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From:   Crt Mori <cmo@melexis.com>
Date:   Sat, 8 Aug 2020 23:57:59 +0200
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Subject: Re: [PATCH v4 2/4] iio:temperature:mlx90632: Adding extended
 calibration option
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        linux-iio <linux-iio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi,
I am very sorry you missed them, I thought you saw it (reply on v3 of
the patch). Maybe something happened to that mail, as it contained
link to datasheet, so I will omit it now.

Except for the order, only the remarks below are still open (did you
get the polling trail I did?)

On Sat, 8 Aug 2020 at 22:04, Andy Shevchenko <andy.shevchenko@gmail.com> wrote:
>
> On Sat, Aug 8, 2020 at 3:11 PM Crt Mori <cmo@melexis.com> wrote:
> >
> > For some time the market wants medical grade accuracy in medical range,
> > while still retaining the declared accuracy outside of the medical range
> > within the same sensor. That is why we created extended calibration
> > which is automatically switched to when object temperature is too high.
> >
> > This patch also introduces the object_ambient_temperature variable which
> > is needed for more accurate calculation of the object infra-red
> > footprint as sensor's ambient temperature might be totally different
> > than what the ambient temperature is at object and that is why we can
> > have some more errors which can be eliminated. Currently this temperature
> > is fixed at 25, but the interface to adjust it by user (with external
> > sensor or just IR measurement of the other object which acts as ambient),
> > will be introduced in another commit.
>
> The kernel doc patch should go before this patch.
>
> ...
>
> > +       *ambient_new_raw = (s16)read_tmp;
>
> > +       *ambient_old_raw = (s16)read_tmp;
>
> Sorry, did I miss your answer about these castings all over the patch?
>

These castings are in fact needed. You read unsigned integer, but the
return value is signed integer. Without the cast it did not extend the
signed bit, but just wrote the value to signed. Also I find it more
obvious with casts, that I did not "accidentally" convert to signed.

> ...
>
> > +       ret = regmap_read(regmap, MLX90632_RAM_1(17), &read_tmp);
> > +       ret = regmap_read(regmap, MLX90632_RAM_2(17), &read_tmp);
> > +       ret = regmap_read(regmap, MLX90632_RAM_1(18), &read_tmp);
> > +       ret = regmap_read(regmap, MLX90632_RAM_2(18), &read_tmp);
> > +       ret = regmap_read(regmap, MLX90632_RAM_1(19), &read_tmp);
> > +       ret = regmap_read(regmap, MLX90632_RAM_2(19), &read_tmp);
>
> What so special about these magic 17, 18, 19? Can you provide definitions?
>
When we started 0 to 19 were all open for access, from userspace, then
only 1 and 2 were used with calculations, and now we use 17, 18 and
19. Matter of fact is, I can't provide a descriptive name as it
depends on DSP version and as you can see now within the same DSP
version, also on the ID part. While RAM3 vs RAM1 and RAM2 could be
named RAM_OBJECT1, RAM_OBJECT2, RAM_AMBIENT, knowing our development
that might not be true in the next configuration, so I rather keep the
naming as in the datasheet.

> ...
>
> > +       int tries = 4;
>
> > +       while (tries-- > 0) {
> > +               ret = mlx90632_perform_measurement(data);
> > +               if (ret < 0)
> > +                       goto read_unlock;
> > +
> > +               if (ret == 19)
> > +                       break;
> > +       }
> > +       if (tries < 0) {
> > +               ret = -ETIMEDOUT;
> > +               goto read_unlock;
> > +       }
>
> Please avoid ping-pong type of changes in the same series (similar way
> as for kernel doc), which means don't introduce something you are
> going to change later on. Patch to move to do {} while () should go
> before this one.

OK, will fix that ordering in v5, but will wait till we solve also
above discussions to avoid adding new versions.

>
> --
> With Best Regards,
> Andy Shevchenko

And about that voodoo stuff with numbers:

Honestly, the equation is in the datasheet[1] and this is just making
floating point to fixed point with proper intermediate scaling
(initially I had defines of TENTOX, but that was not desired). There
is no better explanation of this voodoo.