Re: [PATCH v2] clk: samsung: Prevent potential endless loop in the PLL set_rate ops

[Krzysztof Kozlowski <krzk@kernel.org>]

On Tue, Aug 11, 2020 at 06:41:20PM +0200, Tomasz Figa wrote:
> 2020年8月11日(火) 18:34 Krzysztof Kozlowski <krzk@kernel.org>:
> >
> > On Tue, Aug 11, 2020 at 06:28:18PM +0200, Tomasz Figa wrote:
> > > 2020年8月11日(火) 18:24 Krzysztof Kozlowski <krzk@kernel.org>:
> > > >
> > > > On Tue, Aug 11, 2020 at 02:59:07PM +0200, Tomasz Figa wrote:
> > > > > Hi Sylwester,
> > > > >
> > > > > 2020年8月11日(火) 13:25 Sylwester Nawrocki <s.nawrocki@samsung.com>:
> > > > > >
> > > > > > In the .set_rate callback for some PLLs there is a loop polling state
> > > > > > of the PLL lock bit and it may become an endless loop when something
> > > > > > goes wrong with the PLL. For some PLLs there is already (a duplicated)
> > > > > > code for polling with timeout. This patch replaces that code with
> > > > > > the readl_relaxed_poll_timeout_atomic() macro and moves it to a common
> > > > > > helper function, which is then used for all the PLLs. The downside
> > > > > > 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 shed
> > > > > 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().

Yes, I meant the generic read_poll_timeout_atomic().

> 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(); \
> 

I think udelay does not have it. Delaying by some simple operations
(e.g. multiplication) does not require IO barriers.

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

Hmmm, it is really a generic pattern all over the kernel, so I doubt
that generic macros should target such case.

Best regards,
Krzysztof