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

From: Bjorn Helgaas <helgaas@kernel.org>
To: Mauro Carvalho Chehab <mchehab@kernel.org>
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Subject: Re: [RFC 01/32] Kconfig: introduce and depend on LEGACY_PCI
Date: Wed, 29 Dec 2021 10:03:17 -0600	[thread overview]
Message-ID: <20211229160317.GA1681139@bhelgaas> (raw)
In-Reply-To: <20211229131207.1ac25424@coco.lan>

On Wed, Dec 29, 2021 at 01:12:07PM +0100, Mauro Carvalho Chehab wrote:
> Em Wed, 29 Dec 2021 12:45:38 +0100
> Niklas Schnelle <schnelle@linux.ibm.com> escreveu:
> > ...

> > I do think we agree that once done correctly there is value in
> > such an option independent of HAS_IOPORT only gating inb() etc uses.

I'm not sure I'm convinced about this.  For s390, you could do this
patch series, where you don't define inb() at all, and you add new
dependencies to prevent compile errors.  Or you could define inb() to
return ~0, which is what happens on other platforms when the device is
not present.

> Personally, I don't see much value on a Kconfig var for legacy PCI I/O 
> space. From maintenance PoV, bots won't be triggered if someone use
> HAS_IOPORT instead of the PCI specific one - or vice-versa. So, we
> could end having a mix of both at the wrong places, in long term.
> 
> Also, assuming that PCIe hardware will some day abandon support for 
> "legacy" PCI I/O space, I guess some runtime logic would be needed, 
> in order to work with both kinds of PCIe controllers. So, having a
> Kconfig option won't help much, IMO.
> 
> So, my personal preference would be to have just one Kconfig var, but
> I'm 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.

What if you used the approach from [1] but just dropped the warning?
The inb() would return ~0 if the platform doesn't support I/O port
space.  Drivers should be prepared to handle that because that's what
happens if the device doesn't exist.  

HAS_IOPORT and LEGACY_PCI is a lot of Kconfiggery that basically just
avoids building drivers that aren't useful on s390.  I'm not sure the
benefit outweighs the complication.

Bjorn

[1] https://lore.kernel.org/lkml/CAHk-=wg80je=K7madF4e7WrRNp37e3qh6y10Svhdc7O8SZ_-8g@mail.gmail.com/