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

[Viresh Kumar <viresh.kumar@linaro.org>]

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? 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.

> >>  
> >>  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.

-- 
viresh