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

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Message-ID: <CAKTCnz=jrvbizrzUx1tDxMg__4coJHoxSXOS+K+zXQVAM=WF-g@mail.gmail.com>
Date:   Sun, 17 Dec 2017 14:15:25 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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, 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?

Balbir Singh.