Re: [PATCH v3 2/2] cpufreq: CPPC: add autonomous mode boot parameter support

[Sumit Gupta]

On 18/05/26 19:51, Mario Limonciello wrote:
> External email: Use caution opening links or attachments
>
>
> On 5/18/26 09:15, Sumit Gupta wrote:
> > On 18/05/26 19:20, Mario Limonciello wrote:
> > > External email: Use caution opening links or attachments
> > >
> > >
> > > On 5/18/26 08:44, Sumit Gupta wrote:
> > > > Hi Mario,
> > > >
> > > >
> > > > On 16/05/26 02:43, Mario Limonciello wrote:
> > > > > External email: Use caution opening links or attachments
> > > > >
> > > > >
> > > > > On 5/15/26 07:26, Sumit Gupta wrote:
> > > > > > Add a kernel boot parameter 'cppc_cpufreq.auto_sel_mode' to enable
> > > > > > CPPC autonomous performance selection on all CPUs at system startup.
> > > > > > When autonomous mode is enabled, the hardware automatically adjusts
> > > > > > CPU performance based on workload demands using Energy Performance
> > > > > > Preference (EPP) hints.
> > > > > >
> > > > > > When the parameter is set:
> > > > > > - Configure all CPUs for autonomous operation on first init
> > > > > > - Use HW min/max_perf when available; otherwise initialize from caps
> > > > > > - Initialize desired_perf to max_perf as a starting hint
> > > > > > - Hardware controls frequency instead of the OS governor
> > > > > > - EPP behavior depends on parameter value:
> > > > > >    - performance (or 1): override EPP to performance preference 
> > > > > > (0x0)
> > > > > >    - default_epp (or 2): preserve EPP value programmed by BIOS/
> > > > > > firmware
> > > > > >
> > > > > > The boot parameter is applied only during first policy 
> > > > > > initialization.
> > > > > > Skip applying it on CPU hotplug to preserve runtime sysfs
> > > > > > configuration.
> > > > > >
> > > > > > This patch depends on patch series [1] ("cpufreq: Set policy->min 
> > > > > > and
> > > > > > max as real QoS constraints") so that the policy->min/max set in
> > > > > > cppc_cpufreq_cpu_init() are not overridden by cpufreq_set_policy()
> > > > > > during init.
> > > > > >
> > > > > > Signed-off-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> > > > > > ---
> > > > > > [1] https://lore.kernel.org/lkml/20260511135538.522653-1-
> > > > > > pierre.gondois@xxxxxxx/
> > > > > > ---
> > > > > >   .../admin-guide/kernel-parameters.txt         |  16 +++
> > > > > >   drivers/cpufreq/cppc_cpufreq.c                | 122 +++++++++++++
> > > > > > ++++-
> > > > > >   2 files changed, 133 insertions(+), 5 deletions(-)
> > > > > >
> > > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/
> > > > > > Documentation/admin-guide/kernel-parameters.txt
> > > > > > index 0eb64aab3685..7e4b3a8fd76f 100644
> > > > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > > > @@ -1048,6 +1048,22 @@ Kernel parameters
> > > > > >                       policy to use. This governor must be 
> > > > > > registered
> > > > > > in the
> > > > > >                       kernel before the cpufreq driver probes.
> > > > > >
> > > > > > +     cppc_cpufreq.auto_sel_mode=
> > > > > > +                     [CPU_FREQ] Enable ACPI CPPC autonomous
> > > > > > performance
> > > > > > +                     selection. When enabled, hardware 
> > > > > > automatically
> > > > > > adjusts
> > > > > > +                     CPU frequency on all CPUs based on workload
> > > > > > demands.
> > > > > > +                     In Autonomous mode, Energy Performance
> > > > > > Preference (EPP)
> > > > > > +                     hints guide hardware toward performance (0x0)
> > > > > > or energy
> > > > > > +                     efficiency (0xff).
> > > > > > +                     Requires ACPI CPPC autonomous selection 
> > > > > > register
> > > > > > +                     support.
> > > > > > +                     Accepts:
> > > > > > +                       performance, 1: enable auto_sel + set EPP to
> > > > > > +                                       performance (0x0)
> > > > > > +                       default_epp, 2: enable auto_sel, preserve 
> > > > > > EPP
> > > > > > value
> > > > > > +                                       programmed by BIOS/firmware
> > > > > > +                     Unset: cpufreq governors are used (auto_sel
> > > > > > disabled).
> > > > >
> > > > > Rather than unset doing nothing, have you considered having it take a
> > > > > midpoint like 128?  That's what we do in amd-pstate (default to
> > > > > balance_performance).  I think it turns into a reasonable balance.
> > > >
> > > > Thanks for the suggestion.
> > > > I can add balance_performance that enables auto_sel with EPP=128 in 
> > > > v4.
> > > >
> > > > On changing the driver default (no param behavior) to auto enable
> > > > balance_performance, it would be good to keep the current behavior for
> > > > now since cppc_cpufreq is generic across ARM64/RISC-V platforms where
> > > > EPP and Autonomous Selection registers are optional.
> > > > A default change would affect existing users relying on governors.
> > > >
> > > > Thank you,
> > > > Sumit Gupta
> > >
> > > But couldn't you make the "no module parameter set" follow the behavior
> > > to only set the registers if they're available?
> > >
> > > So the systems that support it start using it, the ones that don't it's
> > > a NOP.
> >
> > Would it work to add balance_performance as a new mode in v4,
> > and discuss changing the default separately as a follow-up?
>
> Sure.
>
> > Runtime detection helps for unsupported platforms. But platforms which
> > support the registers use OS governors today, and silently switching
> > them to autonomous mode on a kernel update is a behavior change for
> > existing users. They would also have no way to boot into sw governor.
>
> But hopefully it should be better battery life/responsiveness for those
> scenarios too, right?

Yes in many cases, but if some workloads rely on specific OS governor
configurations, then that would get impacted.
I will send a separate change later to seek broader consensus on
enabling auto_sel as default without any param.

Thank you,
Sumit Gupta

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