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

[Sumit Gupta <sumitg@nvidia.com>]

On 17/09/26 18:38, Christian Loehle wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 9/17/26 12:01, Sumit Gupta wrote:
>>
>>
>>>>>
>>>>> 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,
> 
> Hi Sumit, 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.
> 
> I basically agree(d) with Jie here when I commented on v3:
> "I think cppc_set_perf() needs some prep first before using it on reset values.
> We assume that reset value may be Autonomous Mode on, right? So we must never
> write MIN>MAX and vice versa. I think we may just have to read and write
> the 'otherwise-offending' value first on reset."
> So I wanted to have this (as prep work) within cppc_set_perf() not in cppc-cpufreq,
> 
>>
>> 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/
> 

Hi Christian, Jie,

Thanks for the clarification. I will drop the preparation from
cppc_cpufreq in v6 and restore the controls directly using
cppc_set_perf(). I will address the MIN/MAX write ordering in
cppc_set_perf() in a separate series soon.

Thanks,
Sumit