linux-kernel - Re: Using irq-crossbar.c

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Message-ID: <20160612110048.3dd5ea17@arm.com>
Date:	Sun, 12 Jun 2016 11:00:48 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Mason <slash.tmp@...e.fr>
Cc:	Sebastian Frias <sf84@...oste.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Mans Rullgard <mans@...sr.com>
Subject: Re: Using irq-crossbar.c

On Sat, 11 Jun 2016 17:37:51 +0200
Mason <slash.tmp@...e.fr> wrote:

> On 11/06/2016 11:58, Marc Zyngier wrote:
> 
> > Mason wrote:
> > 
> >> I think Sebastian is even more baffled by the DT mess
> >> (sorry, intricacies) than I am.
> > 
> > This mess is what has saved us from the apocalypse 5 years ago, and
> > describing a complex system is not easy (what a surprise...).
> 
> The problem with some Linux APIs is that they're logical and obvious
> to people who've been using them for years. For newcomers, it's not
> always so obvious.
> 
> In this specific instance, the problem statement seems rather simple,
> on the surface. An interrupt controller, X=0..127 lines in, Y=0..23
> lines out (connected to GIC interrupt lines 0..23) and "all" we need
> is a way to map Xs to Ys.
> 
> As a first order approximation, it's enough to map all Xs to 0.
> And provide a way for the kernel to check the registers containing
> the bit-vectors indicating which interrupt(s) fired.

If that's what your hardware is, then you are taking the wrong
approach. The irq-crossbar driver does not do that at all: it has x
inputs and y outputs, but connects exactly *one input to one output*.
No multiplexing.

And the hierarchical domain infrastructure enforces a similar property:
a Linux interrupt is dealt with at each level of the hierarchy without
multiplexing: the "irq" is the same, while the "hwirq" varies to
reflect the "input pin" for a given interrupt controller.

In your particular case, you have an evolved chained interrupt
controller, and nothing else.

> 
> > If you just want to apply recipes without understanding the underlying
> > constraints, you're in for a lot of pain.
> 
> For example, the IRQ driver for Tango3/4 calls
> 
> irq_find_mapping
> generic_handle_irq
> irq_desc_get_handler_data
> irq_desc_get_chip
> chained_irq_enter/chained_irq_exit
> irq_setup_alt_chip
> irq_get_domain_generic_chip
> irq_domain_add_linear
> irq_alloc_domain_generic_chips
> irq_set_chained_handler
> irq_set_handler_data
> 
> Taking irq_find_mapping, I see that there's a short comment in
> kernel/irq/irqdomain.c
> 
> /**
>  * irq_find_mapping() - Find a linux irq from an hw irq number.
>  * @domain: domain owning this hardware interrupt
>  * @hwirq: hardware irq number in that domain space
>  */
> 
> Is this Doxygen format? Is there a make target to generate
> some documentation?

Try "make help".

> 
> Other relevant resources, for my own reference:
> 
> https://www.kernel.org/doc/Documentation/IRQ-domain.txt
> https://stackoverflow.com/questions/34371352/what-are-linux-irq-domains-why-are-they-needed
> https://community.nxp.com/thread/332183
> 
> Are there other important kernel documentation?
> 
> >> The base file he was referring to is:
> >>
> >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/tango4-common.dtsi
> > 
> > I know which file that is, it is mentioned in the diff. I was merely
> > trying to point out the glaring mistakes that could be enough for a
> > interrupt controller hierarchy to be completely non-functional:
> 
> Only the name of the file was provided, not the path. I was not aware
> that you already knew where to find it. I made no claim whatsoever on
> the implementation. In fact, I agree with everything Lennart wrote.
> 
> > - Your crossbar doesn't have a #interrupt-cells property. How do you
> >   expect the interrupt specifiers to be interpreted?
> 
> Why do "fundamental" DT properties start with hash?

Because # is a shorthand for "number of".

> 
> > - You've changed the default interrupt controller to be your crossbar.
> >   Which means that all the sub-nodes are inheriting it. Have you
> >   checked that this was valid for all of these nodes?
> 
> I'm not sure I follow. All platform interrupts flow into the platform
> controller. Maybe other platforms have more complex setups, with
> several cascaded controllers?

Most embedded platforms do.

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