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Re: [PATCH V6 1/9] PM / OPP: Introduce "power-domain-opp" property

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  Re: [PATCH V6 1/9] PM / OPP: Introduce "power-domain-opp" property Kevin Hilman <khilman@baylibre.com> - 2017-05-12 17:10 +0200
    Re: [PATCH V6 1/9] PM / OPP: Introduce "power-domain-opp" property Viresh Kumar <viresh.kumar@linaro.org> - 2017-05-12 18:20 +0200

#1640526 — Re: [PATCH V6 1/9] PM / OPP: Introduce "power-domain-opp" property

Viresh Kumar <viresh.kumar@linaro.org> writes:

> On 06-05-17, 11:58, Kevin Hilman wrote:
>> Rob Herring <robh@kernel.org> writes:
>> 
>> > On Wed, Apr 26, 2017 at 04:27:05PM +0530, Viresh Kumar wrote:
>> >> Power-domains need to express their active states in DT and the devices
>> >> within the power-domain need to express their dependency on those active
>> >> states. The power-domains can use the OPP tables without any
>> >> modifications to the bindings.
>> >> 
>> >> Add a new property "power-domain-opp", which will contain phandle to the
>> >> OPP node of the parent power domain. This is required for devices which
>> >> have dependency on the configured active state of the power domain for
>> >> their working.
>> >> 
>> >> For some platforms the actual frequency and voltages of the power
>> >> domains are managed by the firmware and are so hidden from the high
>> >> level operating system. The "opp-hz" property is relaxed a bit to
>> >> contain indexes instead of actual frequency values to support such
>> >> platforms.
>> >> 
>> >> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
>> >> ---
>> >>  Documentation/devicetree/bindings/opp/opp.txt | 74 ++++++++++++++++++++++++++-
>> >>  1 file changed, 73 insertions(+), 1 deletion(-)
>> >> 
>> >> diff --git a/Documentation/devicetree/bindings/opp/opp.txt b/Documentation/devicetree/bindings/opp/opp.txt
>> >> index 63725498bd20..6e30cae2a936 100644
>> >> --- a/Documentation/devicetree/bindings/opp/opp.txt
>> >> +++ b/Documentation/devicetree/bindings/opp/opp.txt
>> >> @@ -77,7 +77,10 @@ This defines voltage-current-frequency combinations along with other related
>> >>  properties.
>> >>  
>> >>  Required properties:
>> >> -- opp-hz: Frequency in Hz, expressed as a 64-bit big-endian integer.
>> >> +- opp-hz: Frequency in Hz, expressed as a 64-bit big-endian integer. In some
>> >> +  cases the exact frequency in Hz may be hidden from the OS by the firmware and
>> >> +  this field may contain values that represent the frequency in a firmware
>> >> +  dependent way, for example an index of an array in the firmware.
>> >
>> > Not really sure OPP binding makes sense here.
>> 
>> I think OPP makes perfect sense here, because microcontroller firmware
>> is managaging OPPs in hardware.  We just may not know the exact voltage
>> and/or frequency (and the firmware/hardware may even be doing AVS for
>> micro-adjustments.)
>
> Yes, AVS is being done for the Qcom SoC as well.
>
>> > What about all the other properties. We expose voltage, but not freq?
>> 
>> I had the same question.  Seems the same comment about an abstract
>> "index" is needed for voltage also.
>
> Why should we do that?

For starters, because the lack of it looks very strange upon first read
(notice that both Rob and I pointed that out), and because you didn't
explain why in the first place, it draws attention.

> Here are the cases that I had in mind while writing this:
>
> - DT only contains the performance-index and nothing else (i.e. voltages aren't
>   exposed).
>
>   We wouldn't be required to fill the microvolt property as it is optional.
>
> - DT contains both performance-index and voltages.
>
>   The microvolts property will contain the actual voltages and opp-hz will
>   contain the index.
>
> I don't see why would we like to put some index value in the microvolts
> property. We are setting the index value in the opp-hz property to avoid adding
> extra fields and making sure opp-hz is still the unique property for the nodes.

What about the case where firmware wants exact frequencies, and
microvolts property is just an index?

