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Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Bjorn Helgaas <helgaas@kernel.org>,
        Arnd Bergmann <arnd@kernel.org>,
        John Garry <john.garry@huawei.com>
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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Date:   Fri, 06 May 2022 11:38:52 +0200
In-Reply-To: <20220505195342.GA509942@bhelgaas>
References: <20220505195342.GA509942@bhelgaas>
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On Thu, 2022-05-05 at 14:53 -0500, Bjorn Helgaas wrote:
> On Thu, May 05, 2022 at 07:39:42PM +0200, Arnd Bergmann wrote:
> > On Thu, May 5, 2022 at 6:10 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > On Wed, May 04, 2022 at 11:31:28PM +0200, Arnd Bergmann wrote:
> > > > The main goal is to avoid c), which is what happens on s390, but
> > > > can also happen elsewhere. Catching b) would be nice as well,
> > > > but is much harder to do from generic code as you'd need an
> > > > architecture specific inline asm statement to insert a ex_table
> > > > fixup, or a runtime conditional on each access.
> > > 
> > > Or s390 could implement its own inb().
> > > 
> > > I'm hearing that generic powerpc kernels have to run both on machines
> > > that have I/O port space and those that don't.  That makes me think
> > > s390 could do something similar.
> > 
> > No, this is actually the current situation, and it makes absolutely no
> > sense. s390 has no way of implementing inb()/outb() because there
> > are no instructions for it and it cannot tunnel them through a virtual
> > address mapping like on most of the other architectures. (it has special
> > instructions for accessing memory space, which is not the same as
> > a pointer dereference here).
> > 
> > The existing implementation gets flagged as a NULL pointer dereference
> > by a compiler warning because it effectively is.
> 
> I think s390 currently uses the inb() in asm-generic/io.h, i.e.,
> "__raw_readb(PCI_IOBASE + addr)".  I understand that's a NULL pointer
> dereference because the default PCI_IOBASE is 0.
> 
> I mooted a s390 inb() implementation like "return ~0" because that's
> what happens on most arches when there's no device to respond to the
> inb().
> 
> The HAS_IOPORT dependencies are fairly ugly IMHO, and they clutter
> drivers that use I/O ports in some cases but not others.  But maybe
> it's the most practical way.
> 
> Bjorn

I fear such stubs are kind of equivalent to my previous patch doing the
same in asm-generic/io.h that was pulled and then unpulled by Linus.
Maybe it would be slightly different if instead of a warning outX()
would just be a NOP and inX() just returned ~0 but we're in essence
pretending that we have these functions when we know they are nonsense.

Another argument I see is that as shown by POWER9 we might start to see
more platforms that just can't do I/O port access. E.g. I would also be
surprised if Apple's M1 has I/O port access. Sooner or later I expect
distributions on some platforms to only support such systems. For
example on ppc a server distribution might only support IBM POWER
without I/O port support before too long. Then having HAS_IOPORT allows
to get rid of drivers that won't work anyway.

There are also reports of probing a driver with I/O ports causing a
system crash on systems without I/O port support. For example in this
answer by John Garry (added so he may supply more information):

https://lore.kernel.org/lkml/db043b76-880d-5fad-69cf-96abcd9cd34f@huawei.com/