linux-kernel - Re: [RFC PATCH v1 3/4] dt-bindings: interrupt-controller: realtek,rtl-intc: replace irq mapping

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Message-ID: <ofvekt.r4rr67.2rw3hx-qmf@polaris.svanheule.net>
Date:   Mon, 27 Dec 2021 10:15:41 +0000
From:   Sander Vanheule <sander@...nheule.net>
To:     maz@...nel.org
Cc:     devicetree@...r.kernel.org, robh+dt@...nel.org,
        mail@...ger-koblitz.de, bert@...t.com, john@...ozen.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [RFC PATCH v1 3/4] dt-bindings: interrupt-controller:
 realtek,rtl-intc: replace irq mapping

Hi Marc,

On Monday, 27 December 2021, Marc Zyngier wrote:
> On Thu, 23 Dec 2021 19:29:23 +0000,
> Sander Vanheule <sander@...nheule.net> wrote:
> > 
> > On Thu, 2021-12-23 at 18:00 +0000, Marc Zyngier wrote:
> > > On Thu, 23 Dec 2021 12:08:33 +0000,
> > > Sander Vanheule <sander@...nheule.net> wrote:
> > > > 
> > > > The binding incorrectly specified the "interrupt-map" property should be
> > > > used, although the use is non-standard. A quirk had to be introduced in
> > > > commit de4adddcbcc2 ("of/irq: Add a quirk for controllers with their own
> > > > definition of interrupt-map") to allow the driver to function again.
> > > 
> > > That's too late. We have released a kernel with this binding, and it
> > > will live on forever until we totally remove the platform from the
> > > tree.
> > > 
> > > DT is an ABI, and only time travel can fix this blunder.
> > 
> > Taking into account your comments on the previous patch, this change
> > wouldn't even be required if I correct the mappings for my
> > devices. But that wouldn't get rid of the assumed mapping between
> > output lines and parent interrupts.
> 
> A driver can always ignore some information from the DT and do its own
> thing. No sure if that addresses your problem though.
> 
> > 
> > To what extent can the binding be updated to get rid of this
> > assumption? Or would that require a completely new binding?
> 
> You can only extend a binding in a two-way fashion: old kernel works
> with new DT, new kernel works old DT. Which means that in practice,
> you can only *add* information to the DT, and have reasonable defaults
> in the driver when you don't find it.

Thanks for clarifying. In that case I don't think it is possible to get rid of the output-to-parent assumption entirely, since the driver would always need to accommodate for the original binding, where there is no output mapping specified in the binding. There are no SoC-specific compatibles (where a mapping could be assumed), and I don't know on how many MIPS platforms Realtek has used this interrupt router/controller.

I don't have much time anymore today, but I'll break my head over it again tomorrow.

Best,
Sander