Re: [PATCH v2] cpufreq: CPPC: add autonomous mode boot parameter support

[Sumit Gupta]

On 08/05/26 08:03, Jie Zhan wrote:
> External email: Use caution opening links or attachments
>
>
> On 5/5/2026 8:29 PM, Sumit Gupta wrote:
> > Hi Jie,
> >
> >
> > On 27/04/26 13:54, Jie Zhan wrote:
> > > External email: Use caution opening links or attachments
> > >
> > >
> > > Hi Sumit,
> > >
> > > In general, I would expect this parameter only toggles on auto_sel by
> > > default.  IIUC, other CPPC configurations (min/max/desired perf, EPP,
> > > enable) are optional and not closely related to this.
> > >
> > > Why including those stuff here?
> > >
> > >
> > > Please see other questions inline.
> > >
> > > Thanks!
> > > Jie
> > These together provide a known, predictable autonomous mode boot.
> > min/max/desired seeding ensures HW has a known starting bound (BIOS
> > may leave them unset).
> > EPP=PERFORMANCE_PREF is needed as BIOS defaults often bias toward
> > energy saving, and admins on many CPU systems shouldn't have to script
> > per CPU sysfs writes at every boot to undo that.
> Hi Sumit,
>
> The min/max/desired perf might be a slightly different case, but the EPP
> value should probably follow the default if there is?
> Otherwise, users may complain that their BIOS defaults don't work and find
> out it's driver's fault.
>
> Alternatively, we may also make an assumption that we ignore BIOS EPP
> config, but not sure if that's suitable?
>
> (Perhaps let's just discuss in the bottom trunk because the main issue is
> pretty much the same.)
> > > On 4/25/2026 4:18 AM, 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 auto_sel_mode=1:
> > > > - Configure all CPUs for autonomous operation on first init
> > > > - Set EPP to performance preference (0x0)
> > > > - Use HW min/max_perf when available; otherwise initialize from caps
> > > > - Clamp desired_perf to bounds before enabling autonomous mode
> > > > - Hardware controls frequency instead of the OS governor
> > > >
> > > > 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 [2] ("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.
> > > >
> > > > Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> (Documentation)
> > > > Signed-off-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> > > > ---
> > > > v[1] -> v2:
> > > > - Call cppc_set_enable() unconditionally so CPPC is enabled for both
> > > >     OS-driven and autonomous modes.
> > > Why adding this in v2?
> > > This looks like a separate issue since setting CPPC Enable reg doesn't seem
> > > to be related with autonomous control.
> > In v2, moved it out of the auto_sel specific check.
> > Agree that cppc_set_enable() is general CPPC enablement and can be split
> > into a separate patch in v3 if preferred.
> Yeah it should be a separate patch I think.
> > > > - Init min/max from caps instead of cppc_cpufreq_update_perf_limits()
> > > >     as policy->min/max aren't yet populated.
> > > >
> > > > [1] https://lore.kernel.org/lkml/20260317151053.2361475-1-sumitg@xxxxxxxxxx/
> > > > [2] https://lore.kernel.org/lkml/20260423084731.1090384-2-pierre.gondois@xxxxxxx/
> > > > ---
> > > >    .../admin-guide/kernel-parameters.txt         | 13 +++
> > > >    drivers/cpufreq/cppc_cpufreq.c                | 89 +++++++++++++++++--
> > > >    2 files changed, 97 insertions(+), 5 deletions(-)
> ...
> > > > +
> > > > +             policy->cur = cppc_perf_to_khz(caps,
> > > > +                                            cpu_data->perf_ctrls.desired_perf);
> > > > +
> > > > +             /* EPP is optional - some platforms may not support it */
> > > > +             ret = cppc_set_epp(cpu, CPPC_EPP_PERFORMANCE_PREF);
> > > Why setting this to PERFORMANCE by default?
> > > A platform can have its own default EPP value.  This would override that.
> > The boot option targets performance oriented use cases on many CPU
> > systems, avoiding sysfs scripting across all CPUs on every boot.
> > The BIOS default EPP (often biased toward energy saving) would otherwise
> > steer HW away from that goal. Admins can still re-tune EPP at runtime via
> > the existing energy_performance_preference_val sysfs.
> Yeah, avoiding scripting makes sense for sure.
> The thing is how do we do this nicely.
>
> For now we need to consider follow either the BIOS default or the driver
> default.
> They seem to be exclusive since there isn't a clear way to support both at
> the same time.
>
> So, are we going to support the BIOS default case?
> For example, users can config a default EPP in the BIOS setup menu.

To consider the BIOS default case, we can extend the parameter to accept
an EPP mode in v3:
- cppc_cpufreq.auto_sel_mode=performance / =1 (EPP=performance)
- cppc_cpufreq.auto_sel_mode=default / =2 (preserve BIOS EPP)

Thank you,
Sumit Gupta

....