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From:   Tomasz Figa <tomasz.figa@gmail.com>
Date:   Tue, 11 Aug 2020 18:41:20 +0200
Message-ID: <CA+Ln22FdFBPU5f0agknRN5xnUtJWOuGARfnsYh3ru_xdjoGC=A@mail.gmail.com>
Subject: Re: [PATCH v2] clk: samsung: Prevent potential endless loop in the
 PLL set_rate ops
To:     Krzysztof Kozlowski <krzk@kernel.org>
Cc:     Sylwester Nawrocki <s.nawrocki@samsung.com>,
        "open list:COMMON CLK FRAMEWORK" <linux-clk@vger.kernel.org>,
        Chanwoo Choi <cw00.choi@samsung.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Mike Turquette <mturquette@baylibre.com>,
        "moderated list:SAMSUNG SOC CLOCK DRIVERS" 
        <linux-samsung-soc@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>
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2020=E5=B9=B48=E6=9C=8811=E6=97=A5(=E7=81=AB) 18:34 Krzysztof Kozlowski <kr=
zk@kernel.org>:
>
> On Tue, Aug 11, 2020 at 06:28:18PM +0200, Tomasz Figa wrote:
> > 2020=E5=B9=B48=E6=9C=8811=E6=97=A5(=E7=81=AB) 18:24 Krzysztof Kozlowski=
 <krzk@kernel.org>:
> > >
> > > On Tue, Aug 11, 2020 at 02:59:07PM +0200, Tomasz Figa wrote:
> > > > Hi Sylwester,
> > > >
> > > > 2020=E5=B9=B48=E6=9C=8811=E6=97=A5(=E7=81=AB) 13:25 Sylwester Nawro=
cki <s.nawrocki@samsung.com>:
> > > > >
> > > > > In the .set_rate callback for some PLLs there is a loop polling s=
tate
> > > > > of the PLL lock bit and it may become an endless loop when someth=
ing
> > > > > goes wrong with the PLL. For some PLLs there is already (a duplic=
ated)
> > > > > code for polling with timeout. This patch replaces that code with
> > > > > the readl_relaxed_poll_timeout_atomic() macro and moves it to a c=
ommon
> > > > > helper function, which is then used for all the PLLs. The downsid=
e
> > > > > of switching to the common macro is that we drop the cpu_relax() =
call.
> > > >
> > > > Tbh. I'm not sure what effect was exactly expected from cpu_relax()=
 in
> > > > the functions which already had timeout handling. Could someone she=
d
> > > > some light on this?
> > >
> > > For us, it should not matter much, except:
> > > 1. when on A9 with ARM_ERRATA_754327, but we do not enable it on our
> > >    platforms,
> > > 2. it is a generic pattern for busy loops.
> > >
> > > On other architectures it could mean something (e.g. yield to other
> > > hyper-threading CPU).
> >
> > Okay, thanks for confirming that it doesn't matter for us.
> >
> > Now, I wonder if the readx_poll_*() helpers are supposed to take all
> > of those into account or on systems which would benefit from such
> > operations, it would be the caller's responsibility.
>
> That's a very good point. In case of ARM_ERRATA_754327, busy waiting
> should have a barrier thus cpu_relax() is desired. I guess the generic
> macro for busy waiting therefore should use them.

Is there yet another macro available somewhere or you mean
read_poll_timeout_atomic()? The latter doesn't include cpu_relax().
Given that udelay() likely already does this kind of an idle call,
perhaps it could be as simple as this?

        if (__delay_us) \
                udelay(__delay_us); \
+       else \
+               cpu_relax(); \

On the other hand, I wonder if there are cases where a call to
cpu_relax() is not desirable.

Best regards,
Tomasz