Re: Using irq-crossbar.c

Lennart Sorensen
2016-06-13T15:42:27Z

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