linux-kernel - Re: [PATCH v2 2/2] cpufreq: qcom-hw: Delay enabling throttle

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Date:   Fri, 28 Jan 2022 10:30:48 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Jonathan Corbet <corbet@....net>, linux-pm@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH v2 2/2] cpufreq: qcom-hw: Delay enabling throttle_irq

On Fri 28 Jan 02:39 PST 2022, Lukasz Luba wrote:

> 
> 
> On 1/28/22 3:25 AM, Bjorn Andersson wrote:
> > In the event that the SoC is under thermal pressure while booting it's
> > possible for the dcvs notification to happen inbetween the cpufreq
> > framework calling init and it actually updating the policy's
> > related_cpus cpumask.
> > 
> > Prior to the introduction of the thermal pressure update helper an empty
> > cpumask would simply result in the thermal pressure of no cpus being
> > updated, but the new code will attempt to dereference an invalid per_cpu
> > variable.
> 
> Just to confirm, is that per-cpu var the 'policy->related_cpus' in this
> driver?
> 

Correct, we boot under thermal pressure, so the interrupt fires before
we return from "init", which means that related_cpus is still 0.

> > 
> > Avoid this problem by using the newly reintroduced "ready" callback, to
> > postpone enabling the IRQ until the related_cpus cpumask is filled in.
> > 
> > Fixes: 0258cb19c77d ("cpufreq: qcom-cpufreq-hw: Use new thermal pressure update function")
> 
> You have 'Fixes' tagging here, which might be picked by the stable tree.
> The code uses the reverted callback .ready(), which might be missing
> there (since patch 1/2 doesn't have tagging). This patch looks like a
> proper fix for the root cause.
> 

Yes, the pair would need to be picked up.

> Anyway, I'm going to send a patch, which adds a check for null cpumask
> in the topology_update_thermal_pressure()
> It was removed after the review comments:
> https://lore.kernel.org/linux-pm/20211028054459.dve6s2my2tq7odem@vireshk-i7/
> 

I attempted that in v1:
https://lore.kernel.org/all/20220118185612.2067031-2-bjorn.andersson@linaro.org/

And while patch 1 is broken, I think Greg and Sudeep made it clear that
they didn't want a condition to guard against the caller passing cpus of
0.

That's why I in v2 reverted to postpone the thermal pressure IRQ until
cpufreq is "ready".

Regards,
Bjorn

> I'll also push that change for the stable tree.
> 
> Regards,
> Lukasz