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Subject: Re: [GIT PULL 1/2] asm-generic: rework PCI I/O space access
To:     Arnd Bergmann <arnd@kernel.org>
CC:     Linus Torvalds <torvalds@linux-foundation.org>,
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From:   John Garry <john.garry@huawei.com>
Message-ID: <dd2d49ef-3154-3c87-67b9-c134567ba947@huawei.com>
Date:   Tue, 10 Aug 2021 10:19:12 +0100
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On 04/08/2021 09:52, Arnd Bergmann wrote:
>>>> On another point, I noticed SCSI driver AHA152x depends on ISA, but is
>>>> not an isa driver - however it does use port IO. Would such dependencies
>>>> need to be changed to depend on HAS_IOPORT?
>>> I'm not sure what you mean here. As far as I can tell, AHA152x is an ISA
>>> driver in the sense that it is a driver for ISA add-on cards. However, it
>>> is not a 'struct isa_driver' in the sense that AHA1542 is, AHA152x  is even
>>> older and uses the linux-2.4 style initialization using a module_init()
>>> function that does the probing.
>> ok, fine. So I just wonder what the ISA kconfig dependency gets us for
>> aha152x. I experimented by removing the kconfig dependency and enabling
>> for the arm64 (which does not have CONFIG_ISA) std defconfig and it
>> built fine.
> The point of CONFIG_ISA is to only build drivers for ISA add-on cards
> on architectures that can have such slots. For ISA drivers in particular,
> we don't want them to be loaded on machines that don't have them
> because of the various ways this can cause trouble with hardwired
> port and irq numbers.
> 
>>>> Yeah, that sounds the same as what I was thinking. Maybe IOPORT_NATIVE
>>>> could work as a name. I would think that only x86/ia64 would define it.
>>>> A concern though is that someone could argue that is a functional
>>>> dependency, rather than just a build dependency.
>>> You can have those on a number of platforms, such as early
>>> PowerPC CHRP or pSeries systems, a number of MIPS workstations
>>> including recent Loongson machines, and many Alpha platforms.
>>>
>> hmmm... if some machines under an arch support "native" port IO and some
>> don't, then if we use a common multi-platform defconfig which defines
>> HARDCODED_IOPORT, then we still build for platforms without "native"
>> port IO, which is not ideal.
> Correct, but that's not a problem I'm trying to solve at this point. The
> machines that have those are extremely rare, so almost all configurations
> that one would encounter in practice do not suffer from it, and solving it
> reliably would be really hard.

Hi Arnd,

This seems a reasonable approach. Do you have a plan for this work? Or 
still waiting for the green light?

I have noticed the kernel test robot reporting the following to me, 
which seems to be the same issue which was addressed in this series 
originally:

config: s390-randconfig-r032-20210802 (attached as .config)
compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 
4f71f59bf3d9914188a11d0c41bedbb339d36ff5)
...
All errors (new ones prefixed by >>):

    In file included from drivers/block/null_blk/main.c:12:
    In file included from drivers/block/null_blk/null_blk.h:8:
    In file included from include/linux/blkdev.h:25:
    In file included from include/linux/scatterlist.h:9:
    In file included from arch/s390/include/asm/io.h:75:
    include/asm-generic/io.h:464:31: warning: performing pointer 
arithmetic on a null pointer has undefined behavior 
[-Wnull-pointer-arithmetic]
            val = __raw_readb(PCI_IOBASE + addr);

So I imagine lots of people are seeing these.

Thanks,
john