linux-kernel - Re: [PATCH v4] dt-bindings: dvfs: Add support for generic performance domains

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Date:   Thu, 21 Oct 2021 14:33:18 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Rob Herring <robh@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Hector Yuan <hector.yuan@...iatek.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [PATCH v4] dt-bindings: dvfs: Add support for generic
 performance domains

On Thu, Oct 21, 2021 at 03:13:30PM +0200, Ulf Hansson wrote:
> On Wed, 20 Oct 2021 at 14:11, Rob Herring <robh@...nel.org> wrote:
> >

[...]

> > 'power domains' in DT is supposed to mean physical power islands in
> > the h/w where as genpd can be whatever you want. Are power and
> > performance domains always 1:1?
>
> I wouldn't say that "power domains" *must* correspond to physical
> power islands. At least, that's not the way the bindings are being
> used. For example, if it makes better sense to keep some of the logic
> in FW, rather than describing a complete topology in DT, that should
> be perfectly fine.
>

I agree. The DT must either have h/w view or f/w view of the topology
and not both(that is inviting more trouble in terms of bindings as well
as handling it in the OSPM).

> Additionally, I am not suggesting that "performance domains" and
> "power domains" must map 1:1. A device can be performance managed
> through one domain and power managed by another, that would be
> perfectly fine to me.

I don't understand what you mean by that. Do you expect to create a genpd
with no power domain ops and just performance ops to deal with scenario
I have been presenting(i.e. power domains for a set of devices(CPUs in
particular) aren't exposed to OSPM while performance domains are).

I really don't like to create psuedo/dummy power domains with no useful
info(as f/w hides or abstracts it) just to represent the performance
domains.

Also with CPUs you can imagine all sort of combinations like:
1. cluster level perf domain + cpu level power domains
3. cluster level perf domain + cluster level power domains
2. cpu level perf domain + cpu level power domains
4. cpu level perf domain + cluster level power domains

+ power domains not available to OSPM in all the 4 combo.

So I am really struggling to visualise a way to represent these based
on your suggestion.

--
Regards,
Sudeep