Re: [PATCH 1/2] asm-generic/io.h: provide default ioremap/iounmap for !HAS_IOMEM

[Richard Weinberger <richard@nod.at>]

Am 29.03.2016 um 22:13 schrieb Rob Herring:
>>> Reuse the !MMU variants for !HAS_IOMEM as they are sufficient for our
>>> needs. This fixes build errors for UM allyesconfig:
>>>
>>> drivers/mfd/syscon.c:89:2: error: implicit declaration of function 'iounmap' [-Werror=implicit-function-declaration]
>>>      iounmap(base);
>>>
>>> Reported-by: Fengguang Wu <fengguang.wu@intel.com>
>>> Cc: Arnd Bergmann <arnd@arndb.de>
>>> Cc: linux-arch@vger.kernel.org
>>> Signed-off-by: Rob Herring <robh@kernel.org>
>>
>> (adding Richard and the UML list to cc)
>>
>> I actually prototyped a patch that did the opposite: remove the readl/writel/...
>> definitions when HAS_IOMEM is unset. I didn't get far enough to submit it,
>> but see below for what I did.
> 
> Ewww. Why do the opposite of what we do for every other Kconfig symbol
> which is provide empty functions? It really only functions as a
> disable on UML flag which runs counter to enabling drivers to build
> for all arches.
> 
> I actually started a patch to remove the HAS_IOMEM dependency
> everywhere (or just the per driver cases). It didn't break as bad as I
> expected, but became more than I wanted to fix. Mainly, all the devm_
> variants also need empty versions or to be always enabled.

The root cause of the problem is COMPILE_TEST. People who use it need to
get forced to think about the consequences.
COMPILE_TEST means that the driver will also be build on not so fancy archs
like UML, s390 and m68k where you don't have IOMEM or also DMA in every
possible configuration.
So a quick build test on x86 is not sufficient.

This is why I'm absolutely not a fan of having stubs for iounmap() an friends
for UML. It only tries to bypass the root cause.
What is next? Stubs for DMA? PCI? For everything else that does not build?

With COMPILE_TEST we have created a monster and now we have to deal with it in terms of
having correct dependencies when COMPILE_TEST is being used.

Thanks,
//richard