linux-kernel - Re: [PATCH v3 1/5] arch_topology: validate input frequencies to arch_set_freq

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Date:   Thu, 27 Aug 2020 11:40:22 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     rjw@...ysocki.net, dietmar.eggemann@....com,
        catalin.marinas@....com, sudeep.holla@....com, will@...nel.org,
        valentin.schneider@....com, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] arch_topology: validate input frequencies to
 arch_set_freq_scale()

On 25-08-20, 12:31, Ionela Voinescu wrote:
> On Tuesday 25 Aug 2020 at 11:26:18 (+0530), Viresh Kumar wrote:
> > On 24-08-20, 22:02, Ionela Voinescu wrote:
> > > The current frequency passed to arch_set_freq_scale() could end up
> > > being 0, signaling an error in setting a new frequency. Also, if the
> > > maximum frequency in 0, this will result in a division by 0 error.
> > > 
> > > Therefore, validate these input values before using them for the
> > > setting of the frequency scale factor.
> > > 
> > > Signed-off-by: Ionela Voinescu <ionela.voinescu@....com>
> > > Cc: Sudeep Holla <sudeep.holla@....com>
> > > Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> > > ---
> > >  drivers/base/arch_topology.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> > > index 75f72d684294..1aca82fcceb8 100644
> > > --- a/drivers/base/arch_topology.c
> > > +++ b/drivers/base/arch_topology.c
> > > @@ -33,6 +33,9 @@ void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> > >  	unsigned long scale;
> > >  	int i;
> > >  
> > > +	if (!cur_freq || !max_freq)
> > 
> > We should probably use unlikely() here.
> > 
> > Rafael: Shouldn't this have a WARN_ON_ONCE() as well ?
> > 
> 
> I'll add the unlikely() as it's definitely useful.
> 
> I'm somewhat on the fence about WARN_ON_ONCE() here. Wouldn't it work
> better in cpufreq_driver_fast_switch()? It would cover scenarios where
> the default arch_set_freq_scale() is used and flag potential hardware
> issues with setting frequency that are currently just ignored both here
> and in sugov_fast_switch().

I think validation and the WARN (if required) must all happen at the
same place. Considering that there can be many callers of a routine,
like this one, it is better to put all that in the end function only.

Maybe we can add the same in the dummy arch_set_freq_scale() if
required.

-- 
viresh