linux-kernel - Re: [PATCH v5 3/5] iio:temperature:mlx90632: Convert polling while loop to do-while

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Message-ID: <CAKv63uvQBoD=a2ADG7XBoQd2JRt6ggK0UB-g6cSuWLE7EV+qww@mail.gmail.com>
Date:   Fri, 14 Aug 2020 09:32:45 +0200
From:   Crt Mori <cmo@...exis.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/5] iio:temperature:mlx90632: Convert polling while
 loop to do-while

On Thu, 13 Aug 2020 at 21:41, Andy Shevchenko <andy.shevchenko@...il.com> wrote:
>
> On Thu, Aug 13, 2020 at 4:04 PM Crt Mori <cmo@...exis.com> wrote:
> > On Thu, 13 Aug 2020 at 13:24, Andy Shevchenko <andy.shevchenko@...il.com> wrote:
> > > On Thu, Aug 13, 2020 at 2:14 PM Crt Mori <cmo@...exis.com> wrote:
> > > > On Thu, 13 Aug 2020 at 13:03, Andy Shevchenko <andy.shevchenko@...il.com> wrote:
> > > > > On Thu, Aug 13, 2020 at 10:53 AM Crt Mori <cmo@...exis.com> wrote:
>
> ...
>
> > > > > I don't see how it prevents using iopoll.h. It uses usleep_range()
> > > > > under the hood in the same way you did here, but open coded.
> > > > >
> > > >
> > > > One loop is indeed 10ms and that is not the problem, the problem is
> > > > that timeout is at least 3 calls of this data ready (3 channels), so
> > > > that is at minimum 30ms of timeout, or it could even be 4 in worse
> > > > case scenario and that is outside of the range for usleep to measure.
> > > > So in case of the other loop, where we wait 200ms for channel refresh
> > > > it is also out of scope. Timeout should be in number of tries or in
> > > > msleep range if you ask me.
> > >
> > > I still didn't buy it. You have in both cases usleep_range(). Why in
> > > your case it's okay and in regmap_read_poll_timeout() is not?
> > >
> >
> > I tried and it did not work, so then I read the manual. Looking into
> >
> > * regmap_read_poll_timeout_atomic - Poll until a condition is met or a
> > timeout occurs
>
> Why _atomic?!

I just pasted something, it is the same as for non _atomic
>
> > ...
> >  * @delay_us: Time to udelay between reads in us (0 tight-loops).
> >  *            Should be less than ~10us since udelay is used
> >  *            (see Documentation/timers/timers-howto.rst).
> >  * @timeout_us: Timeout in us, 0 means never timeout
> >
> >
> > So I went to read Documentation/timers/timers-howto.rst
> >
> > SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
> > * Use usleep_range
> >
> > - Why not msleep for (1ms - 20ms)?
> > Explained originally here:
> > http://lkml.org/lkml/2007/8/3/250
> >
> > msleep(1~20) may not do what the caller intends, and
> > will often sleep longer (~20 ms actual sleep for any
> > value given in the 1~20ms range). In many cases this
> > is not the desired behavior.
> >
> > Since I am above the 20ms range, it is too much for usleep_range and
> > that proved to be a case as I got -ETIMEOUT and the desired channels
> > were not read.
> > > > > ...
> > > > >
> > > > > > -       while (tries-- > 0) {
> > > > > > +       do {
> > > > > >                 ret = regmap_read(data->regmap, MLX90632_REG_STATUS,
> > > > > >                                   &reg_status);
> > > > > >                 if (ret < 0)
> > > > > >                         return ret;
> > > > > > -               if (reg_status & MLX90632_STAT_DATA_RDY)
> > > > > > -                       break;
> > > > > >                 usleep_range(10000, 11000);
>
> You use here usleep_range(). The same is used for
> regmap_read_poll_timeout(). What's the difference?
>
> Since it uses 1/4 of the range you probably need to update tries and
> timeout_us to make it work.
>

Timeout_us here needs to be in one case 100 * 10ms (maybe not
realistic as we could live with number of around 40 * 10ms), but this
is a lot more than proposed range of usleep which Is up to 20ms. Even
in best case this timeout should be 40 ms to give enough time to
measure 2 channels for sure. So with the current timeout_us
requirement we are outside of the range of the udelay timer and that
is why I would need a macro with number of tries, not with the timeout
value (or timeout value of ms).


> > > > > > -       }
> > > > > > +       } while (!(reg_status & MLX90632_STAT_DATA_RDY) && tries--);
> > > > > >
> > > > > >         if (tries < 0) {
> > > > > >                 dev_err(&data->client->dev, "data not ready");
>
> --
> With Best Regards,
> Andy Shevchenko