Re: [RFC 01/32] Kconfig: introduce and depend on LEGACY_PCI

[John Garry <john.garry@xxxxxxxxxx>]

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

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?

[0] 
https://lore.kernel.org/lkml/CAK8P3a0MNbx-iuzW_-=0ab6-TTZzwV-PT_6gAC1Gp5PgYyHcrA@xxxxxxxxxxxxxx/

> IMO inb() should
> be present but do something innocuous like return ~0, as it would if
> I/O port space is supported but there's no device at that address.
>
> [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/megaraid.c?id=v5.15#n4210

That driver would prob not be used on systems which does not support 
PIO, and so could have a HAS_IOPORT dependency. But it is not strictly 
necessary.

Anyway, it would be good to have an idea of how much ifdeffery is 
required in drivers.

Thanks,
John