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

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Message-Id: <20171218083820.GA28881@in.ibm.com>
Date:   Mon, 18 Dec 2017 14:08:20 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        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

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.


> 
> Balbir Singh.
> 

--
Thanks and Regards
gautham.