Date:   Wed, 23 Nov 2016 16:55:38 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Kevin Hilman <khilman@...libre.com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        "linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Lina Iyer <lina.iyer@...aro.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Nayak Rajendra <rnayak@...eaurora.org>
Subject: Re: [PATCH 1/2] PM / Domains: Introduce domain-performance-state binding

On 23 November 2016 at 16:51, Kevin Hilman <khilman@...libre.com> wrote:
> Vincent Guittot <vincent.guittot@...aro.org> writes:
>
>> On 22 November 2016 at 19:12, Kevin Hilman <khilman@...libre.com> wrote:
>>> Viresh Kumar <viresh.kumar@...aro.org> writes:
>>>
>>>> On 21-11-16, 09:07, Rob Herring wrote:
>>>>> On Fri, Nov 18, 2016 at 02:53:12PM +0530, Viresh Kumar wrote:
>>>>> > Some platforms have the capability to configure the performance state of
>>>>> > their Power Domains. The performance levels are represented by positive
>>>>> > integer values, a lower value represents lower performance state.
>>>>> >
>>>>> > The power-domains until now were only concentrating on the idle state
>>>>> > management of the device and this needs to change in order to reuse the
>>>>> > infrastructure of power domains for active state management.
>>>>> >
>>>>> > This patch introduces a new optional property for the consumers of the
>>>>> > power-domains: domain-performance-state.
>>>>> >
>>>>> > If the consumers don't need the capability of switching to different
>>>>> > domain performance states at runtime, then they can simply define their
>>>>> > required domain performance state in their node directly. Otherwise the
>>>>> > consumers can define their requirements with help of other
>>>>> > infrastructure, for example the OPP table.
>>>>> >
>>>>> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
>>>>> > ---
>>>>> >  Documentation/devicetree/bindings/power/power_domain.txt | 6 ++++++
>>>>> >  1 file changed, 6 insertions(+)
>>>>> >
>>>>> > diff --git a/Documentation/devicetree/bindings/power/power_domain.txt b/Documentation/devicetree/bindings/power/power_domain.txt
>>>>> > index e1650364b296..db42eacf8b5c 100644
>>>>> > --- a/Documentation/devicetree/bindings/power/power_domain.txt
>>>>> > +++ b/Documentation/devicetree/bindings/power/power_domain.txt
>>>>> > @@ -106,6 +106,12 @@ domain provided by the 'parent' power controller.
>>>>> >   - power-domains : A phandle and PM domain specifier as defined by bindings of
>>>>> >                     the power controller specified by phandle.
>>>>> >
>>>>> > +Optional properties:
>>>>> > +- domain-performance-state: A positive integer value representing the minimum
>>>>> > +  performance level (of the parent domain) required by the consumer for its
>>>>> > +  working. The integer value '1' represents the lowest performance level and the
>>>>> > +  highest value represents the highest performance level.
>>>>>
>>>>> How does one come up with the range of values?
>>>>
>>>> Why would we need a range here? The value here represents the minimum 'state'
>>>> and the assumption is that everything above that level would be fine. So the
>>>> range is automatically: domain-performance-state -> MAX.
>>>>
>>>>> It seems like you are
>>>>> just making up numbers. Couldn't the domain performance level be an OPP
>>>>> in the sense that it is a collection of clock frequencies and voltage
>>>>> settings?
>>>>
>>>> The clock is going to be handled by the device itself (at least for the case we
>>>> have today) and the performance-state lies with the power-domain which is
>>>> configured separately. If the performance level includes both clk and voltage,
>>>> then why would we need to show the clock rates in the DT ? Wouldn't a
>>>> performance level be enough in such cases?
>>>
>>> I think the question is: what does the performance-level of a domain
>>> actually mean?  Or, what are the units?
>>>
>>> Depending on the SoC, there's probably a few things this could mean.  It
>>> might mean is that an underlying bus/interconnect can be configured to
>>> guarantee a specific bandwidth or throughput.  That in turn might mean
>>> that that bus/interconnect might have to be set at a specific
>>> frequency/voltage.
>>>
>>> In your case, IIUC, you're just passing some magic value to some
>>> firmware running on a micro-controller, but under the hood that uC is
>>> probably configuring a frequency/voltage someplace.
>>
>> In the case described by Viresh, it's only about setting the voltage
>> of a power domain that is shared between different devices. these
>> devices wants to run at different frequency (set by the devices) but
>> we have to select a Volateg value that will match with the constraint
>> of all devices (in this case the highest voltage)
>
> Then, at least for this use case, we're talking about voltage, not some
> unspecified units.
>
> But that makes me wonder, this performance state sounds like something
> that is changing dynamically at runtime, so why do you want to describe
> this statically in DT?
>
> This sounds to me like the job of the genpd.  When any device in the
> domain does its pm_runtime_get(), the domain could check the device
> frequency and see if it needs to change the domain voltage in order for
> that device to operate at that frequency.  When the device goes away
> (using pm_runtime_put()) the domain can check again if it could lower
> the voltage and still meet the requirements of the remaining devices.

That's only part of the job. The device can change its frequency and
as a result ask for a new voltage index while it is already running

Vincent

>
> Kevin
>
>
>

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