linux-kernel - Re: [PATCH 02/12] dt-bindings: interrupt-controller: stm32-exti: Add irq nexus child node

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Date: Fri, 23 Feb 2024 14:39:25 +0100
From: Antonio Borneo <antonio.borneo@...s.st.com>
To: Rob Herring <robh@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski
	<krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue
	<alexandre.torgue@...s.st.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        Fabrice Gasnier
	<fabrice.gasnier@...s.st.com>
Subject: Re: [PATCH 02/12] dt-bindings: interrupt-controller: stm32-exti:
 Add irq nexus child node

On Thu, 2024-02-22 at 16:43 -0700, Rob Herring wrote:
> On Fri, Feb 16, 2024 at 10:47:47AM +0100, Antonio Borneo wrote:
> > The mapping of EXTI interrupts to its parent interrupt controller
> > is both SoC and instance dependent.
> > The current implementation requires adding a new table to the
> > driver's code and a new compatible for each new EXTI instance.
> > 
> > Add to the binding an interrupt nexus child node that will be
> > used on the new EXTI instances and can be optionally used on the
> > existing instances.
> > The property 'interrupt-map' in the nexus node maps each EXTI
> > interrupt to the parent interrupt.
> > Align #address-cells and #interrupt-cells between the EXTI node
> > and its nexus node.
> 
> Looks like an abuse of interrupt-map. You avoid adding yourself to the 
> abuser list by putting it in a child node. Clever. (See list in 
> drivers/of/irq.c if you don't know what I'm talking about)

Hi Rob,
thanks for the review.

Yes, I know already about the abuser list but, from the commit
message and the associated comment, I interpret it as an incorrect
use of the property interrupt-map with custom syntax thus relying 
on custom parsing code.
The child nexus node in this series allows using the default parser
in kernel.

>From your reply, looks like my interpretation is incorrect and I
missed the real concern about the abuser list.
Could you please explain why this use of interrupt-map is incorrect
and/or which are the correct use cases?

> I assume the EXTI has 0..N interrupts. Just define 'interrupts' with N 
> entries with each entry mapping EXTI interrupt N to 'interrupts' entry 
> N.

Yes, EXTI has 0..N interrupts that can be mapped to multiple
parent interrupt controllers and the mapping table has holes.
While the DT in this series only use one interrupt parent, a second
parent will follow.
So 'interrupts-extended' property would be a better matching than
'interrupts' to handle the multiple parents.

But how to code the missing entries in an 'interrupts-extended' list?
As in the example in Documentation/devicetree/bindings/dma/apple,admac.yaml ?

The 'interrupt-map' contains the matching EXTI index, thus allowing
a 'sparse' map where holes are simply ignored.

Best Regards,
Antonio