linux-kernel - Re: [v3 PATCH 2/3] powernv-cpufreq: Fix pstate_to

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Date:   Wed, 3 Jan 2018 23:47:58 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Abhishek <huntbag@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        linux-pm@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [v3 PATCH 2/3] powernv-cpufreq: Fix pstate_to_idx() to handle
 non-continguous pstates

On Wed, Jan 3, 2018 at 11:07 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Monday, December 18, 2017 9:38:20 AM CET Gautham R Shenoy wrote:
>> Hi Balbir,
>>
>> On Sun, Dec 17, 2017 at 02:15:25PM +1100, Balbir Singh wrote:
>> > On Wed, Dec 13, 2017 at 5:57 PM, Gautham R. Shenoy
>> > <ego@...ux.vnet.ibm.com> wrote:
>> > > From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>> > >
>> > > The code in powernv-cpufreq, makes the following two assumptions which
>> > > are not guaranteed by the device-tree bindings:
>> > >
>> > >     1) Pstate ids are continguous: This is used in pstate_to_idx() to
>> > >        obtain the reverse map from a pstate to it's corresponding
>> > >        entry into the cpufreq frequency table.
>> > >
>> > >     2) Every Pstate should always lie between the max and the min
>> > >        pstates that are explicitly reported in the device tree: This
>> > >        is used to determine whether a pstate reported by the PMSR is
>> > >        out of bounds.
>> > >
>> > > Both these assumptions are unwarranted and can change on future
>> > > platforms.
>> >
>> > While this is a good thing, I wonder if it is worth the complexity. Pstates
>> > are contiguous because they define transitions in incremental value
>> > of change in frequency and I can't see how this can be broken in the
>> > future?
>>
>> In the future, we can have the OPAL firmware give us a smaller set of
>> pstates instead of expose every one of them. As it stands today, for
>> most of the workloads, we will need at best 20-30 pstates and not
>> beyond that.
>
> I'm not sure about the status here.
>
> Is this good to go as is or is it going to be updated?
>

I have no major objections, except some of the added complexity, but
Gautham makes a point that this is refactoring for the future

Balbir Singh.