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

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Date:   Fri, 17 Jun 2022 14:22:22 +0000
From:   Dmitry Rokosov <DDRokosov@...rdevices.ru>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
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 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:
> 
> ...
> 
> > > Not sure why you put those blank lines herey, it makes code not compact.
> >
> > Here I use blank lines to split fields from different registers.
> > In other words, in the msa311_fields enum one line contains fields from one
> > register. But for some heavy registers (like TAP_ACTIVE_STS) we have so many
> > fields and their declaration doesn't fit to 80 symbols.
> > So I've made a decision to split registers using blank lines.
> 
> Better is to add a comment explaining what register is described
> below, and not just a blank line.
> 
> ...
> 

Agreed, I'll do that in the v4.

...

> > > > +       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:
> >
> >         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.

> > > > +                       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.

> > > > +       mutex_lock(&msa311->lock);
> > > > +       err = regmap_field_write(msa311->fields[F_NEW_DATA_INT_EN], state);
> > > > +       mutex_unlock(&msa311->lock);
> > >
> > > > +
> > >
> > > No need.
> >
> > Sorry, I don't understand. We do not need to call it under lock, right?
> > I think we have to wrap it by msa311 lock, because other msa311
> > operations should not be executed when we enable or disable new data
> > interrupt (for example ODR value changing or something else).
> 
> The blank line is not needed, I specifically commented on the
> emphasized paragraph (by delimiting it with blank lines and leaving
> the rest for the better context for you to understand, it seems it did
> the opposite...).
> 
> ...
> 

Yep, didn't get you properly... Sorry for that...

-- 
Thank you,
Dmitry