Date:	Wed, 30 Mar 2016 10:13:53 +0200
From:	Richard Weinberger <richard@....at>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Rob Herring <robh@...nel.org>, Lee Jones <lee.jones@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH 1/2] asm-generic/io.h: provide default ioremap/iounmap for
 !HAS_IOMEM

Am 30.03.2016 um 10:04 schrieb Arnd Bergmann:
> On Wednesday 30 March 2016 09:50:22 Richard Weinberger wrote:
>> 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@...el.com>
>>>>> Cc: Arnd Bergmann <arnd@...db.de>
>>>>> Cc: linux-arch@...r.kernel.org
>>>>> Signed-off-by: Rob Herring <robh@...nel.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.
>>
> 
> One way out of this would be accept that UML is just too different
> from the other architectures, and make COMPILE_TEST itself depend
> on !UML.
> 
> COMPILE_TEST is highly valuable for me, because it means that all the
> ARM specific drivers can now be build-tested on x86 and they show up
> for anyone doing allmodconfig builds. If we introduce a regression
> in any of the drivers, it gets caught much quicker because many
> subsystem maintainers run an x86 allmodconfig build after pulling
> downstream maintainer trees. There is also stuff like Coverity that
> only ever runs on x86.
> 
> It does make sense to have COMPILE_TEST imply that we are able
> to build things on all architectures, not just x86 in addition to
> the platform a driver is meant for, but the value of compile-testing
> a random driver with UML in particular approaches zero, and instead
> causes lots of work.

I fully understand your point of view. COMPILE_TEST is a monster that
can do the heavy lifting for you, but monsters also have claws and fangs. ;-)

Having COMPILE_TEST having depend on !UML works for me. But don't
we have other archs without io mem? At least a few years ago while
porting nandsim to UML I found s390 that lacks of io mem too.

Maybe we depend COMPILE_TEST on HAS_IOMEM?

Thanks,
//richard

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