linux-kernel - Re: [PATCH 09/16] dt-bindings: firmware: arm,scmi: Extend bindings for protocol@13

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Date:   Sat, 15 Jul 2023 14:35:38 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>,
        Nikunj Kela <nkela@...cinc.com>,
        Prasad Sodagudi <psodagud@...cinc.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH 09/16] dt-bindings: firmware: arm,scmi: Extend bindings
 for protocol@13

On Fri, 14 Jul 2023 at 19:15, Rob Herring <robh@...nel.org> wrote:
>
> On Thu, Jun 15, 2023 at 3:11 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Thu, 15 Jun 2023 at 01:00, Rob Herring <robh@...nel.org> wrote:
> > >
> > > On Wed, Jun 07, 2023 at 02:46:21PM +0200, Ulf Hansson wrote:
> > > > The protocol@13 node is describing the performance scaling option for the
> > > > ARM SCMI interface, as a clock provider. This is unnecessary limiting, as
> > > > performance scaling is in many cases not limited to switching a clock's
> > > > frequency.
> > > >
> > > > Therefore, let's extend the binding so the interface can be modelled as a
> > > > generic "performance domain" too. The common way to describe this, is to
> > > > use the "power-domain" bindings, so let's use that.
> > >
> > > What's wrong with the performance-domain binding?
> >
> > In my opinion I think the performance-domain binding is superfluous.
> > We already have plenty of power-domains that do performance scaling
> > too - and they stick with the power-domain binding, as it's
> > sufficient.
>
> IMO, power-domains should be for power islands which can be power
> gated. I know they are abused though. Of course, when things are
> hidden in firmware, you don't really know. A power-domain could be
> just a clock or a clock could be multiple physical clocks.

I would also like to point out that it's perfectly possible that a
power-domain can be a combination of a "power-island" and a
performance-domain. In fact we have those cases already (Qcom, Tegra).

>
> > That said, I would rather follow the defacto standard that has been
> > used for many years in the kernel. Do you have a preference that we
> > should stick to?
>
> If power-domains are sufficient, then why do we have
> performance-domains? We need clear rules for when to use what.

Well, I think we invented the performance-domains bindings, especially
with CPUs in mind. So far, that's the only use-case too (Mediatek,
Apple). Even if I think the power-domains bindings would have worked
fine for these cases too, maybe we should limit the
performance-domains binding to be used for CPUs?

Anyway, for the more generic use-case, I think the power-domains DT
binding is better to stick with (it's what we have used for many years
by now), as it provides us with the flexibility of hooking up an
opp-table to the power-domain, allowing us to make the domain
"performance-capable" too.

Kind regards
Uffe