Re: [PATCH 1/2] PM / Domains: Introduce domain-performance-state binding

From: Vincent Guittot
Date: Wed Nov 23 2016 - 10:56:15 EST

On 23 November 2016 at 16:51, Kevin Hilman <khilman@xxxxxxxxxxxx> wrote:
> Vincent Guittot <vincent.guittot@xxxxxxxxxx> writes:
>> On 22 November 2016 at 19:12, Kevin Hilman <khilman@xxxxxxxxxxxx> wrote:
>>> Viresh Kumar <viresh.kumar@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>> > ---
>>>>> > 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


> Kevin