linux-kernel - Re: [PATCH v4 01/26] dt-bindings: interrupt-controller: Add Arm GICv5

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Message-ID: <CAL_JsqJ5N2ZUBeAes=wexq=EstRSZ5=heF1_6crAw76yZ9uXog@mail.gmail.com>
Date: Tue, 3 Jun 2025 10:15:25 -0500
From: Rob Herring <robh@...nel.org>
To: Lorenzo Pieralisi <lpieralisi@...nel.org>
Cc: Peter Maydell <peter.maydell@...aro.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Marc Zyngier <maz@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Conor Dooley <conor+dt@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, andre.przywara@....com, 
	Arnd Bergmann <arnd@...db.de>, Sascha Bischoff <sascha.bischoff@....com>, 
	Timothy Hayes <timothy.hayes@....com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Mark Rutland <mark.rutland@....com>, Jiri Slaby <jirislaby@...nel.org>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	devicetree@...r.kernel.org, suzuki.poulose@....com
Subject: Re: [PATCH v4 01/26] dt-bindings: interrupt-controller: Add Arm GICv5

On Tue, Jun 3, 2025 at 2:48 AM Lorenzo Pieralisi <lpieralisi@...nel.org> wrote:
>
> On Thu, May 29, 2025 at 02:17:26PM +0100, Peter Maydell wrote:
> > On Thu, 29 May 2025 at 13:44, Lorenzo Pieralisi <lpieralisi@...nel.org> wrote:
> > >
> > > [+Andre, Peter]
> > >
> > > On Tue, May 13, 2025 at 07:47:54PM +0200, Lorenzo Pieralisi wrote:
> > > > +      reg:
> > > > +        minItems: 1
> > > > +        items:
> > > > +          - description: IRS control frame
> > >
> > > I came across it while testing EL3 firmware, raising the topic for
> > > discussion.
> > >
> > > The IRS (and the ITS) has a config frame (need to patch the typo
> > > s/control/config, already done) per interrupt domain supported, that is,
> > > it can have up to 4 config frames:
> > >
> > > - EL3
> > > - Secure
> > > - Realm
> > > - Non-Secure
> > >
> > > The one described in this binding is the non-secure one.
> > >
> > > IIUC, everything described in the DT represents the non-secure address
> > > space.
> >
> > The dt bindings do allow for describing Secure-world devices:
> > Documentation/devicetree/bindings/arm/secure.txt has the
> > details. We use this in QEMU so we can provide a DTB to
> > guest EL3 firmware that tells it where the hardware is
> > (and which EL3 can then pass on to an NS kernel). It would
> > be helpful for the GICv5 binding to be defined in a way that
> > we can do this for a GICv5 system too.
>
> It would be good to understand what DT {should/should not} describe and
> whether this DT usage to configure firmware is under the DT maintainers
> radar or it is an attempt at reusing it to avoid implementing a
> configuration scheme.
>
> Rob, Krzysztof,
>
> Any thoughts on the matter please ?

I'm all for firmware using DT, but using a single DT for all
components with an ABI between all components is an impractical dream.
You can take that a step further even with a single DT for all
processors in a system (aka System DT). Ultimately, the DT is a view
of the system for a client (OS). Different views may need different
DTs.

u-boot and Linux sharing a DT makes sense as they have the same world
view. Secure and NS not so much.

> [...]
>
> > The tempting thing to do is to have regs[] list the frames
> > in some given order, but the spec makes them not simple
> > supersets, allowing all of:
> >  * NS
> >  * S
> >  * NS, S, EL3
> >  * NS, Realm, EL3
> >  * NS, Realm, S, EL3
>
> Maybe reg-names can help ? Even though first we need to understand
> what resources should be described in DT.
>
> Current bindings are reviewed and I am not keen on dragging this
> discussion on forever - the information the kernel requires is there,
> I'd like to bring this to a close.
>
> Thanks,
> Lorenzo
>
> >
> > secure.txt says:
> > # The general principle of the naming scheme for Secure world bindings
> > # is that any property that needs a different value in the Secure world
> > # can be supported by prefixing the property name with "secure-". So for
> > # instance "secure-foo" would override "foo".

Today I would say a 'secure-' prefix is a mistake. To my knowledge,
it's never been used anyways. But I don't have much visibility into
what secure world firmware is doing.

> >
> > So maybe we could have
> >  reg : the NS frame(s)
> >  secure-reg : the S frame(s)
> >  realm-reg : the Realm frame(s)
> >  root-reg : the EL3 frame(s)

Here's why. It really doesn't scale.

Rob