The point is, you have a very specific SoC and use-case in mind, but the
goal of a binding change like this is to make something that could be
generically useful.

>> >>  
>> >>  Optional properties:
>> >>  - opp-microvolt: voltage in micro Volts.
>> >> @@ -154,6 +157,13 @@ properties.
>> >>  
>> >>  - status: Marks the node enabled/disabled.
>> >>  
>> >> +- power-domain-opp: Phandle to the OPP node of the parent power-domain. The
>> >> +  parent power-domain should be configured to the OPP whose node is pointed by
>> >> +  the phandle, in order to configure the device for the OPP node that contains
>> >> +  this property. The order in which the device and power domain should be
>> >> +  configured is implementation defined. The OPP table of a device can set this
>> >> +  property only if the device node contains "power-domains" property.
>> >> +
>> 
>> I do understand the need to map a device OPP to a parent power-domain
>> OPP, but I really don't like another phandle.
>> 
>> First, just because a device OPP changes does not mean that a
>> power-domain OPP has to change.  What really needs to be specified is a
>> minimum requirement, not an exact OPP.  IOW, if a device changes OPP,
>> the power-domain OPP has to be *at least* an OPP that can guarantee that
>> level of performance, but could also be a more performant OPP, right?
>
> Right and that's how the code is interpreting it right now. Yes, the description
> above should have been more clear on that though.
>
>> Also, the parent power-domain driver will have a list of all its
>> devices, and be able to get OPPs from those devices.
>> 
>> IMO, we should do the first (few) implementations of this feature from
>> the power-domain driver itself, and not try to figure out how to define
>> this for everyone in DT until we have a better handle on it (pun
>> intended) ;)
>
> Hmm, I am not sure how things are going to work in that case. The opp-hz value
> read from the phandle is passed to the QoS framework in this series, which makes
> sure that we select the highest requested performance point for a particular
> power-domain. The index value is required to be present with the OPP framework
> to make it all work, at least based on the way I have designed it for now.

IMO, this kind of dependency isn't the job of the OPP framework, it's
the job of the power-domain governor.

Kevin

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#1640566

On 12 May 2017 at 20:29, Kevin Hilman <khilman@baylibre.com> wrote:
> Viresh Kumar <viresh.kumar@linaro.org> writes:

>> Why should we do that?
>
> For starters, because the lack of it looks very strange upon first read
> (notice that both Rob and I pointed that out), and because you didn't
> explain why in the first place, it draws attention.

:)

>> I don't see why would we like to put some index value in the microvolts
>> property. We are setting the index value in the opp-hz property to avoid adding
>> extra fields and making sure opp-hz is still the unique property for the nodes.
>
> What about the case where firmware wants exact frequencies, and
> microvolts property is just an index?
>
> The point is, you have a very specific SoC and use-case in mind, but the
> goal of a binding change like this is to make something that could be
> generically useful.

I agree, but I am not sure of having such a case in very near future at least.
Wouldn't it be wise to not touch opp-microvolt for now and update it only
when needed? Its not a big change anyway..

>> Hmm, I am not sure how things are going to work in that case. The opp-hz value
>> read from the phandle is passed to the QoS framework in this series, which makes
>> sure that we select the highest requested performance point for a particular
>> power-domain. The index value is required to be present with the OPP framework
>> to make it all work, at least based on the way I have designed it for now.
>
> IMO, this kind of dependency isn't the job of the OPP framework, it's
> the job of the power-domain governor.

Okay. So the way it will work with the current suggestions is:

- OPP framework gets DVFS update request for device X
- OPP framework finds that the device has a power-domain and so it asks
  the power-domain framework to set the device in a particular state
  corresponding to the OPP (if we are going to a higher OPP).
- If the power-domain supports state selection, it does that or returns error.
  (Actually we can optimize this by asking the genpd initially if
state selection
   is possible, only then OPP core calls the genpd API).
- The genpd API will manage a list of all devices in the domain (which it
  already does) and also the states selected for them. It finds the max of
  the requested states and selects that.
- Note that the QoS framework isn't there in the picture anymore.

Will that be fine ?

--
viresh

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