linux-kernel - Re: [RFC PATCH v1] irqchip: add support for SMP irq router

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Date:	Tue, 5 Jul 2016 17:48:24 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Sebastian Frias <sf84@...oste.net>,
	Jason Cooper <jason@...edaemon.net>
Cc:	Mason <slash.tmp@...e.fr>, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH v1] irqchip: add support for SMP irq router

On 05/07/16 17:38, Sebastian Frias wrote:
> Hi Jason,
> 
> On 07/05/2016 05:53 PM, Jason Cooper wrote:
>>>
>>> Thanks for your comments.
>>> So, aside from some naming issues, do you think the driver is ok?
>>
>> Well, it's going to be few days before I can really dig in to this.
>> Until then, what I can say I see is that it looks like you're using
>> devicetree to tell Linux how to lay out the irq domains.  That's not
>> right :(
> 
> Ok, so that replies my questions 1 and 2, thanks.
> 
>>
>> The devicetree should *only* describe the hardware. Would *BSD be able
>> to use the description in the dtb effectively?
>>
>> iiuc, I think irq-crossbar.c may be a similar enough in task to give you
>> an idea or two.
> 
> I already did something like that, you can see it here:
> 
> https://marc.info/?l=linux-kernel&m=146592235919308&w=2
> 
> the problem with that code is that it cannot handle more than 24 IRQs (the
> number of outputs of the router), because they are not being shared.
> 
> Maybe I need a sort of hybrid approach by reintroducing part of
> "irq-crossbar.c" code to replace the irq domain layout that is currently
> being done using DT properties ?
> 
> However, I have not seen any examples of how to describe, using the DT,
> an association between a device HW irq, and the GIC hwirq where it goes to,
> nor how to express in the DT that multiple devices should share a given GIC
> hwirq.
> Basically, when a device requests the IRQ specified in its DT, I need:
> - to know which GIC hwirq line should I route it to (or the GIC to tell
> me which one it expects)

You really don't need to describe this. The configuration that is
applied to your router in entirely under software control, and none of
that should appear in the DT. You could decide to mux all the interrupts
to a single one, or decide that the 23 first interrupts you discover get
their own private line to the GIC and that everything else is muxed.

So given that this is completely defined by software, it has no place in
DT. There may not be an example of such an interrupt controller in the
tree, but this doesn't look too hard to implement.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...