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

[Sebastian Frias <sf84@laposte.net>]

Hi Marc,

On 07/05/2016 07:13 PM, Marc Zyngier wrote:
>>> You really don't need to describe this. The configuration that is
>>> applied to your router in entirely under software control,
>>
>> With "entirely under software control" do you mean this driver's code?
> 
> Yes.

Ok.

> 
>>
>>> 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. 
>>
>> I think I'm missing something, what is the difference between the domains
>> described by nodes in the DT for irq-tango.c (arch/arm/boot/dts/tango4-common.dtsi)
>> and the DT from my RFC?
> 
> The fundamental difference is that with your new fancy controller, you
> can decide what is going where, while the previous one is completely set
> in stone (the output line is a direct function of the input line).

I think that's where part the misunderstanding comes from.
IMHO the output line is not a direct function of the input line.
Any of the 64 IRQ lines entering the "old controller" (irq-tango.c) can be
routed to any of its 3 outputs.
The only thing fixed is which GIC input is connected to those 3 outputs, ie:
GIC inputs 2, 3 and 4.

In the the "new controller" (irq-tango_v2.c, this RFC), any of 128 IRQ lines
can be routed to any of 24 outputs, connected to GIC inputs 0...23.

In a nutshell:
- "old controller": routes [0...N] => GIC inputs [2...4]
- "new controller": routes [0...M] => GIC inputs [0...23]

So, when we think about it, if the "new DT" specified 24 domains, it would
be equivalent of the "old DT" with 3 domains, right?

That's why it seemed more or less natural to keep describing the domains in
the DT, the main reason for that being that it allowed the user to specify
the IRQ sharing in the DT, and this is precisely the key point of this.

So, putting aside routing considerations and the discussion above, I think
a simpler question is: if the domains should not be described in the DT,
how can we define the IRQ sharing in the DT?

Best regards,

Sebastian