Re: [PATCH v4 2/2] cpufreq: acpi-cpufreq: fix P-state index mismatch in get_cur_freq_on_cpu()

[Rafael J. Wysocki (Intel)]

On Thu, Sep 10, 2026 at 1:13 PM lirongqing <lirongqing@xxxxxxxxx> wrote:
>
> From: Li RongQing <lirongqing@xxxxxxxxx>
>
> get_cur_freq_on_cpu() uses perf->state, which is an index into
> perf->states[], to index policy->freq_table[]. However, freq_table[]
> is built by filtering _PSS entries that are not lower in frequency
> than the previous one, so its index space no longer matches
> perf->states[]. The original P-state index for each remaining
> freq_table entry is stored in driver_data.
>
> Once an entry has been skipped, using perf->state as an index into
> freq_table[] can therefore select the frequency of a different
> P-state.
>
> The reported current frequency itself remains correct because it is
> obtained from extract_freq(). The mismatch only affects the cached
> frequency used by get_cur_freq_on_cpu() to detect a firmware frequency
> change behind our back.
>
> If the wrong table entry contains a frequency different from the one
> the CPU is actually running at, the check falsely detects a frequency
> change and sets data->resume. The next ->target() call then performs a
> redundant control-register write even if the requested P-state is
> already the current P-state.
>
> Conversely, if the wrong table entry happens to contain the frequency
> to which firmware has changed the CPU, the frequency change is missed
> and data->resume remains clear. A subsequent ->target() call for the
> P-state that the cpufreq core believes to be current can then
> short-circuit without rewriting the control register, leaving the CPU
> at the firmware-selected frequency until a different P-state is
> requested.
>
> Fix this by taking the cached frequency directly from
> perf->states[perf->state].core_frequency. perf->state and
> perf->states[] use the same P-state index space, and converting
> core_frequency to kHz yields the same value stored in the corresponding
> freq_table entry during initialization.

Sashiko has comments on this patch and it has a point IMV:

https://sashiko.dev/#/patchset/20260910111329.2220-1-lirongqing%40baidu.com

Can you please tell me what you think?

> Fixes: e56a727b023d ("[CPUFREQ] Make acpi-cpufreq more robust against BIOS freq changes behind our back.")
> Reported-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
> Suggested-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> Reviewed-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
> ---
>  drivers/cpufreq/acpi-cpufreq.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> index a797bb2..3313c74 100644
> --- a/drivers/cpufreq/acpi-cpufreq.c
> +++ b/drivers/cpufreq/acpi-cpufreq.c
> @@ -353,6 +353,7 @@ static u32 get_cur_val(const struct cpumask *mask, struct acpi_cpufreq_data *dat
>
>  static unsigned int get_cur_freq_on_cpu(unsigned int cpu)
>  {
> +       struct acpi_processor_performance *perf;
>         struct acpi_cpufreq_data *data;
>         struct cpufreq_policy *policy;
>         unsigned int freq;
> @@ -368,7 +369,9 @@ static unsigned int get_cur_freq_on_cpu(unsigned int cpu)
>         if (unlikely(!data || !policy->freq_table))
>                 return 0;
>
> -       cached_freq = policy->freq_table[to_perf_data(data)->state].frequency;
> +       perf = to_perf_data(data);
> +       cached_freq = perf->states[perf->state].core_frequency * 1000;
> +
>         freq = extract_freq(policy, get_cur_val(cpumask_of(cpu), data));
>         if (freq != cached_freq) {
>                 /*
> --
> 2.9.4
>