Re: [Qemu-devel] [PATCH v2 07/11] versatile_pci: Implement the correct PCI IRQ mapping

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 26 March 2013, Peter Maydell wrote:
> 
> Implement the correct IRQ mapping for the Versatile PCI controller; it
> differs between realview and versatile boards, but the previous QEMU
> implementation was correct only for the first PCI card on a versatile
> board, since we weren't swizzling IRQs based on the slot number.
> 
> Since this change would otherwise break any uses of PCI on Linux kernels
> which have an equivalent bug (since they have effectively only been
> tested against QEMU, not real hardware), we implement a mechanism
> for automatically detecting those broken kernels and switching back
> to the old mapping. This works by looking at the values the kernel
> writes to the PCI_INTERRUPT_LINE register in the config space, which
> is effectively the interrupt number the kernel expects the device
> to be using. If this doesn't match reality we use the broken mapping.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Yes, very good.  We will probably introduce sparse irq support on
versatile in the near future, and then the value we write into the
PCI_INTERRUPT_LINE field will become arbitrary from qemu's point
of view, but I will make sure that we fix the interrupt mapping
in the kernel at the same time so we always fall into the
"s->broken_irq_mapping = false;" case.

We also need to find a way to make the new kernel work with
an old qemu, and I think we can do that by using the versatile-dt
board type with a PCI device node that sets all four lines to
27, while using the actual interrupt lines for the default
versatile device tree.

	Arnd