Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary

[Michael Schmitz]

Hi Arnd,

Am 29.12.2021 um 16:41 schrieb Arnd Bergmann:
> On Tue, Dec 28, 2021 at 8:20 PM Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
> > Am 28.12.2021 um 23:08 schrieb Geert Uytterhoeven:
> > > On Mon, Dec 27, 2021 at 5:44 PM Niklas Schnelle <schnelle@xxxxxxxxxxxxx> wrote:
> > > > We introduce a new HAS_IOPORT Kconfig option to gate 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. On these platforms
> > > > inb()/outb() etc are currently just stubs in asm-generic/io.h which when
> > > > called will cause a NULL pointer access which some compilers actually
> > > > detect and warn about.
> > > >
> > > > The dependencies on HAS_IOPORT in drivers as well as ifdefs for
> > > > HAS_IOPORT specific sections will be added in subsequent patches on
> > > > a per subsystem basis. Then a final patch will ifdef the I/O access
> > > > functions on HAS_IOPORT thus turning any use not gated by HAS_IOPORT
> > > > into a compile-time warning.
> > > >
> > > > Link: https://lore.kernel.org/lkml/CAHk-=wg80je=K7madF4e7WrRNp37e3qh6y10Svhdc7O8SZ_-8g@xxxxxxxxxxxxxx/
> > > > Co-developed-by: Arnd Bergmann <arnd@xxxxxxxxxx>
> > > > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxxxx>
> > > > Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> > >
> > > Thanks for your patch!
> > >
> > > > --- a/arch/m68k/Kconfig
> > > > +++ b/arch/m68k/Kconfig
> > > > @@ -16,6 +16,7 @@ config M68K
> > > >         select GENERIC_CPU_DEVICES
> > > >         select GENERIC_IOMAP
> > > >         select GENERIC_IRQ_SHOW
> > > > +       select HAS_IOPORT
> > > >         select HAVE_AOUT if MMU
> > > >         select HAVE_ASM_MODVERSIONS
> > > >         select HAVE_DEBUG_BUGVERBOSE
> > >
> > > This looks way too broad to me: most m68k platform do not have I/O
> > > port access support.
> > >
> > > My gut feeling says:
> > >
> > >     select HAS_IOPORT if PCI || ISA
> > >
> > > but that might miss some intricate details...
> >
> > In particular, this misses the Atari ROM port ISA adapter case -
> >
> >         select HAS_IOPORT if PCI || ISA || ATARI_ROM_ISA
> >
> > might do instead.
>
> Right, makes sense. I had suggested to go the easy way and assume that
> each architecture would select HAS_IOPORT if any configuration supports
> it, but it looks like for m68k there is a clearly defined set of platforms that
> do.
>
> Note that for the platforms that don't set any of the three symbols, the
> fallback makes inb() an alias for readb() with a different argument type,
> so there may be m68k specific drivers that rely on this, but those would
> already be broken if ATARI_ROM_ISA is set.

I'd hope not - we spent some effort to make sure setting ATARI_ROM_ISA 
does not affect other m68k platforms when e.g. building multiplatform 
kernels.

Replacing inb() by readb() without any address translation won't do much 
good for m68k though - addresses in the traditional ISA I/O port range 
would hit the (unmapped) zero page.

Cheers,

	Michael

>           Arnd