Re: [Qemu-devel] [PATCH 00/10] Fix versatile_pci (and break versatilepb linux guests!)

[Peter Maydell]

On 24 March 2013 21:16, Arnd Bergmann <address@hidden> wrote:
> On Sunday 24 March 2013, Peter Maydell wrote:
>> Yeah, ideally being able to detect the buggy kernel would be good;
>> I can't see anything at the controller level that would do though,
>> and I don't really know enough about PCI to know about generic
>> PCI stuff that would work. (Why would the OS need to tell the
>> device anything about its IRQ if it's hardwired?)
>
> I think it actually does on versatile and other platforms on which
> the kernel probes the PCI bus itself, rather than relying on firmware
> to have resources assigned in advance.
>
> IIRC, the PCI_INTERRUPT_LINE pci config space byte (0x3c) is purely
> informational and used as a way to communicate the interrupt number
> from the bus scan code (assumed to be a PC BIOS in the PCI spec,
> but drivers/pci/setup-irq.c in case of versatile+linux) to a device
> driver.
>
> So the kernel should actually write the proper interrupt number in
> there. In future kernels, this may not necessarily be the hardware
> number, but today it is. Can you try out what the kernel writes into
> that register in qemu, with and without my patches?

OK, I'll give that a go tomorrow.

While you're here, do you know what the point of the PCI_SELFID
register is? The h/w docs are clear that the OS has to write
the slot number of the board itself in to this register, but
again I don't see why the OS has to tell the hardware something
it already knows. On QEMU I just implemented this register as
a no-op and everything seems to still function...

thanks
-- PMM