linux-kernel - Re: [PATCH v3 2/3] iio: add MEMSensing MSA311 3-axis accelerometer driver

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Date:   Fri, 17 Jun 2022 18:54:14 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Dmitry Rokosov <DDRokosov@...rdevices.ru>
Cc:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "stano.jakubek@...il.com" <stano.jakubek@...il.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "jic23@...nel.org" <jic23@...nel.org>,
        "lars@...afoo.de" <lars@...afoo.de>,
        "stephan@...hold.net" <stephan@...hold.net>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] iio: add MEMSensing MSA311 3-axis accelerometer driver

On Fri, Jun 17, 2022 at 4:22 PM Dmitry Rokosov <DDRokosov@...rdevices.ru> wrote:
>
> On Thu, Jun 16, 2022 at 08:38:46PM +0200, Andy Shevchenko wrote:
> > On Thu, Jun 16, 2022 at 7:02 PM Dmitry Rokosov <DDRokosov@...rdevices.ru> wrote:
> > > On Thu, Jun 16, 2022 at 02:18:52PM +0200, Andy Shevchenko wrote:
> > > > On Thu, Jun 16, 2022 at 12:42 PM Dmitry Rokosov
> > > > <DDRokosov@...rdevices.ru> wrote:

...

> > > > > +       wait_ms = (USEC_PER_SEC * MSEC_PER_SEC) / freq_uhz;
> > > >
> > > > This looks very odd from a physics perspective: sec * sec * sec == sec ?!
> > > >
> > > > Perhaps you meant some HZ* macros from units.h?
> > >
> > > I suppose because of UHZ calculation I have to use NANO instead of
> > > USEC_PER_SEC in the following line:
> > >
> > >         freq_uhz = msa311_odr_table[odr].val * USEC_PER_SEC +
> > >                    msa311_odr_table[odr].val2;
> > >
> > > But below line is right from physics perspective. 1sec = 1/Hz, so
> > > msec = (USEC_PER_SEC / freq_uhz) * MSEC_PER_SEC:

I believe the first one should be HZ_PER_MHZ, then it will be fine.

> > >         wait_ms = (USEC_PER_SEC * MSEC_PER_SEC) / freq_uhz;
> > >
> > > Or do you mean that I should change MSEC_PER_SEC to just MILLI?
> >
> > 1 / Hz = 1 sec. That's how physics defines it. Try to figure out what
> > you meant by above multiplications / divisions and come up with the
> > best that fits your purposes.
>
> From my point of view, I've already implemented the best way to calculate
> how much time I need to wait for the next data chunk based on ODR Hz
> value :-)
>
> ODR value from the table has val integer part and val2 in microHz.
> By this line we calculate microHz conversion to take into account val2
> part:
>
>     freq_uhz = msa311_odr_table[odr].val * USEC_PER_SEC +
>                msa311_odr_table[odr].val2;
>
> By the next line we try to calculate miliseconds for msleep() from ODR
> microHz value:
>
>     wait_ms = (USEC_PER_SEC * MSEC_PER_SEC) / freq_uhz;
>
> (USEC_PER_SEC / freq_uhz) => seconds

> seconds * MSEC_PER_SEC => milliseconds>

> USEC_PER_SEC and MSEC_PER_SEC are just coefficients, they are not
> measured in "seconds" units.

Nope, it's a mistake. Those multipliers imply the unit. The rest are
the numbers. See above how to fix this (as far as I can tell).

...

> > > > > +                       if (err) {
> > > > > +                               dev_err(dev, "cannot update freq (%d)\n", err);
> > > > > +                               goto failed;
> > > > > +                       }
> > > >
> > > > Why is this inside the loop and more important under lock? Also you
> > > > may cover the initial error code by this message when moving it out of
> > > > the loop and lock.
> > > >
> > > > Ditto for other code snippets in other function(s) where applicable.
> > >
> > > Yes, I can move dev_err() outside of loop. But all ODR search loop
> > > should be under lock fully, because other msa311 operations should not
> > > be executed when we search proper ODR place.
> >
> > I didn't suggest getting rid of the lock.

> Sorry, I didn't get you... But I fully agree with you about dev_err()
> movement.

Yes, that's what I'm talking about. The dev_err() should be outside of
critical section, for example:

  mutex_unlock();
  if (ret) {
    dev_err(...);
    return ret;
  }
  ...
  return 0;

-- 
With Best Regards,
Andy Shevchenko