Re: [PATCH] iio: accel: bmc150: Use more consistent and accurate scale values

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 11 Jun 2021 21:41:41 +0300
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> On Fri, Jun 11, 2021 at 9:27 PM Stephan Gerhold <stephan@gerhold.net> wrote:
> >
> > It is quite strange that BMA222 and BMA222E have very close, yet
> > subtly different values in their scale tables. Comparing the datasheets
> > this is simply because the "Resolution" for the different measurement
> > ranges are documented with different precision.
> >
> > For example, for +-2g the BMA222 datasheet [1] suggests a resolution
> > of 15.6 mg/LSB, while the BMA222E datasheet [2] suggests 15.63 mg/LSB.
> >
> > Actually, there is no need to rely on the resolution given by the Bosch
> > datasheets. The resolution and scale can be calculated more consistently
> > and accurately using the range (e.g. +-2g) and the channel size (e.g. 8 bits).
> >
> > Distributing 4g (-2g to 2g) over 8 bits results in an exact resolution
> > of (4g / 2^8) = 15.625 mg/LSB which is the same value as in both datasheets,
> > just slightly more accurate. Multiplying g = 9.80665 m/s^2 we get a more
> > accurate value for the IIO scale table.
> >
> > Generalizing this we can calculate the scale tables more accurately using
> > (range / 2^bits) * g * 10^6 (because of IIO_VAL_INT_PLUS_MICRO).
> >
> > Document this and make the scale tables more consistent and accurate
> > for all the variants using that formula. Now the scale tables for
> > BMA222 and BMA222E are consistent and probably slightly more accurate.  
> 
> Oh, oh, one more HW vendor to blame for not being mathematically precise!
> We already had the very same issue in Intel and AMD documentation :-)
> 
> In full support of such fixes!
> Reviewed-by: Andy Shevchenko <andy.shevchenko@gnail.com>
> 
> > [1]: https://media.digikey.com/pdf/Data%20Sheets/Bosch/BMA222.pdf
> > [2]: https://www.mouser.com/datasheet/2/783/BST-BMA222E-DS004-06-1021076.pdf
> >
> > Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
> > Signed-off-by: Stephan Gerhold <stephan@gerhold.net>

Given these sensors are very inaccurate at the best of times
(bma222 is +- 100mg) this is unlikely to have any meaningful effect.
But consistency is good so I've applied it anyway :)

Applied to the togreg branch of iio.git and pushed out
as testing to see if 0-day is in a spiteful mood.

