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

[Jie Zhan]

On 9/15/2026 8:05 PM, Sumit Gupta wrote:
> 
> 
> On 01/09/26 13:54, Jie Zhan wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> Hi Sumit,
>>
>> Sorry for catching up late.
>>
>> A few questions inline.
>>
>> Regards,
>> Jie
> 
> 
> Hi Jie,
> 
> Thanks for the review and sorry for the late reply.
> 
> 
>>
>> 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@xxxxxxxxxx>
>>> ---
>>>   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.
> 
> I added this in response to Christian's comment on v3 [1], so I would
> prefer to keep it here for now. If the generic fix is still wanted, I
> can send the core API changes separately and we can discuss it there.
> 
>>> +     if (ret)
>>> +             pr_debug("Failed to reorder perf restore on CPU%u (%d)\n",
>>> +                      cpu, ret);
>>> +
>>> +     ret = cppc_set_perf(cpu, &cpu_data->perf_ctrls);
>>> +     if (ret)
>>> +             pr_debug("Failed to reapply perf request on CPU%u (%d)\n",
>>> +                      cpu, ret);
>>> +
>>> +     return 0;
>>> +}
>>> +
>>>   static void cppc_cpufreq_cpu_exit(struct cpufreq_policy *policy)
>>>   {
>>>        struct cppc_cpudata *cpu_data = policy->driver_data;
>> For neatness, can we place the above functions after cppc_cpufreq_cpu_exit()?
>> such that the order of source functions would be the same as the following
>> structure, i.e. init, exit, online, offline, and perhaps, suspend, resume.
> 
> Will change the order in v5, matching cppc_cpufreq_driver.
> 
> Thanks,
> Sumit
> 
> ....
> 
>