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

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Date:   Thu, 15 Jun 2023 11:39:06 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     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, Rob Herring <robh+dt@...nel.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 Thu, 15 Jun 2023 at 10:44, Sudeep Holla <sudeep.holla@....com> 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.
> >
> > Cc: Rob Herring <robh+dt@...nel.org>
> > Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
> > Cc: Conor Dooley <conor+dt@...nel.org>
> > Cc: devicetree@...r.kernel.org
> > Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> > ---
> >  Documentation/devicetree/bindings/firmware/arm,scmi.yaml | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/devicetree/bindings/firmware/arm,scmi.yaml b/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
> > index 5824c43e9893..cff9d1e4cea1 100644
> > --- a/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
> > +++ b/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
> > @@ -145,8 +145,8 @@ properties:
> >        '#clock-cells':
> >          const: 1
> >
> > -    required:
> > -      - '#clock-cells'
>
> I am yet to look at the patches, just looked at this binding changes for now.
>
> Won't this break compatibility with existing DTBs. IMO, this is strict
> no no, you can't drop #clock-cells. I wanted to add performance-domains
> here as alternative but decided to not as I knew you were working on this.

Thanks for reviewing!

The point with the suggested change was to allow any kind of
combination of using #clock-cells and/or #power-domain-cells. Honestly
I didn't really know how to best express that in the binding, but
maybe someone can help me out here?

I think enforcing #clock-cells to be used is unnecessary. Making it
optional should not break existing DTBs, right?

Moreover, currently it seems to be only Juno that uses "protocol@13"
and the "#clock-cells" (at least by looking at the DTSes in-kernel).
So, I wonder if it's really such a big deal to update the DT bindings
for "protocol@13" at this point, but I may not have the complete
picture.

Kind regards
Uffe