linux-kernel - Re: [PATCH v4] iio: chemical: Add support for Bosch BME680 sensor

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Message-ID: <CAEnQRZC80FWN9EADvE9geu773=vwRmE0JEwSfy_DMjig0d=qqw@mail.gmail.com>
Date:   Sat, 21 Jul 2018 20:45:34 +0300
From:   Daniel Baluta <daniel.baluta@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Himanshu Jha <himanshujha199640@...il.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald <pmeerw@...erw.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-iio <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v4] iio: chemical: Add support for Bosch BME680 sensor

On Sat, Jul 21, 2018 at 6:43 PM, Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Sat, Jul 21, 2018 at 6:36 PM, Himanshu Jha
> <himanshujha199640@...il.com> wrote:
>
>>> > +   /* Look up table 1 for the possible gas range values */
>>> > +   u32 lookupTable1[16] = {2147483647u, 2147483647u, 2147483647u,
>>> > +                           2147483647u, 2147483647u, 2126008810u,
>>> > +                           2147483647u, 2130303777u, 2147483647u,
>>> > +                           2147483647u, 2143188679u, 2136746228u,
>>> > +                           2147483647u, 2126008810u, 2147483647u,
>>> > +                           2147483647u};
>
> This one needs perhaps a bit of though, but...
>
>>> > +   /* Look up table 2 for the possible gas range values */
>>> > +   u32 lookupTable2[16] = {4096000000u, 2048000000u, 1024000000u,
>>> > +                           512000000u, 255744255u, 127110228u, 64000000u,
>>> > +                           32258064u, 16016016u, 8000000u, 4000000u,
>>> > +                           2000000u, 1000000u, 500000u, 250000u, 125000u};
>
> ...this one obviously just a not needed one. You may replace it with a
> one constant and simple calculation to get either value (index from
> value, or value from index).

Indeed this can be reduce to:

125.000 << (15 - idx).

The real question here is if we approximate 255.744.255u to 256.00.00u how
much different is the result. Being a gas sensor I think it is very
hard to appreciate.

We can go with this formula + adding a comment with the table with the
exact coefficients.

thanks,
Daniel.