RE: [PATCH v4] Kconfig: introduce HAS_IOPORT option and select it as necessary

[David Laight <David.Laight@ACULAB.COM>]

From: Geert Uytterhoeven
> Sent: 11 April 2023 09:50
> 
> Hi David,
> 
> On Wed, Apr 5, 2023 at 11:37 PM David Laight <David.Laight@aculab.com> wrote:
> > From: Linuxppc-dev Arnd Bergmann
> > > Sent: 05 April 2023 21:32
> > >
> > > On Wed, Apr 5, 2023, at 22:00, H. Peter Anvin wrote:
> > > > On April 5, 2023 8:12:38 AM PDT, Niklas Schnelle <schnelle@linux.ibm.com> wrote:
> > > >>On Thu, 2023-03-23 at 17:33 +0100, Niklas Schnelle wrote:
> > > >>> We introduce a new HAS_IOPORT Kconfig option to indicate support for I/O
> > > >>> Port access. In a future patch HAS_IOPORT=n will disable compilation of
> > > >>> the I/O accessor functions inb()/outb() and friends on architectures
> > > >>> which can not meaningfully support legacy I/O spaces such as s390.
> > > >>> >>
> > > >>Gentle ping. As far as I can tell this hasn't been picked to any tree
> > > >>sp far but also hasn't seen complains so I'm wondering if I should send
> > > >>a new version of the combined series of this patch plus the added
> > > >>HAS_IOPORT dependencies per subsystem or wait until this is picked up.
> > > >
> > > > You need this on a system supporting not just ISA but also PCI.
> > > >
> > > > Typically on non-x86 architectures this is simply mapped into a memory window.
> > >
> > > I'm pretty confident that the list is correct here, as the HAS_IOPORT
> > > symbol is enabled exactly for the architectures that have a way to
> > > map the I/O space. PCIe generally works fine without I/O space, the
> > > only exception are drivers for devices that were around as early PCI.
> >
> > Isn't there a difference between cpu that have inb()/outb() (probably
> > only x86?) and architectures (well computer designs) that can generate
> > PCI 'I/O' cycles by some means.
> > It isn't even just PCI I/O cycles, I've used an ARM cpu (SA1100)
> > that mapped a chuck of physical address space onto PCMCIA I/O cycles.
> >
> > If the hardware can map a PCI 'IO' bar into normal kernel address
> > space then the bar and accesses can be treated exactly like a memory bar.
> > This probably leaves x86 as the outlier where you need (IIRC) io_readl()
> > and friends that can generate in/out instructions for those accesses.
> >
> > There are also all the x86 ISA devices which need in/out instructions.
> > But (with the likely exception of the UART) they are pretty much
> > platform specific.
> >
> > So, to my mind at least, HAS_IOPORT is just the wrong question.
> 
> Not all PCI controllers support mapping the I/O bar in MMIO space, so
> in general you cannot say that CONFIG_PCI=y means CONFIG_HAS_IOPORT=y.

But a CONFIG_HAS_PCI_IO=y would imply CONFIG_HAS_IOPORT=y.
It is the former that is more interesting for driver support.

	David

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