio: accel: Support Kionix/ROHM KX022A
 accelerometer
Message-ID: <20231018203423.06f20a6c@jic23-huawei>
In-Reply-To: <CAM+2Eu+Xp6j1ppLd+zHMTu6jfc6DQKBShfe-nAyokVi0MUmoSA@mail.gmail.com>
References: <cover.1666614295.git.mazziesaccount@gmail.com>
        <758b00d6aea0a6431a5a3a78d557d449c113b21e.1666614295.git.mazziesaccount@gmail.com>
        <CAM+2Eu+Xp6j1ppLd+zHMTu6jfc6DQKBShfe-nAyokVi0MUmoSA@mail.gmail.com>
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On Wed, 18 Oct 2023 01:37:12 +0530
Jagath Jog J <jagathjog1996@gmail.com> wrote:

> Hi Matti,
>=20
> On Mon, Oct 24, 2022 at 6:10=E2=80=AFPM Matti Vaittinen
> <mazziesaccount@gmail.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 IR=
Q,
> > tap/motion detection, wake-up & back-to-sleep events, four acceleration
> > ranges (2, 4, 8 and 16g), and probably some other cool features. =20
>=20
> This is a nice driver, and I found it very helpful as a reference.
> One question regarding scale please see below.
>=20
> > + * range is typically +-2G/4G/8G/16G, distributed over the amount of b=
its.
> > + * The scale table can be calculated using
> > + *     (range / 2^bits) * g =3D (range / 2^bits) * 9.80665 m/s^2
> > + *     =3D> KX022A uses 16 bit (HiRes mode - assume the low 8 bits are=
 zeroed
> > + *     in low-power mode(?) )
> > + *     =3D> +/-2G  =3D> 4 / 2^16 * 9,80665 * 10^6 (to scale to micro)
> > + *     =3D> +/-2G  - 598.550415
> > + *        +/-4G  - 1197.10083
> > + *        +/-8G  - 2394.20166
> > + *        +/-16G - 4788.40332
> > + */
> > +static const int kx022a_scale_table[][2] =3D {
> > +       { 598, 550415 },
> > +       { 1197, 100830 },
> > +       { 2394, 201660 },
> > +       { 4788, 403320 },
> > +}; =20
>=20
> 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/s=C2=B2 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


>=20
> > +
> > +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 =3D (const int *)kx022a_accel_samp_freq_table;
> > +               *length =3D ARRAY_SIZE(kx022a_accel_samp_freq_table) *
> > +                         ARRAY_SIZE(kx022a_accel_samp_freq_table[0]);
> > +               *type =3D IIO_VAL_INT_PLUS_MICRO;
> > +               return IIO_AVAIL_LIST;
> > +       case IIO_CHAN_INFO_SCALE:
> > +               *vals =3D (const int *)kx022a_scale_table;
> > +               *length =3D ARRAY_SIZE(kx022a_scale_table) *
> > +                         ARRAY_SIZE(kx022a_scale_table[0]);
> > +               *type =3D IIO_VAL_INT_PLUS_MICRO;
> > +               return IIO_AVAIL_LIST;
> > +       default:
> > +               return -EINVAL;
> > +       }
> > +} =20
>=20
> Regards
> Jagath