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

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Message-ID: <CAKv63utpidP12iQXWU59=F+cVCq6oShSAitmKhoC4W4aaPtnRA@mail.gmail.com>
Date:   Fri, 14 Aug 2020 11:42:07 +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 Fri, 14 Aug 2020 at 11:32, Andy Shevchenko <andy.shevchenko@...il.com> wrote:
>
> On Fri, Aug 14, 2020 at 10:33 AM Crt Mori <cmo@...exis.com> wrote:
> > 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
>
> OK.
>
> ...
>
> > > >  * @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
>
> ...
>
> > > > > > > >                 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).
>
> I do not understand. The regmap_read_poll_timeout() is a macro which
> unrolls in the very similar loop you have now in the code.
> What prevents it from using it?
>
> I think there is a big misunderstanding about the parameters of that macro.
> delay_us (must be small enough), timeout_us can be any long.
>
I tested on Beaglebone with the 100 * 10000 as timeout_us and I always
got the -ETIMEDOUT error. I also tested in the other case where
delay_us is 250000 and then timeout_us would be 4*250000 and I have
also received -ETIMEDOUT as a response.

I can prepare a patch with the iopoll.h API and maybe you will spot
the mistake, as after rechecking timeout_us is indeed 64bit and is
only used in the time comparison operations and not with timers.

> --
> With Best Regards,
> Andy Shevchenko