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

[Pengjie Zhang]

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@xxxxxxxxxx>
> > > 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@xxxxxxxxxx/
> > > >
> > > >    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