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

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Message-ID: <20200825113131.GB12506@arm.com>
Date:   Tue, 25 Aug 2020 12:31:31 +0100
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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 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().

Thanks,
Ionela.