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Date: Sun, 8 Dec 2002 21:59:27 -0600 (CST)
From: Patrick Mochel <mochel@osdl.org>
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To: Linus Torvalds <torvalds@transmeta.com>
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       Petr Vandrovec <VANDROVE@vc.cvut.cz>, <linux-kernel@vger.kernel.org>,
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Subject: Re: /proc/pci deprecation?
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> > It just reports what's in the PCI_INTERRUPT_LINE register.
> 
> That's wrong, then.

Actually, it doesn't. IIUC, it gets it from /proc/bus/pci/devices. That 
info is formatted in drivers/pci/proc.c::show_device().

> > At least on Alpha, we wrote into this register during pci
> > configuration, so the value matches what's in /proc/interrupts.
> > I guess I always assumed we did the same on x86, but I've
> > never checked.
> 
> It _shouldn't_ be done. The PCI_INTERRUPT_LINE register is just a byte,
> which isn't even _enough_ to identify the irq "for real" on many
> architectures. It's also a totally different namespace at least on x86
> machines: the PCI_INTERRUPT_LINE register should contain the "legacy
> interrupt", while the remapped interrupt will depend on whether the
> io-apic is enabled or not.
> 
> Writing it back is actively _bad_, since it will make it very hard to
> re-boot the machine without the BIOS re-enumarating the PCI bus and
> filling it in again (ie it would definitely screw up using things like
> kexec() on PC's, if the kernel we boot _from_ is an APIC kernel, but the
> kernel we boot _into_ is not).

I couldn't find any place that ia32 writes the PCI_INTERRUPT_LINE register 
during boot. And, manually reading in various places (before, during, and 
after enabling a device) always returned the same info. 

> So the rule should be:
>  - the PCI config space is _not_ the same as "pci->irq"
>  - we should _never_ update the PCI_INTERRUPT_LINE register, because it
>    destroys boot loader information (the same way we need to not overwrite
>    BIOS extended areas and ACPI areas etc in order to be able to reboot
>    cleanly)
>  - we need _some_ way of getting the "kernel irq line" from /sys or /proc.
> 
> Sounds like /proc/pci still does something for us. Or is the info
> available in /proc/bus/pci/devices?

AFAICT, lspci is getting it from /proc/bus/pci/devices. I'm getting the 
source now to sift through and verify. Alternatively, the data is also 
available in the 'irq' file in each PCI device's sysfs directory. 

	-pat

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