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

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Date:	Mon, 13 Jun 2016 11:42:23 -0400
From:	"Lennart Sorensen" <lsorense@...lub.uwaterloo.ca>
To:	Sebastian Frias <sf84@...oste.net>
Cc:	Mason <slash.tmp@...e.fr>, Marc Zyngier <marc.zyngier@....com>,
	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 Mon, Jun 13, 2016 at 04:57:13PM +0200, Sebastian Frias wrote:
> Actually we have 128 inputs and 24 outputs, the 24 outputs go straight to the GIC.
> The HW block is a many-to-many router.
> There are 128 32bit registers which specify, for each of the corresponding 128 inputs, to which of the 24 outputs it would be routed to.
> 
> There are 4 32bit registers that can show the RAW status of the 128 inputs, but they do not latch on the inputs.
> That's why our understanding is that on Linux terms it is not an interrupt controller, but just a many-to-many mux, the only real interrupt-controller (where one can set if the line is active high or low for example) is the GIC.

Well that does just sound like a mux.  But that does mean you either
can't use more than 24 inputs at once, or you will be sharing interrupts.

I really hate shared interrutps so I would never design something that
way, but it is simpler.

> Thanks for the background on the i8259 and the cascaded interrupts.
> However, our understanding is that it would only be required if more than 24 devices request IRQ lines, in which case, some of them would have to share a single GIC IRQ line, right?
> Shall we worry about that now?

Well if you are sure you never need more than 24 devices registered at
once, then it shouldn't be a problem.

> This is interesting.
> We have one interrupt controller already upstream, drivers/irqchip/irq-tango.c, and our understanding is that it dispatches one IRQ at the time, see tangox_dispatch_irqs() function, is that what you are discussing?

That does look like a proper interrupt controller that could be cascaded
of another one if needed.

-- 
Len Sorensen