view sources/patches/linux-arm-qemuirq.patch @ 1684:7b4566efd8bd draft 1.2.8

Silence some asynchronous messages linux produces on top of the shell prompt.
author Rob Landley <>
date Sun, 07 Sep 2014 10:39:51 -0500
parents 1b2017d3ecf5
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Modify arm IRQ setup to work with all versions of qemu since 1.0.

For many years, the kernel's versatile board setup expected all devices
to share IRQ 27, and QEMU 1.2 helpfully emulated hardware that did that
(or used IRQ 59 if the IRQ controller was told to shift everything up 32
places). This wasn't what the actual hardware did, but nobody noticed for
over a decade because this reference board was long-gone and only useful
as a base for virtualization.

Then the kernel developers noticed that the default IRQ range could
(theoretically) use IRQ 0 as an actual IRQ, so they poked the "move everything
up 32 places" register... and got the math wrong calculating the new IRQ
to expect stuff on. (Because just adding 32 wouldn't be the right thing to
do, you've gotta do a multi-stage process going through three different
functions with a callback.)

When informed they'd broken qemu, they looked up old Versatile documentation
and realized what they'd done had never matched real hardware, and adjusted
it to some other random value: still getting the math wrong. Then they finally
fixed the wrong math, but it turns out the documentation they were using
didn't match what actual hardware was doing, or something. All in all they
changed the IRQ mapping at least _4_TIMES_ before finally unearthing an
actual Versatile board out of some landfill or other to test it out on.
Needless to say, "this breaks QEMU" was not considered a valid data point.

Meanwhile, QEMU 1.2 and earlier only supported the "everything on irq 27" mode,
and the next several releases supported random potluck things the kernel du
jour did, but current ones don't.

BUT: if you the kernel requests irq 27 from the controller as
its first action, QEMU thinks you're running an old "everying on 27" kernel
and triggers an emulation mode that puts everything on IRQ 27 (or 59 if
you flip the add 32 bit in the controller, meaning current kernels aren't
bothered by IRQ 0 being potentially used). And this runs on all the qemu
versions since 1.0. (Although you still don't want to use QEMU 1.3 and 1.4
because they had a bug in TCG that got the size of translated blocks wrong
causing spurious but highly intermittent segfaults...)

Note, I broke this patch out of the big arm patch after the FIFTH time
they made changes that prevented this patch from cleanly applying.

diff --git a/arch/arm/mach-versatile/pci.c b/arch/arm/mach-versatile/pci.c
index c97be4e..da0342d 100644
--- a/arch/arm/mach-versatile/pci.c
+++ b/arch/arm/mach-versatile/pci.c
@@ -305,7 +305,7 @@ int __init pci_versatile_setup(int nr, struct pci_sys_data *sys)
 	 * real hardware behaviour and it need not be backwards
 	 * compatible for us. This write is harmless on real hardware.
 	 * Do not to map Versatile FPGA PCI device into memory space
@@ -346,7 +346,7 @@ static int __init versatile_map_irq(const struct pci_dev *dev, u8 slot, u8 pin)
 	 * 30	PCI0	PCI1	PCI2	PCI3
 	 * 29	PCI3	PCI0	PCI1	PCI2
-	irq = IRQ_SIC_PCI0 + ((slot + 2 + pin - 1) & 3);
+	irq = 59;
 	return irq;