Re: [PATCH v4 1/4] cpufreq: CPPC: Keep the policy across CPU hotplug

[Sumit Gupta <sumitg@nvidia.com>]

>>>
>>> On 8/7/2026 4:08 AM, Sumit Gupta wrote:
>>>> Without online()/offline() callbacks, the cpufreq core fully tears
>>>> down a policy during exit() when its last online CPU is offlined, and
>>>> rebuilds it during init() when it comes back.
>>>>
>>>> Add lightweight online()/offline() callbacks so the core instead keeps
>>>> the policy live and reuses the driver's cpu_data across CPU hotplug.
>>>> This avoids re-reading the CPPC capabilities on every offline/online,
>>>> making CPU hotplug faster.
>>>>
>>>> Move what init() and exit() did on hotplug into the new callbacks:
>>>>
>>>>     - offline() requests the lowest desired performance, as exit() did.
>>>>     - online() re-enables CPPC and restores the performance controls, as
>>>>       the platform may have reset them. Failures are logged, not returned,
>>>>       as the core would free the policy.
>>>>     - online() also resyncs the frequency invariance counters, so that the
>>>>       first tick does not measure across the offline window.
>>>>
>>>> The restore in online() uses cppc_set_perf(), which writes MIN before
>>>> MAX. If the platform lowered MAX while the CPU was offline, writing the
>>>> saved MIN could briefly leave MIN above MAX on registers not accessed
>>>> through PCC, as PCC delivers the writes in one transaction. Raise MAX
>>>> ahead of the restore when the saved MIN is above it.
>>>>
>>>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>>>> ---
>>>>    drivers/cpufreq/cppc_cpufreq.c | 128 +++++++++++++++++++++++++++++++++
>>>>    1 file changed, 128 insertions(+)
>>>>
>>>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>>>> index 80893844353c..4b3da9a3e122 100644
>>>> --- a/drivers/cpufreq/cppc_cpufreq.c
>>>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>>>> @@ -211,6 +211,29 @@ static void cppc_cpufreq_cpu_fie_exit(struct cpufreq_policy *policy)
>>>>         }
>>>>    }
>>>>
>>>> +/*
>>>> + * Resync the counter snapshot, as the policy is kept across CPU hotplug and
>>>> + * the first tick after online would otherwise span the offline window.
>>>> + */
>>>> +static void cppc_cpufreq_cpu_fie_resync(struct cpufreq_policy *policy)
>>>> +{
>>>> +     struct cppc_freq_invariance *cppc_fi;
>>>> +     int cpu, ret;
>>>> +
>>>> +     if (fie_disabled)
>>>> +             return;
>>>> +
>>>> +     /* policy->cpus still holds related_cpus here, so skip offline CPUs. */
>>>> +     for_each_cpu_and(cpu, policy->cpus, cpu_online_mask) {
>>>> +             cppc_fi = &per_cpu(cppc_freq_inv, cpu);
>>>> +
>>>> +             ret = cppc_get_perf_ctrs(cpu, &cppc_fi->prev_perf_fb_ctrs);
>>>> +             if (ret)
>>>> +                     pr_debug("%s: failed to read perf counters for cpu:%d: %d\n",
>>>> +                              __func__, cpu, ret);
>>>> +     }
>>>> +}
>>>> +
>>>>    static void cppc_fie_kworker_init(void)
>>>>    {
>>>>         struct sched_attr attr = {
>>>> @@ -281,6 +304,10 @@ static inline void cppc_cpufreq_cpu_fie_exit(struct cpufreq_policy *policy)
>>>>    {
>>>>    }
>>>>
>>>> +static inline void cppc_cpufreq_cpu_fie_resync(struct cpufreq_policy *policy)
>>>> +{
>>>> +}
>>>> +
>>>>    static inline void cppc_freq_invariance_init(void)
>>>>    {
>>>>    }
>>>> @@ -735,6 +762,105 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
>>>>         return ret;
>>>>    }
>>>>
>>>> +/*
>>>> + * With offline() defined, the cpufreq core keeps the policy alive when
>>>> + * a CPU is hotplugged out.
>>>> + */
>>>> +static int cppc_cpufreq_cpu_offline(struct cpufreq_policy *policy)
>>>> +{
>>>> +     struct cppc_cpudata *cpu_data = policy->driver_data;
>>>> +     struct cppc_perf_ctrls perf_ctrls = cpu_data->perf_ctrls;
>>>> +     unsigned int cpu = policy->cpu;
>>>> +     int ret;
>>>> +
>>>> +     /*
>>>> +      * Request the lowest desired performance while the policy has no online
>>>> +      * CPU. Zeroing MIN and MAX makes cppc_set_perf() leave them unchanged.
>>>> +      */
>>>> +     perf_ctrls.desired_perf = cpu_data->perf_caps.lowest_perf;
>>>> +     perf_ctrls.min_perf = 0;
>>>> +     perf_ctrls.max_perf = 0;
>>>> +
>>>> +     ret = cppc_set_perf(cpu, &perf_ctrls);
>>>> +     if (ret)
>>>> +             pr_debug("Err setting perf value:%u on CPU:%u. ret:%d\n",
>>>> +                      cpu_data->perf_caps.lowest_perf, cpu, ret);
>>>> +
>>>> +     return 0;
>>>> +}
>>>> +
>>>> +/*
>>>> + * Raise MAX ahead of the full restore when the requested MIN is above the
>>>> + * current MAX. cppc_set_perf() writes MIN before MAX, so the platform would
>>>> + * otherwise briefly see MIN above MAX on registers not accessed through PCC.
>>>> + * Lowering MAX is safe, as the MIN written first is never above it.
>>>> + */
>>>> +static int
>>>> +cppc_cpufreq_prepare_perf_restore(unsigned int cpu,
>>>> +                               const struct cppc_perf_ctrls *target)
>>>> +{
>>>> +     struct cppc_perf_ctrls cur = {}, prep = {};
>>>> +     int ret;
>>>> +
