linux-kernel - Q40 interrupts (and genirq)

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Date:	Tue, 23 Aug 2011 11:16:40 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Richard Zidlicky <rz@...ux-m68k.org>
Cc:	"Linux/m68k" <linux-m68k@...r.kernel.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Q40 interrupts (and genirq)

Hi Richard,

I'm a bit puzzled by the Q40 interrupt architecture.

arch/m68k/q40/q40ints.c says:

 * Q40 IRQs are defined as follows:
 *            3,4,5,6,7,10,11,14,15 : ISA dev IRQs

Yep, that's _9_ interrupts.

 *            16-31: reserved
 *            32   : keyboard int
 *            33   : frame int (50/200 Hz periodic timer)
 *            34   : sample int (10/20 KHz periodic timer)

According to the Q40_IRQ*_MASK definitions (8 bits in the mask) and the code
(e.g. q40_irq_startup()), ISA IRQ 11 is not implemented.

In the "new" interrupt code, by Roman Zippel, all interrupts sources are handled
through q40_irq_handler().
Only autovector IRQs IRQ_AUTO_2 and IRQ_AUTO_4 are enabled.

In the old (pre-2006) interrupt code, only internal (master) and ISA interrupts
go through q40_irq2_handler on IRQ_AUTO_2.
Q40_IRQ_SAMPLE goes via both IRQ_AUTO_4 and IRQ_AUTO_6.

Why doesn't the new code use IRQ_AUTO_6? Does the new code work?

For the genirq conversion, I was first thinking about using a custom
chain-alike flow handler.
But it looks like this is not possible, as the Q40 handler remaps
autovector interrupts to
conflicting numbers (it handles IRQ_AUTO_4 = 4, but ISA interrupt 4 is also 4).
So if the custom flow handler would call generic_handle_irq(4) for ISA
interrupt 4,
it would recurse into the flow handler set up for autovector IRQ 4 :-(
Changing the numbers is not trivial, as ISA assumes interrupts 1-15 everywhere.

So I'm afraid we're stuck with the current m68k_setup_auto_interrupt() hack,
which overwrites the lowest-level asm interrupt code, and keep the current
q40_irq_handler() (This is similar to process_int() for 68328 on nommu).

Thanks for your clarifications and suggestions!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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