linux-kernel - Re: [RFC/PATCH 0/5] DVFS in the OPP core

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Message-ID: <CAJZ5v0jY5Zjg0R7uC6ewJT5Pi-15Mohotj=im9k8w4THJSM0Dw@mail.gmail.com>
Date:   Thu, 31 Jan 2019 10:58:26 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Michael Turquette <mturquette@...libre.com>,
        grahamr@...eaurora.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        linux-serial@...r.kernel.org,
        linux-spi <linux-spi@...r.kernel.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Doug Anderson <dianders@...omium.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC/PATCH 0/5] DVFS in the OPP core

On Thu, Jan 31, 2019 at 10:23 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> Adding few folks to the thread who might be interested in this stuff.
>
> On 28-01-19, 17:55, Stephen Boyd wrote:
> > This patch series is an RFC around how we can implement DVFS for devices
> > that aren't your typical OPPish device (i.e. GPU/CPU). They don't have a
> > strict set of frequencies that they have been tested at to derive some
> > operating performance point. Instead they have a coarser set of
> > frequency max or 'fmax' OPPs that describe the maiximum frequency the
> > device can operate at with a given voltage.
> >
> > The approach we take is to let the devm_pm_opp_set_rate() API accept 0
> > as a valid frequency indicating the frequency isn't required anymore and
> > to make the same API use the clk framework to round the frequency passed
> > in instead of relying on the OPP table to specify each frequency that
> > can be used. Once we have these two patches in place, we can use the OPP
> > API to change clk rates instead of clk_set_rate() and use all the recent
> > OPP enhancements that have been made around required-opps and genpd
> > performance states to do DVFS for all devices.
>
> Generally speaking I am fine with such an approach but I am not sure
> about what others would say on this as they had objections to using
> OPP core for setting the rate itself.
>
> FWIW, I suggested exactly this solution sometime back [1]
>
> - Drivers need to use two API sets to change clock rate (OPP helpers)
>   and enable/disable them (CLK framework helpers) and this leads us to
>   exactly that combination. Is that acceptable ? It doesn't look great
>   to me as well..

I agree here.

> - Do we expect the callers will disable clk before calling
>   opp-set-rate with 0 ? We should remove the regulator requirements as
>   well along with perf-state.

Well, disabling clk affects HW in general, doesn't it?

> - What about enabling/disabling clock as well from OPP framework. We
>   can enable it on the very first call to opp-set-rate and disable
>   when freq is 0. That will simplify the drivers as well.

That sounds compelling, but I guess there are cases in which you can
gate the clock regardless of the frequency setting.  How would that
work then?