>>>> +     ret = cppc_get_perf(cpu, &cur);
>>>> +     if (ret)
>>>> +             return ret;
>>>> +
>>>> +     if (!cur.max_perf || target->min_perf <= cur.max_perf)
>>>> +             return 0;
>>>> +
>>>> +     prep.desired_perf = target->desired_perf;
>>>> +     prep.min_perf = 0;      /* Zero leaves MIN unchanged. */
>>>> +     prep.max_perf = target->max_perf;
>>>> +
>>>> +     return cppc_set_perf(cpu, &prep);
>>>> +}
>>>> +
>>>> +/*
>>>> + * Restore what the CPU may have lost while offline, as the platform may have
>>>> + * disabled CPPC and reset the performance controls. Never fail the callback,
>>>> + * or the core would free the policy and leave the CPU without cpufreq. The
>>>> + * governor redoes the control writes, so they are best effort, unlike the
>>>> + * enable, which only a later online() can retry.
>>> Sorry, I don't quite understand the last sentence.
>>
>>
>> Will rewrite in v5 as below:
>>
>>   Report failures without returning them, or the core would free the
>>   policy and leave the CPU without cpufreq. A failed write to the
>>   performance controls is not fatal, as the governor's next request
>>   programs them again. A failed CPPC enable stops the restore, as the
>>   writes that follow may not reach the platform.
>>
>>>> + */
>>>> +static int cppc_cpufreq_cpu_online(struct cpufreq_policy *policy)
>>>> +{
>>>> +     struct cppc_cpudata *cpu_data = policy->driver_data;
>>>> +     unsigned int cpu = policy->cpu;
>>>> +     int ret;
>>>> +
>>>> +     cppc_cpufreq_cpu_fie_resync(policy);
>>>> +
>>>> +     ret = cppc_set_enable(cpu, true);
>>>> +     if (ret && ret != -EOPNOTSUPP) {
>>>> +             pr_warn("Failed to re-enable CPPC for CPU%u (%d)\n", cpu, ret);
>>>> +             return 0;
>>>> +     }
>>>> +
>>>> +     /*
>>>> +      * The platform may reset the controls while the CPU is offline, so
>>>> +      * recompute min/max, clamp desired_perf into range, and reprogram them.
>>>> +      */
>>>> +     cppc_cpufreq_update_perf_limits(cpu_data, policy);
>>>> +
>>>> +     cpu_data->perf_ctrls.desired_perf =
>>>> +             clamp_t(u32, cpu_data->perf_ctrls.desired_perf,
>>>> +                     cpu_data->perf_ctrls.min_perf,
>>>> +                     cpu_data->perf_ctrls.max_perf);
>>>> +
>>>> +     ret = cppc_cpufreq_prepare_perf_restore(cpu, &cpu_data->perf_ctrls);
>>> Actually, I don't quite think this is necessary?
>>>
>>> The motivation of doing this is fair (as mentioned in v3), but what's the
>>> real consequence of transiently setting min_perf larger than max_perf?
>>> Platforms should be able to handle this.
>>>
>>> Even if we have to fix it, it's supposed to be done in cppc_acpi.c.  The
>>> current ABI wraps many things up.  cppc_get_perf() reads 4 values -
>>> min_perf, max_perf, energy_perf, auto_sel.  cppc_set_perf writes 3
>>> values - desired_perf, min_perf, max_perf.  The cppc_cpufreq driver would
>>> be able to handle performance setting cleaner if those are separated.
>>>
>>> I don't suggest we complicate the driver for now?
>>
>> Agreed that it is not hotplug specific and can be done in the
>> generic API.
>>
>> cppc_set_perf() would have to know the programmed MIN and MAX to pick
>> the write order. Separate accessors would let it read only those two,
>> but that would add a read before every write, including fast_switch().
> fast_switch() doesn't have to touch min/max_perf, but it did at the moment.
>> Caching what was last written would avoid that, but the platform can
>> reset the registers while the CPU is offline or suspended.
> Yeah, understood.  My question is still whether it's practically useful and
> we're complicating this.
> 
> Two reasons.
> 
> 1. Platform should be able to handle min_perf being trasiently larger than
> max_perf, otherwise it would be fragile.
> 
> 2. It depends on the reset values of the two registers.
> 
> I went over the ACPI Spec and didn't manage to find a descprition on what
> the default/reset values of min/max perf registers should be.
> 
> For a sensisble design, min_perf defaults to be 0 or lowest perf, and
> max_perf defaults to be all 1s or highest perf.  In those cases, we are
> safe to directly restore the saved values.

Hi Jie,

Agreed, those values would be safe, although they are not required
reset defaults. A platform could reset MAX to a lower value, such as
lowest_perf, while the saved policy MIN is higher.
Restoring MIN first would then temporarily result in MIN greater than
MAX on non-PCC systems.

This preparation was added in response to Christian’s v3 comment [1].
I had already posted v5 [2] before receiving this reply, and it retains
the preparation.

Christian, are you okay with dropping it and restoring the controls
directly with cppc_set_perf(), as in v3?
Any general requirement for ordered MIN/MAX updates can then be handled
in a separate CPPC core series.

[1] 
https://lore.kernel.org/lkml/40d72385-0b3f-46e5-9f32-a27be3842d7c@arm.com/
[2] https://lore.kernel.org/lkml/20260916103820.1760297-1-sumitg@nvidia.com/

Thanks,
Sumit
....