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

Niklas Schnelle
2021-12-28T11:00:32Z

On Tue, 2021-12-28 at 10:15 +0100, Mauro Carvalho Chehab wrote:
> Em Tue, 28 Dec 2021 09:21:23 +0100
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> escreveu:
>
> > On Mon, Dec 27, 2021 at 05:42:46PM +0100, Niklas Schnelle wrote:
> > > --- a/drivers/pci/Kconfig
> > > +++ b/drivers/pci/Kconfig
> > > @@ -23,6 +23,17 @@ menuconfig PCI
> > >
> > > if PCI
> > >
> > > +config LEGACY_PCI
> > > + bool "Enable support for legacy PCI devices"
> > > + depends on HAVE_PCI
> > > + help
> > > + This option enables support for legacy PCI devices. This includes
> > > + PCI devices attached directly or via a bridge on a PCI Express bus.
> > > + It also includes compatibility features on PCI Express devices which
> > > + make use of legacy I/O spaces.
>
> This Kconfig doesn't seem what it is needed there, as this should be an
> arch-dependent feature, and not something that the poor user should be
> aware if a given architecture supports it or not. Also, the above will keep
> causing warnings or errors with randconfigs.
>
> Also, the "depends on HAVE_CPI" is bogus, as PCI already depends on
> HAVE_PCI:

Ah yes you're right.

>
> menuconfig PCI
> bool "PCI support"
> depends on HAVE_PCI
> help
> This option enables support for the PCI local bus, including
> support for PCI-X and the foundations for PCI Express support.
> Say 'Y' here unless you know what you are doing.
>
> So, instead, I would expect that a new HAVE_xxx option would be
> added at arch/*/Kconfig, like:
>
> config X86
> ...
> select HAVE_PCI_DIRECT_IO
>
> It would also make sense to document it at Documentation/features/.

I'll look into that, thanks.

>
> > All you really care about is the "legacy" I/O spaces here, this isn't
> > tied to PCI specifically at all, right?
> >
> > So why not just have a OLD_STYLE_IO config option or something like
> > that, to show that it's the i/o functions we care about here, not PCI at
> > all?
> >
> > And maybe not call it "old" or "legacy" as time constantly goes forward,
> > just describe it as it is, "DIRECT_IO"?
>
> Agreed. HAVE_PCI_DIRECT_IO (or something similar) seems a more appropriate
> name for it.
>
> Thanks,
> Mauro

Hmm, I might be missing something here but that sounds a lot like the
HAS_IOPORT option added in patch 02.

We add both LEGACY_PCI and HAS_IOPORT to differentiate between two
cases. HAS_IOPORT is for PC-style devices that are not on a PCI card
while LEGACY_PCI is for PCI drivers that require port I/O. This
includes pre-PCIe devices as well as PCIe devices which require
features like I/O spaces. The "legacy" naming is comes from the PCIe
spec which in section 2.1.1.2 says "PCI Express supports I/O Space for
compatibility with legacy devices which require their use. Future
revisions of this specification may deprecate the use of I/O Space."

These two separate config options allow us to compile without support
for these legacy PCI devices even on a system where inb()/outb() and
friends are required for some PC style devices and for example ACPI.