linux-kernel - Re: [PATCH 3/5] thermal: devfreq_cooling: add new registration functions with Energy Model

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Message-ID: <20201201140520.GA7206@arm.com>
Date:   Tue, 1 Dec 2020 14:05:20 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, amit.kucheria@...durent.com,
        airlied@...ux.ie, daniel.lezcano@...aro.org, steven.price@....com,
        alyssa.rosenzweig@...labora.com, rui.zhang@...el.com,
        orjan.eide@....com
Subject: Re: [PATCH 3/5] thermal: devfreq_cooling: add new registration
 functions with Energy Model

Hi,

On Thursday 22 Oct 2020 at 12:17:31 (+0100), Lukasz Luba wrote:
[..]

> > > +/**
> > > + * devfreq_cooling_em_register_power() - Register devfreq cooling device with
> > > + *		power information and attempt to register Energy Model (EM)
> > 
> > It took me a while to understand the differences between devfreq
> > register functions and it left me with a nagging feeling that we don't
> > need all of them. Also, looking over the cpufreq cooling devices, they
> > keep their registering interfaces quite simple.
> 
> This was discussed in previous series, related to EM core changes.
> It was requested to have a helper registration function which would
> create EM automatically.
> 
> > 
> > With the functions added by this patch, the devfreq cooling devices will have:
> >   - old:
> >         of_devfreq_cooling_register_power
> >         of_devfreq_cooling_register
> >         devfreq_cooling_register
> >         devfreq_cooling_unregister
> >   - new:
> >         devfreq_cooling_em_register_power
> >         devfreq_cooling_em_register
> > 
> > My question is whether we actually need the two new
> > devfreq_cooling_em_register_power() and devfreq_cooling_em_register()?
> 
> It is just for consistency, with older scheme. It is only a wrapper, one
> line, with default NULL. This scheme is common in thermal and some other
> frameworks.
> 
> > 
> > The power_ops and the em are dependent on one another, so could we
> > extend the of_devfreq_cooling_register_power() to do the additional em
> > registration. We only need a way to pass the em_cb and I think that
> > could fit nicely in devfreq_cooling_power.
> 
> No, they aren't 'dependent on one another'. The EM usage doesn't depend
> on presence of power_ops. Drivers might not support power_ops, but want
> the framework still use EM and do power estimation.
> 

Okay, wrong choice of words. There's only a one way dependency: you can't
use power_ops without an em, according to
of_devfreq_cooling_register_power().

Correct me if I'm wrong, but I see this as being okay as you still need
an em to give you the maximum power of a device in a certain state.

With this in mind, and taking in detail the possible calls of the
devfreq cooling register functions:

1. Register devfreq cooling device with energy model.
   (used in patch 5/5)

 -> devfreq_cooling_em_register()
    -> devfreq_cooling_em_register_power(dfc_power = NULL, em obtained
                                      through various methods)
      -> of_devfreq_cooling_register_power(same as above)

2. Register devfreq cooling device with power_ops and em:
   (not used)

 -> devfreq_cooling_em_register_power(dfc_power != NULL, em obtained
                                     through various methods)
   -> of_devfreq_cooling_register_power(same as above)

3. Register a devfreq cooling devices with power_ops but no em
   (not used)

 -> of_devfreq_cooling_register_power(dfc_power != NULL)

4. Register a devfreq cooling devices without any kind of power
   information (em or dfc_power/power_ops)

 -> devfreq_cooling_register() or of_devfreq_cooling_register()
   -> of_devfreq_cooling_register_power(dfc_power = NULL)

Given this, aren't we ending up with some possible calls to these
registration functions that don't make sense? That is case 3, as
of_devfreq_cooling_register_power() could not assign and later use
power_ops without an em. For this usecase, 2 should be used instead.

Therefore, can't the same be achieved by collapsing
devfreq_cooling_em_register_power() into
of_devfreq_cooling_register_power()? (with the user having the
possibility to provide the em callback similarly to how get_real_power()
is provided - in devfreq_cooling_power).

IMO is cleaner to unify the functionality (registration and callbacks)
of cooling devices with power capabilities (based on em alone or together
with power_ops). Otherwise we just create confusion for users registering
cooling devices not knowing which function to call.

If this has been discussed previously and I'm missing some details,
please provide some links to the discussions.

Thank you for the patience :).

Ionela.