linux-kernel - Re: [PATCH] cpufreq: CPPC: Mark driver with NEED_UPDATE

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Message-ID: <20250722060352.pdwk2nrrth4uph2s@vireshk-i7>
Date: Tue, 22 Jul 2025 11:33:52 +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>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] cpufreq: CPPC: Mark driver with NEED_UPDATE_LIMITS flag

On 22-07-25, 05:55, Prashant Malani wrote:
> AMU counters on certain CPPC-based platforms tend to yield inaccurate
> delivered performance measurements on systems that are idle/mostly idle.
> This results in an inaccurate frequency being stored by cpufreq in its
> policy structure when the CPU is brought online. [1]
> 
> Consequently, if the userspace governor tries to set the frequency to a
> new value, there is a possibility that it would be the erroneous value
> stored earlier. In such a scenario, cpufreq would assume that the
> requested frequency has already been set and return early, resulting in
> the correct/new frequency request never making it to the hardware.
> 
> Since the operating frequency is liable to this sort of inconsistency,
> mark the CPPC driver with CPUFREQ_NEED_UPDATE_LIMITS so that it is always
> invoked when a target frequency update is requested.
> 
> [1] https://lore.kernel.org/linux-pm/20250619000925.415528-3-pmalani@google.com/
> 
> Cc: Beata Michalska <beata.michalska@....com>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Signed-off-by: Prashant Malani <pmalani@...gle.com>
> ---
>  drivers/cpufreq/cppc_cpufreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index a1fd0ff22bc5..4a17162a392d 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -910,7 +910,7 @@ static struct freq_attr *cppc_cpufreq_attr[] = {
>  };
>  
>  static struct cpufreq_driver cppc_cpufreq_driver = {
> -	.flags = CPUFREQ_CONST_LOOPS,
> +	.flags = CPUFREQ_CONST_LOOPS | CPUFREQ_NEED_UPDATE_LIMITS,
>  	.verify = cppc_verify_policy,
>  	.target = cppc_cpufreq_set_target,
>  	.get = cppc_cpufreq_get_rate,

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

-- 
viresh