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

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Date:   Mon, 8 May 2017 11:06:54 +0530
From:   Rajendra Nayak <rnayak@...eaurora.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Kevin Hilman <khilman@...libre.com>
Cc:     Rob Herring <robh@...nel.org>, Rafael Wysocki <rjw@...ysocki.net>,
        ulf.hansson@...aro.org, Viresh Kumar <vireshk@...nel.org>,
        Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        lina.iyer@...aro.org, sudeep.holla@....com,
        devicetree@...r.kernel.org
Subject: Re: [PATCH V6 1/9] PM / OPP: Introduce "power-domain-opp" property



On 05/08/2017 09:45 AM, Viresh Kumar wrote:
> On 06-05-17, 11:58, Kevin Hilman wrote:
>> Rob Herring <robh@...nel.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@...aro.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? 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.

So the performance-index is specified in opp-hz property?
What if the microcontroller firmware maps the performance-index to voltage but
expects linux to scale the frequency? There is no way to specify a performance-index
*and* a frequency for a OPP now I guess?

> 
> - DT contains both performance-index and voltages.
> 
>   The microvolts property will contain the actual voltages and opp-hz will
>   contain the index.

So this is for cases where the performance-index maps to a freq managed by the
microcontroller and voltages managed by linux? I have a case of exact opposite
and I don't see now how to handle it now with these bindings.

> 
> 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.

Maybe to handle the case like what I described above?

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