Re: [PATCH v2] cpufreq: CPPC: Enable boost by default when supported

[Pengjie Zhang <zhangpengjie2@huawei.com>]

On 11/5/2026 12:12 AM, Christian Loehle wrote:
> On 11/4/26 07:43, Pengjie Zhang wrote:
>> On 10/29/2026 4:51 AM, Christian Loehle wrote:
>>> On 10/22/26 08:59, Pengjie Zhang wrote:
>>>> The CPPC driver leaves boost disabled by default when Highest Performance
>>>> exceeds Nominal Performance. Userspace must explicitly enable boost
>>>> before governors can use the full advertised performance range.
>>>>
>>>> Enable boost by default when at least one possible CPU reports support,
>>>> while users who prefer to stay within the sustained nominal limit can
>>>> still disable it via the boost knob without determining scaling_max_freq
>>>> for each policy.
>>>>
>>>> Check the performance capabilities before registering the driver so that
>>>> the cpufreq core enables boost for each policy that supports it.
>>>>
>>>> Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
>>> Just to be clear I'm not blocking this per se, but continuing my comments:
>>>
>>> I just wonder if this is a configuration worth keeping.
>>> You mentioned your platforms have a BIOS to set nominal==highest or not.
>>> If we removed interpreting highest - nominal as boost entirely that
>>> would also make that BIOS setting obsolete. Userspace could still
>>> query nominal and restrict cpufreq to that if it wishes to do that,
>>> we could add that to cpupower for convenience (i.e. set all policies'
>>> max to nominal).
>>>
>>> With the changes that are coming with CPCv4 we would save ourselves
>>> some headache (like keeping track of boost status when ospm_nominal_perf
>>> is being written).
>>>
>>> The question is also how often the combination of "enable nominal < highest"
>>> and then explicitly toggling boost at runtime actually happens...
>>>
>>>> ---
>>>> Changes in v2:
>>>> - Drop "even" and clarify the commit message (Christian).
>>>> - Link to v1: https://lore.kernel.org/all/20261015015812.867988-1-zhangpengjie2@huawei.com/
>>>>
>>>>    drivers/cpufreq/cppc_cpufreq.c | 20 ++++++++++++++++++++
>>>>    1 file changed, 20 insertions(+)
>>>>
>>>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>>>> index 80893844353c..0a4255a36f60 100644
>>>> --- a/drivers/cpufreq/cppc_cpufreq.c
>>>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>>>> @@ -1052,6 +1052,25 @@ static struct cpufreq_driver cppc_cpufreq_driver = {
>>>>        .name = "cppc_cpufreq",
>>>>    };
>>>>    +static void __init cppc_cpufreq_default_boost_init(void)
>>>> +{
>>>> +    struct cppc_perf_caps caps;
>>>> +    bool enable = false;
>>>> +    int cpu;
>>>> +
>>>> +    for_each_possible_cpu(cpu) {
>>>> +        if (cppc_get_perf_caps(cpu, &caps))
>>>> +            continue;
>>>> +
>>>> +        if (caps.highest_perf > caps.nominal_perf) {
>>>> +            enable = true;
>>>> +            break;
>>>> +        }
>>>> +    }
>>>> +
>>>> +    cppc_cpufreq_driver.boost_enabled = enable;
>>> Any reason for the scan instead of just setting
>>> boost_enabled = true? Platforms without boost would
>>> also expose the toggle, but that's mostly in line with
>>> the other drivers? (also again the ospm_nominal_perf
>>> angle)
>> The reason for scanning here is to keep the initial sysfs state
>> consistent on platforms where highest_perf == nominal_perf on all CPUs
>> (i.e. policy->boost_supported is false everywhere). If we just set
>> boost_enabled = true unconditionally, the global cpufreq/boost file will
>> read 1 on boot while every policyX/boost reads 0 (and rejects writes
>> with -EINVAL).
> FWIW, amd-pstate can already expose global boost=1 while all per-policy
> boost knobs read 0 and writes to either knob then fail with -EINVAL AFAICS?
>
> AFAIU the global sysfs knob represents permission to boost where
> supported, rather than meaning boost is supported, too?

Yes, Documentation/admin-guide/pm/cpufreq.rst also states that 1 in the
global boost file "only means a permission to use the frequency boost
mechanism (which still may never be used for other reasons)", while 0
means it "is disabled and cannot be used at all".

> PS:
> Also another fun one I discovered...
> power-orion-o6-01# cat /sys/devices/system/cpu/cpufreq/boost
> 0
> power-orion-o6-01# echo 1 > /sys/devices/system/cpu/cpufreq/boost
> echo: write error: invalid argument
> power-orion-o6-01# cat /sys/devices/system/cpu/cpufreq/boost
> 0
> power-orion-o6-01# echo 0 > /sys/devices/system/cpu/cpufreq/boost
> echo: write error: invalid argument
> power-orion-o6-01# cat /sys/devices/system/cpu/cpufreq/boost
> 1
>
> I'll go and fix that...
>
>> Also, since ospm_nominal_perf has a valid range of [lowest_perf,
>> nominal_perf], it actually pairs quite well with the boost knob: on
>> platforms with highest_perf > nominal_perf, ospm_nominal_perf will
>> always stay below highest_perf, and even on platforms with
>> nominal_perf == highest_perf, lowering ospm_nominal_perf could
>> dynamically define a nominal ceiling for when boost is disabled.
> I don't think I agree with the latter. OSPM nominal has its own (platform)
> semantics, but making boost=0 impose that value as a frequency ceiling
> is just another way of setting scaling_max_freq.
> I don't see why these two need to be coupled?
>
> Also I think the fact that caps.highest_perf == caps.nominal_perf
> but ospm_nominal_perf can change boost supported later is a reason
> against the boot-time scan? (which would need to exclude
> ospm_nominal_perf platforms?)

Fair enough, ospm_nominal_perf doesn't need to be coupled with boost.

Viresh, Rafael, any preference here? Should we keep the boot-time check
in v2 (so global boost stays 0 on platforms without boost support), or
drop the scan and just set .boost_enabled = true statically in
cppc_cpufreq_driver?

Thanks,
Pengjie