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

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Date:   Wed, 03 Jan 2018 13:07:28 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     ego@...ux.vnet.ibm.com, Balbir Singh <bsingharora@...il.com>
Cc:     Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        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 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?

Thanks,
Rafael