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From:   Chunyan Zhang <zhang.lyra@gmail.com>
Date:   Fri, 3 Sep 2021 17:40:13 +0800
Message-ID: <CAAfSe-v0DbOS5p7nF0db4SdTrsy28gJNdW93ds4capVxuUO1xw@mail.gmail.com>
Subject: Re: [PATCH] s390/io: Fix ioremap and iounmap undefinded issue for s390
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     Vasily Gorbik <gor@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        linux-s390@vger.kernel.org, kernel test robot <lkp@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Chunyan Zhang <chunyan.zhang@unisoc.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, 3 Sept 2021 at 16:24, Niklas Schnelle <schnelle@linux.ibm.com> wrote:
>
> On Fri, 2021-09-03 at 16:03 +0800, Chunyan Zhang wrote:
> > From: Chunyan Zhang <chunyan.zhang@unisoc.com>
> >
> > There would not be ioremap and iounmap implementations if CONFIG_PCI is
> > not set for s390, so add default declarations of these two functions
> > for the case to avoid 'undefined reference' issue.
> >
> > Fixes: 71ba41c9b1d9 ("s390/pci: provide support for MIO instructions")
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
> > ---
> > The issue was reported from https://lkml.org/lkml/2021/8/1/18
>
> Thanks for the patch but I'm a little skeptical about adding
> ioremap()/iounmap() stubs that don't do anything useful and on top ofn
> that would do so silently.
>
> In the above discussion you said that TIMER_OF should depend on
> HAS_IOMEM. In arch/s390/Kconfig HAS_IOMEM is set if and only if
> CONFIG_PCI is set so that sounds to me like it would prevent the
> undefined reference without the risk of someone trying to use io*map()
> without CONFIG_PCI.

Humm... you can ignore my reply on that time, I later found that's not
correct :)

TIMER_OF would be selected by other configs and it seems not able to
depends on HAS_IOMEM, unless all configs which select TIMER_OF depend
on HAS_IOMEM, that would be a big change.

Actually HAS_IOMEM is set as default on other architectures, but not
for s390 which redefined it.

>
> At the very least I think the functions should do a WARN_ONCE() but
> then we have the same situation as discussed below with Linus making it
> pretty clear that he prefers these cases to be compile time checked:

Ok, if I understand correctly, if io*map is not implemented for some
case, there should be a *compile-time* error rather than adding a stub
function to make this kind of errors disappeared.

Please correct me if I missed something.

Thanks,
Chunyan

>
> https://lkml.org/lkml/2021/7/2/511
>
>
> > ---
> >  arch/s390/include/asm/io.h | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/s390/include/asm/io.h b/arch/s390/include/asm/io.h
> > index e3882b012bfa..9438370c6445 100644
> > --- a/arch/s390/include/asm/io.h
> > +++ b/arch/s390/include/asm/io.h
> > @@ -23,11 +23,8 @@ void unxlate_dev_mem_ptr(phys_addr_t phys, void *addr);
> >  #define IO_SPACE_LIMIT 0
> >
> >
> .. snip ..
>