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Subject: Re: [PATCH] cpufreq: cppc: drop invariance when FIE is disabled
To: Penghe Geng <pgeng@nvidia.com>
CC: Viresh Kumar <viresh.kumar@linaro.org>, Ionela Voinescu
	<ionela.voinescu@arm.com>, <linux-pm@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <stable@vger.kernel.org>, "Rafael J. Wysocki"
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References: <20260220224920.416602-1-pgeng@nvidia.com>
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From: Jie Zhan <zhanjie9@hisilicon.com>
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On 2/21/2026 6:49 AM, Penghe Geng wrote:
> CONFIG_ACPI_CPPC_CPUFREQ_FIE gates CPPC counter-based frequency
> invariance support. When FIE is disabled, the CPPC driver does not
> register a frequency scale source, but cpufreq_register_driver() still
> enables cpufreq_freq_invariance for target/fast-switch drivers.
> 
> Disable cpufreq frequency invariance after CPPC driver registration when
> FIE is disabled. This avoids scheduler behavior mismatch when no
> invariance updates are provided, which can cause major performance
> regressions on sensitive platforms.
Hi Penghe,

IIUC, even if CPPC FIE is not there, cpufreq still updates frequency scales
in cpufreq_freq_transition_end() and cpufreq_driver_fast_switch(), so the
frequency scale number is still somewhat meaningful and the
'cpufreq_freq_invariance' static key reflects that correctly.

Any numbers or evidence of that "performance regressions" so that we can
understand the issue better?

Thanks,
Jie
> 
> Export cpufreq_disable_freq_invariance() so modular cppc_cpufreq can call
> it.
> 
> Fixes: 1eb5dde674f5 ("cpufreq: CPPC: Add support for frequency invariance")
> Cc: stable@vger.kernel.org
> Signed-off-by: Penghe Geng <pgeng@nvidia.com>
> ---
>  drivers/cpufreq/cppc_cpufreq.c | 4 ++++
>  drivers/cpufreq/cpufreq.c      | 8 ++++++++
>  include/linux/cpufreq.h        | 3 +++
>  3 files changed, 15 insertions(+)
> 
...