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

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Message-Id: <20180110085545.GA13666@in.ibm.com>
Date:   Wed, 10 Jan 2018 14:25:45 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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

Hi Rafael,

On Wed, Jan 03, 2018 at 11:47:58PM +1100, Balbir Singh wrote:
> 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

I have tested this across POWER8 and POWER9. The additional complexity
introduced by the second patch is required for the future when we are
going to reduce the number of pstates.


> 
> Balbir Singh.
> 
--
Thanks and Regards
gautham.