linux-kernel - Re: [PATCH v3 1/3] cpufreq: mediatek: using global lock avoid race condition

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Message-ID: <20250324054333.sgwwksawnybx3lp4@vireshk-i7>
Date: Mon, 24 Mar 2025 11:13:33 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Chun-Jen Tseng (曾俊仁) <Chun-Jen.Tseng@...iatek.com>
Cc: "cw00.choi@...sung.com" <cw00.choi@...sung.com>,
	"rafael@...nel.org" <rafael@...nel.org>,
	Project_Global_Chrome_Upstream_Group <Project_Global_Chrome_Upstream_Group@...iatek.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	"myungjoo.ham@...sung.com" <myungjoo.ham@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kyungmin.park@...sung.com" <kyungmin.park@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v3 1/3] cpufreq: mediatek: using global lock avoid race
 condition

Hi,

Thanks for sharing the details this time, it makes it much clearer
now.

On 24-03-25, 03:21, Chun-Jen Tseng (曾俊仁) wrote:
> I think the best configuration sequence is as follows:
>   cpufreq policy -> set frequency -> CCI governor get
> CPUFREQ_POSTCHANGE NB -> choose CCI frequency -> set CCI frequency
> 
> However, in drivers/devfreq/governor_passive.c#L77,
> get_target_freq_with_cpufreq() retrieves the current frequency of each
> policy,
> and it determines the CCI frequency based on the frequency of each
> policy.
> 
> But if policy-0 and policy-6 enter simultaneously, the CCI governor
> might get an incorrect frequency.

Yes it may fetch the current frequency (or last known one), but that
shouldn't be a problem as the postchange notification for policy-6
should get called right after and should fix the issue. Right ?

I don't think this is a race and if this requires fixing. clk_get()
for any device, will always return the last configured value, while
the clock might be changing at the same time.

What's important is that you don't get an incorrect frequency (as in
based on intermediate values of registers, etc). Note that the last
configured frequency isn't an incorrect frequency.

> cpufreq policy-0 -> set frequency -> CCI governor get
> CPUFREQ_POSTCHANGE NB -> choose CCI frequency -> set CCI frequency
> => during this time, the CCI governor gets policy-0 and policy-6, BUT
> policy-6 may change frequency by cpufreq driver at the same time.

Sure, and I don't see a problem with that. The issue is there only if
we can reach a state where CCI is left configured in the wrong state.
Which I don't think would happen here as the postchange notifier will
get called again, forcing a switch of frequency again.

-- 
viresh