RE: [PATCH v2 0/7] CPPC optional registers AMD support

[Ghannam, Yazen]

> -----Original Message-----
> From: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> Sent: Tuesday, April 16, 2019 4:34 AM
> To: Natarajan, Janakarajan <Janakarajan.Natarajan@xxxxxxx>
> Cc: Natarajan, Janakarajan <Janakarajan.Natarajan@xxxxxxx>; linux-acpi@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> pm@xxxxxxxxxxxxxxx; devel@xxxxxxxxxx; Rafael J . Wysocki <rjw@xxxxxxxxxxxxx>; Len Brown <lenb@xxxxxxxxxx>; Viresh Kumar
> <viresh.kumar@xxxxxxxxxx>; Robert Moore <robert.moore@xxxxxxxxx>; Erik Schmauss <erik.schmauss@xxxxxxxxx>; Ghannam, Yazen
> <Yazen.Ghannam@xxxxxxx>
> Subject: Re: [PATCH v2 0/7] CPPC optional registers AMD support
> 
> On Tue, Apr 16, 2019 at 12:35 AM Janakarajan Natarajan <jnataraj@xxxxxxx> wrote:
> >
> > On 4/4/19 4:25 PM, Natarajan, Janakarajan wrote:
> > > CPPC (Collaborative Processor Performance Control) offers optional
> > > registers which can be used to tune the system based on energy and/or
> > > performance requirements.
> > >
> > > Newer AMD processors add support for a subset of these optional CPPC
> > > registers, based on ACPI v6.1.
> > >
> > > The following are the supported CPPC registers for which sysfs entries
> > > are created:
> > > * enable                (NEW)
> > > * max_perf              (NEW)
> > > * min_perf              (NEW)
> > > * energy_perf
> > > * lowest_perf
> > > * nominal_perf
> > > * desired_perf          (NEW)
> > > * feedback_ctrs
> > > * auto_sel_enable       (NEW)
> > > * lowest_nonlinear_perf
> > >
> > > The CPPC driver is updated to enable the OSPM and the userspace to
> > > access
> > > the newly supported registers.
> > >
> > > The purpose of exposing the registers via the sysfs entries is to allow
> > > the userspace to:
> > > * Tweak the values to fit its workload.
> > > * Apply a profile from AMD's optimization guides.
> > >
> > > Profiles will be documented in the performance/optimization guides.
> > >
> > > Note:
> > > * AMD systems will not have a policy applied in the kernel at this time.
> > > * By default, acpi_cpufreq will still be used.
> > >
> > > TODO:
> > > * Create a linux userspace tool that will help users generate a CPPC
> > > * profile
> > >    for their target workload.
> > > * Create or update a driver to apply a general CPPC policy in the
> > > * kernel.
> > >
> > > v1->v2:
> > > * Add macro to ensure BUFFER only registers have BUFFER type.
> > > * Add support macro to make the right check based on register type.
> > > * Remove support checks for registers which are mandatory.
> >
> >
> > Are there any concerns regarding this patchset?
> 
> Yes, there are.
> 
> Unfortunately, it is generally problematic.
> 
> First off, the behavior of existing sysfs files cannot be changed at
> will, as there may be users of them out there already depending on the
> current behavior.
> 

The intent is to add new sysfs files without changing the existing files. Is that okay?

Or is the addition of new files also not acceptable?

> Second, at least some CPPC control registers are used by cpufreq
> drivers (cppc_cpufreq and intel_pstate), so modifying them behind the
> drivers' backs is not a good idea in general.  For this reason, adding
> new sysfs attributes to allow user space to do that is quite
> questionable.
> 

Yes, good point.

What if a check is added so that writes only succeed if the CPUFREQ governor is set to userspace?

Thanks,
Yazen