linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Force intel_pstate to load when HWP disabled in firmware

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Date:   Thu, 13 May 2021 12:10:04 +0200
From:   Giovanni Gherdovich <ggherdovich@...e.cz>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Len Brown <lenb@...nel.org>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: intel_pstate: Force intel_pstate to load when
 HWP disabled in firmware

On Thu, 2021-05-13 at 02:24 -0700, Srinivas Pandruvada wrote:
> On Thu, 2021-05-13 at 09:59 +0200, Giovanni Gherdovich wrote:
> > On CPUs succeeding SKX, eg. ICELAKE_X, intel_pstate doesn't load
> > unless
> > CPUID advertises support for the HWP feature. Some OEMs, however, may
> > offer
> > users the possibility to disable HWP from the BIOS config utility by
> > altering the output of CPUID.
>
> Is someone providing a utility? What is the case for broken HWP?

Yes, I know of at least one server manufacturer that ships a BIOS config
utility where the user can disable HWP.

On such server machine, which has an ICELAKE_X CPU, if the user unchecks HWP
via BIOS then intel_pstate will refuse to load saying:

    intel_pstate: CPU model not supported

because ICELAKE_X is not in the list intel_pstate_cpu_ids (defined in
intel_pstate.c) of CPUs that intel_pstate supports when HWP is absent from
CPUID; that list ends at SKYLAKE_X.

An alternative approach to register intel_pstate in the case I'm describing
would be to add ICELAKE_X (and every CPU model after that, forever?) to the
list intel_pstate_cpu_ids.

> It is possible that some user don't want to use HWP, because there
> workloads works better without HWP. But that doesn't mean HWP is
> broken.

That's true, a user may legitimate want to disable HWP, and we have the
intel_pstate=no_hwp option for that. But for that option to work CPUID must
still show that the CPU is HWP-capable; when disablement happens in BIOS, it's
not the case.

The wording "hwp_broken_firmware" deliberately has a negative connotation (the
intended meaning is: "firmware is broken, regarding HWP"), carrying the
not-so-subtle message "OEM folks, please don't do this". My understanding is
that the preferred way to disable HWP is with intel_pstate=no_hwp, the
firmware should stay out of it.

I hope this clarifies the problem (there is an ICELAKE_X somewhere out there
that can't load intel_pstate, which is not nice) and the intention
(discouraging disablement of HWP via firmware).

Giovanni