On Thu, 2022-01-06 at 17:41 +0000, John Garry wrote:

On 05/01/2022 19:47, Bjorn Helgaas wrote:

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ok if the PCI maintainers decide otherwise.

I don't really like the "LEGACY_PCI" Kconfig option. "Legacy" just means something old and out of favor; it doesn't say*what* that something is.

I think you're specifically interested in I/O port space usage, and it seems that you want all PCI drivers that*only* use I/O port space to depend on LEGACY_PCI? Drivers that can use either I/O or memory space or both would not depend on LEGACY_PCI? This seems a little murky and error-prone.

I'd like to hear Arnd's opinion on this but you're the PCI maintainer so of course your buy-in would be quite important for such an option.

I'd like to hear Arnd's opinion, too. If we do add LEGACY_PCI, I think we need a clear guide for when to use it, e.g., "a PCI driver that uses inb() must depend on LEGACY_PCI" or whatever it is.

I must be missing something because I don't see what we gain from this. We have PCI drivers, e.g., megaraid [1], for devices that have either MEM or I/O BARs. I think we want to build drivers like that on any arch that supports PCI.

If the arch doesn't support I/O port space, devices that only have I/O BARs won't work, of course, and hopefully the PCI core and driver can figure that out and gracefully fail the probe.

But that same driver should still work with devices that have MEM BARs. If inb() isn't always present, I guess we could litter these drivers with #ifdefs, but that would be pretty ugly.

I think this is the big question here. If we do go with a compile-time solution as requested by Linus we will either get a lot of #ifdeffery, coarse driver dependencies or as proposed by Alan Stern for the USB #ifdefs might end up turning inb() into a compile-time nop.

The originally proposed change that returned ~0 from inb() and printed a warning clearly is the simpler change and sure we could also drop the warning. I'm honestly torn, I do agree with Linus that we shouldn't have run-time things that we know at compile-time will not work but I also dislike all the #ifdeffery a compile-time solution requires. Sadly C really doesn't give us any better tools here.

Also I 100% agree with you Bjorn how likely it is to see a device on a platform really shouldn't matter. Without going into details, on s390 we have already beneffited from PCI drivers working with 0 changes to support devices we previously didn't have on the platform or anticipated we would get in the future. Consequently drivers that could work in principle should be built.

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There were some ifdefs added to the 8250 drivers in Arnd's original patch [0], but it does not seem included here.

Niklas, what happened to the 8250 and the other driver changes?

I missed it during the rebase, likely because the changed files compile depend on !S390 via config SERIAL_8250 and thus didn't cause any errors for my allyesconfig. That !S390 dependency is of course not really what we want if the driver can use MEM BARs.