Jonathan



> > ---
> >  drivers/iio/accel/bmc150-accel-core.c | 46 ++++++++++++++-------------
> >  1 file changed, 24 insertions(+), 22 deletions(-)
> >
> > diff --git a/drivers/iio/accel/bmc150-accel-core.c b/drivers/iio/accel/bmc150-accel-core.c
> > index 43aecd4bf3a4..5ce384ebe6c7 100644
> > --- a/drivers/iio/accel/bmc150-accel-core.c
> > +++ b/drivers/iio/accel/bmc150-accel-core.c
> > @@ -1088,19 +1088,21 @@ static const struct iio_chan_spec bmc150_accel_channels[] =
> >  static const struct iio_chan_spec bma280_accel_channels[] =
> >         BMC150_ACCEL_CHANNELS(14);
> >
> > +/*
> > + * The range for the Bosch sensors is typically +-2g/4g/8g/16g, distributed
> > + * over the amount of bits (see above). The scale table can be calculated using
> > + *     (range / 2^bits) * g = (range / 2^bits) * 9.80665 m/s^2
> > + * e.g. for +-2g and 12 bits: (4 / 2^12) * 9.80665 m/s^2 = 0.0095768... m/s^2
> > + * Multiply 10^6 and round to get the values listed below.
> > + */
> >  static const struct bmc150_accel_chip_info bmc150_accel_chip_info_tbl[] = {
> >         {
> >                 .name = "BMA222",
> >                 .chip_id = 0x03,
> >                 .channels = bma222e_accel_channels,
> >                 .num_channels = ARRAY_SIZE(bma222e_accel_channels),
> > -               /*
> > -                * The datasheet page 17 says:
> > -                * 15.6, 31.3, 62.5 and 125 mg per LSB.
> > -                * IIO unit is m/s^2 so multiply by g = 9.80665 m/s^2.
> > -                */
> > -               .scale_table = { {152984, BMC150_ACCEL_DEF_RANGE_2G},
> > -                                {306948, BMC150_ACCEL_DEF_RANGE_4G},
> > +               .scale_table = { {153229, BMC150_ACCEL_DEF_RANGE_2G},
> > +                                {306458, BMC150_ACCEL_DEF_RANGE_4G},
> >                                  {612916, BMC150_ACCEL_DEF_RANGE_8G},
> >                                  {1225831, BMC150_ACCEL_DEF_RANGE_16G} },
> >         },
> > @@ -1109,9 +1111,9 @@ static const struct bmc150_accel_chip_info bmc150_accel_chip_info_tbl[] = {
> >                 .chip_id = 0xF8,
> >                 .channels = bma222e_accel_channels,
> >                 .num_channels = ARRAY_SIZE(bma222e_accel_channels),
> > -               .scale_table = { {153277, BMC150_ACCEL_DEF_RANGE_2G},
> > -                                {306457, BMC150_ACCEL_DEF_RANGE_4G},
> > -                                {612915, BMC150_ACCEL_DEF_RANGE_8G},
> > +               .scale_table = { {153229, BMC150_ACCEL_DEF_RANGE_2G},
> > +                                {306458, BMC150_ACCEL_DEF_RANGE_4G},
> > +                                {612916, BMC150_ACCEL_DEF_RANGE_8G},
> >                                  {1225831, BMC150_ACCEL_DEF_RANGE_16G} },
> >         },
> >         {
> > @@ -1119,30 +1121,30 @@ static const struct bmc150_accel_chip_info bmc150_accel_chip_info_tbl[] = {
> >                 .chip_id = 0xF9,
> >                 .channels = bma250e_accel_channels,
> >                 .num_channels = ARRAY_SIZE(bma250e_accel_channels),
> > -               .scale_table = { {38344, BMC150_ACCEL_DEF_RANGE_2G},
> > -                                {76590, BMC150_ACCEL_DEF_RANGE_4G},
> > -                                {153277, BMC150_ACCEL_DEF_RANGE_8G},
> > -                                {306457, BMC150_ACCEL_DEF_RANGE_16G} },
> > +               .scale_table = { {38307, BMC150_ACCEL_DEF_RANGE_2G},
> > +                                {76614, BMC150_ACCEL_DEF_RANGE_4G},
> > +                                {153229, BMC150_ACCEL_DEF_RANGE_8G},
> > +                                {306458, BMC150_ACCEL_DEF_RANGE_16G} },
> >         },
> >         {
> >                 .name = "BMA253/BMA254/BMA255/BMC150/BMI055",
> >                 .chip_id = 0xFA,
> >                 .channels = bmc150_accel_channels,
> >                 .num_channels = ARRAY_SIZE(bmc150_accel_channels),
> > -               .scale_table = { {9610, BMC150_ACCEL_DEF_RANGE_2G},
> > -                                {19122, BMC150_ACCEL_DEF_RANGE_4G},
> > -                                {38344, BMC150_ACCEL_DEF_RANGE_8G},
> > -                                {76590, BMC150_ACCEL_DEF_RANGE_16G} },
> > +               .scale_table = { {9577, BMC150_ACCEL_DEF_RANGE_2G},
> > +                                {19154, BMC150_ACCEL_DEF_RANGE_4G},
> > +                                {38307, BMC150_ACCEL_DEF_RANGE_8G},
> > +                                {76614, BMC150_ACCEL_DEF_RANGE_16G} },
> >         },
> >         {
> >                 .name = "BMA280",
> >                 .chip_id = 0xFB,
> >                 .channels = bma280_accel_channels,
> >                 .num_channels = ARRAY_SIZE(bma280_accel_channels),
> > -               .scale_table = { {2392, BMC150_ACCEL_DEF_RANGE_2G},
> > -                                {4785, BMC150_ACCEL_DEF_RANGE_4G},
> > -                                {9581, BMC150_ACCEL_DEF_RANGE_8G},
> > -                                {19152, BMC150_ACCEL_DEF_RANGE_16G} },
> > +               .scale_table = { {2394, BMC150_ACCEL_DEF_RANGE_2G},
> > +                                {4788, BMC150_ACCEL_DEF_RANGE_4G},
> > +                                {9577, BMC150_ACCEL_DEF_RANGE_8G},
> > +                                {19154, BMC150_ACCEL_DEF_RANGE_16G} },
> >         },
> >  };
> >
> > --
> > 2.32.0
> >  
> 
>