Message-ID: <20201029134953.k3ag72zujqsdt3fm@platonic>
Date:   Thu, 29 Oct 2020 08:49:53 -0500
From:   Nishanth Menon <nm@...com>
To:     Lukasz Luba <lukasz.luba@....com>
CC:     <linux-kernel@...r.kernel.org>, <linux-pm@...r.kernel.org>,
        <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <vireshk@...nel.org>,
        <robh+dt@...nel.org>, <sboyd@...nel.org>, <rafael@...nel.org>,
        <sudeep.holla@....com>, <daniel.lezcano@...aro.org>,
        <Dietmar.Eggemann@....com>
Subject: Re: [PATCH 1/4] dt-bindings: opp: Introduce opp-sustainable bindings

On 13:33-20201029, Lukasz Luba wrote:
> 
> 
> On 10/29/20 12:59 PM, Nishanth Menon wrote:
> > On 10:04-20201029, Lukasz Luba wrote:
> > > 
> > > 
> > > On 10/28/20 9:47 PM, Nishanth Menon wrote:
> > > > On 14:08-20201028, Lukasz Luba wrote:
> > > > > Add opp-sustainable as an additional property in the OPP node to describe
> > > > > the sustainable performance level of the device. This will help to
> > > > > estimate the sustainable performance of the whole system.
> > > > > 
> > > > > Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> > > > > ---
> > > > >    Documentation/devicetree/bindings/opp/opp.txt | 4 ++++
> > > > >    1 file changed, 4 insertions(+)
> > > > > 
> > > > > diff --git a/Documentation/devicetree/bindings/opp/opp.txt b/Documentation/devicetree/bindings/opp/opp.txt
> > > > > index 9847dfeeffcb..cd01028de305 100644
> > > > > --- a/Documentation/devicetree/bindings/opp/opp.txt
> > > > > +++ b/Documentation/devicetree/bindings/opp/opp.txt
> > > > > @@ -154,6 +154,10 @@ Optional properties:
> > > > >    - opp-suspend: Marks the OPP to be used during device suspend. If multiple OPPs
> > > > >      in the table have this, the OPP with highest opp-hz will be used.
> > > > > +- opp-sustainable: Marks the OPP as sustainable. This property can be used for
> > > > > +  estimating sustainable performance of the whole system. If multiple OPPs in
> > > > > +  the table have this, the OPP with highest opp-hz will be used.
> > > > 
> > > > 
> > > > By "sustainable", do you mean sustainable across Process, Voltage and
> > > > Temperature corners upto the max rated operational Power-ON hours
> > > > without IDLE state being achieved on the processor?
> > > 
> > > Yes, in case of CPU: running 100% without idle at that particular OPP.
> > > Running above that OPP would lead to cross control temperature.
> > 
> > We need to tighten the definitions a lot more here and add that to the
> > binding. What we are stating, if I am not misunderstanding is an OPP
> > that is guaranteed by SoC vendor that across Process Voltage and
> > Temperature corners - aka across the entire production spectrum
> > for the part number, *all* devices will operate at this OPP for the
> > mandated power-on-hours rating without hitting IDLE.
> > 
> > Example: So -40C to 125C, across the process (hot/cold/nominal), 100s of
> > thousands/millions of units can operate upto 125,0000 power-on-hours
> > while running a tight deadloop OR maybe high processing function or even
> > cpuburn[1]?
> 
> I think I know what you mean. But this would lead to redefining a lot
> more that just this optional field. This wide range -40C to 125C is for
> automotive chips, then what about opp-suspend, when the device cannot
> even reach that OPP under some stress test e.g. outside temp
> ~100-110C...
> Or opp-turbo, shell all the OPPs have multidimensional table to reflect
> the temperature dependency for all affected optional fields?

yes, and down the rabbit hole we will go :)

> 
> > 
> > 
> > Can you give me one SoC vendor and part that guarantees this? I am
> > wondering if this is all theoretical... There are tons of parameters
> > that come into play for "reliability" "sustainability" etc. Those are
> > tricky terminology that typically makes legal folks pretty happy to
> > debate for decades..
> 
> Yes, but the outside temperature is probably most important for this use
> case.
> 
> > 
> > just my 2 cents.
> > > 
> > > > 
> > > > OR do you mean to leave it up to interpretation?
> > > 
> > > I can tell how I would use them. There is thermal governor IPA, which
> > > needs sustainable power either form DT or uses internal algorithm to
> > > estimate it based on lowest allowed freq OPPs. Then it estimated
> > > internal coefficients based on that value, which is not optimal
> > > for lowest OPPs. When some higher OPP could be marked as sustainable,
> > > it would lead to better estimation and better power budget split.
> > 
> > Seeing your series, I got an idea about how you plan on using it, I
> > just think we need to be more precise in our definition..
> 
> Thank you for having a look on that and understanding the motivation
> behind this series.
> 
> How about adding a description that this sustainable OPP is considered
> for normal room temp (20-25C)?

You could.. but then, practically as we go into smaller process nodes,
the 20-25C reliability is just theoretical. I mean, we Texans in summer
or Finns in winter would probably define "normal room temperature" as
something different in practise (ISO not withstanding ;) ).. Challenge
of reliability has always been on the edge of the PVT ranges. To ignore
that OR to have a scheme that does not scale to describe that, IMHO is a
lacking definition.

My entire point is, if we can avoid getting into rabbit hole
definitions, we probably should.. IMHO.. keep things as simple as
possible.

> 
> BTW, in the Arm SCMI spec definition of that value (used in patch 4/4),

You mean [1] Table 11 Performance Domain Levels with Special
	Significance
> there is no specific temperature for it, just:
> 'This is the maximum performance level that the platform can
> sustain under normal conditions. In exceptional circumstances,
> such as thermal runaway, the platform might not be be able to
> guarantee this level.'
> 

Hehe.. Vincent and SCMI teams have been having fun there :)... But, I
think the definition has little practical significance for the very
reasons I made above IMHO, and with full respect to SCMI team(defining
SCMI is not an easy task, I admit) - it is at best a theoretical,
"works at the engineer's cube definition", as typical "nominal
operation conditions" escape clause tend to be, OR at the worst
ignoring to define the parameters that constitute what would bound
things in a closed box precisely (example: does'nt mention process, so
just nominal OR considers all process corners - what does omission of
that factor really mean?).


> I can put this whole description into the DT binding, if you like.

Will leave it to Viresh and others to comment and guide, the terminology
got my attention, since I almost got bit by a similar usage.. my 2 cents:
I dont think that suffices unfortunately. what it lacks are the
parameters of what that terminology really means,

One actual production part that demonstrates this will probably help
guide the discussion, I guess..

/me goes back to OPP hibernation

> 

[1] https://developer.arm.com/documentation/den0056/b

-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D

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