linux-kernel - Re: [RFC 01/32] Kconfig: introduce and depend on LEGACY

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Date:   Tue, 28 Dec 2021 11:58:55 +0100
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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        Bjorn Helgaas <bhelgaas@...gle.com>,
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Subject: Re: [RFC 01/32] Kconfig: introduce and depend on LEGACY_PCI

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@...uxfoundation.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.