linux-kernel - Re: [PATCH 1/2] cpufreq: Add driver flag to avoid initial frequency verification

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Message-ID: <20250825045641.o2thjvs3xyuxavyk@vireshk-i7>
Date: Mon, 25 Aug 2025 10:26:41 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Prashant Malani <pmalani@...gle.com>
Cc: open list <linux-kernel@...r.kernel.org>,
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Beata Michalska <beata.michalska@....com>
Subject: Re: [PATCH 1/2] cpufreq: Add driver flag to avoid initial frequency
 verification

On 23-08-25, 00:17, Prashant Malani wrote:
> Some cpufreq drivers have a get() function which can return an unreliable
> frequency. This can cause issues when switching governors. For instance,
> a CPU would be on performance governor and have it's frequency (and
> policy->cur) set to max. When the governor is switched to userspace, the
> policy->cur is re-used, but it is checked against the frequency returned
> by the driver's get() function. If it's different, the frequency will
> get set to the new (incorrect) value.
> 
> To avoid this, add a flag that avoids this verify step on governor start
> if the cpufreq driver opts in to it.
> 
> Since there are no users of this flag, no functional changes are
> introduced here.
> 
> Cc: Beata Michalska <beata.michalska@....com>
> Signed-off-by: Prashant Malani <pmalani@...gle.com>
> ---
>  drivers/cpufreq/cpufreq.c |  3 ++-
>  include/linux/cpufreq.h   | 10 ++++++++++
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index b8937737d096..72e6552a40ea 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2482,7 +2482,8 @@ int cpufreq_start_governor(struct cpufreq_policy *policy)
>  
>  	pr_debug("%s: for CPU %u\n", __func__, policy->cpu);
>  
> -	cpufreq_verify_current_freq(policy, false);
> +	if (!(cpufreq_driver->flags & CPUFREQ_DONT_VERIFY_FREQ_ON_GOVERNOR_START))
> +		cpufreq_verify_current_freq(policy, false);

I don't think it is okay to do this to hide a driver's failure to
return the right frequency. What about all the other places where
get() is still used ? Won't that cause similar issues elsewhere ?

The driver must be fixed for this, not the core. The core is doing the
right thing here, asking the driver to return the current frequency.
If the driver is unsure, it can just return the current frequency from
policy->cur instead.

-- 
viresh