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Date:   Thu, 30 May 2019 09:47:10 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     x86 <x86@kernel.org>, Linux PM <linux-pm@vger.kernel.org>,
        Ido Schimmel <idosch@idosch.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Len Brown <len.brown@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Laura Abbott <labbott@fedoraproject.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Simon Schricker <sschricker@suse.de>,
        Borislav Petkov <bp@suse.de>, Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH] x86: intel_epb: Do not build when CONFIG_PM is unset
Message-ID: <20190530074710.GA68696@gmail.com>
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* Rafael J. Wysocki <rjw@rjwysocki.net> wrote:

> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> Commit 9ed0985332a6 ("x86: intel_epb: Take CONFIG_PM into account")
> prevented the majority of the Performance and Energy Bias Hint (EPB)
> handling code from being built when CONFIG_PM is unset to fix a
> regression introduced by commit b9c273babce7 ("PM / arch: x86:
> MSR_IA32_ENERGY_PERF_BIAS sysfs interface").
> 
> In hindsight, however, it would be better to skip all of the EPB
> handling code for CONFIG_PM unset as there really is no reason for
> it to be there in that case.  Namely, if the EPB is not touched
> by the kernel at all with CONFIG_PM unset, there is no need to
> worry about modifying the EPB inadvertently on CPU online and since
> the system will not suspend or hibernate then, there is no need to
> worry about possible modifications of the EPB by the platform
> firmware during system-wide PM transitions.
> 
> For this reason, revert the changes made by commit 9ed0985332a6
> and only allow intel_epb.o to be built when CONFIG_PM is set.
> 
> Note that this changes the behavior of the kernels built with
> CONFIG_PM unset as they will not modify the EPB on boot if it is
> zero initially any more, so it is not a fix strictly speaking, but
> users building their kernels with CONFIG_PM unset really should not
> expect them to take energy efficiency into account.  Moreover, if
> CONFIG_PM is unset for performance reasons, leaving EPB as set
> initially by the platform firmware will actually be consistent
> with the user's expectations.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> 
> This is complementary to the EPB handling changes made in the current
> development cycle, so IMO it would be good to do it in this cycle too
> if there are no technical concerns or objections regarding it.

Sure:

Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo