linux-kernel - Re: [PATCH] drivers: Let several drivers depends on HAS_IOMEM for 'devm_ioremap

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Date:	Mon, 14 Jul 2014 10:18:10 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Lars-Peter Clausen <lars@...afoo.de>,
	Richard Weinberger <richard@....at>, dmitry.torokhov@...il.com,
	linux-iio@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>, teg@...m.no,
	Lennox Wu <lennox.wu@...il.com>,
	Chen Gang <gang.chen.5i5j@...il.com>,
	Marek Vasut <marex@...x.de>,
	Liqin Chen <liqin.linux@...il.com>, msalter@...hat.com,
	linux-pwm@...r.kernel.org, devel@...verdev.osuosl.org,
	linux-watchdog@...r.kernel.org, linux-input@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	knaack.h@....de, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Mischa.Jonker@...opsys.com, jic23@...nel.org
Subject: Re: [PATCH] drivers: Let several drivers depends on HAS_IOMEM for
 'devm_ioremap_resource'

On Sun, Jul 13, 2014 at 12:22:02PM -0700, Greg Kroah-Hartman wrote:
> On Sun, Jul 13, 2014 at 04:25:06PM +0200, Lars-Peter Clausen wrote:
> > On 07/13/2014 04:03 PM, Richard Weinberger wrote:
> > >Am 13.07.2014 15:56, schrieb Lars-Peter Clausen:
> > >>On 07/13/2014 03:40 PM, Richard Weinberger wrote:
> > >>>Am 13.07.2014 15:26, schrieb Lars-Peter Clausen:
> > >>>>On 07/13/2014 11:45 AM, Richard Weinberger wrote:
> > >>>>>Am 13.07.2014 11:27, schrieb Lennox Wu:
> > >>>>>>As I said before, some configurations don't make sense.
> > >>>>>
> > >>>>>If such a configuration can be achieved using allmod/yesconfig it has to be fixed.
> > >>>>>Chen's fixes seem reasonable as not all architectures support iomem.
> > >>>>
> > >>>>Maybe we should stub out ioremap() and friends when COMPILE_TEST is enabled to avoid these linker errors. That's in my opinion better than turning most of the 'depends on
> > >>>>COMPILE_TEST' into 'depends on COMPILE_TEST && HAS_IOMEM'. The issue comes up quite a lot and it is often overlooked when adding a driver that can be build when COMPILE_TEST is
> > >>>>enabled.
> > >>>
> > >>>And what should this stub do?
> > >>>Except calling BUG()...
> > >>
> > >>return NULL;
> > >>
> > >>It's for compile testing, it's not meant to work at runtime.
> > >
> > >Hm, I really don't like the idea of having a non-working kernel.
> > >IMHO either it should build _and_ run and nothing else.
> > >Greg, what do you think?
> > 
> > The kernel will still be working fine and you can run it on a system. The
> > drivers which use ioremap() or similar are probably not instantiated on a
> > system that does not provide HAS_IOMEM. But even if it was the driver should
> > handle ioremap() returning NULL gracefully and abort the driver probe. That
> > said you'll probably not run a kernel that was built with COMPILE_TEST on
> > your real hardware since it contains so many drivers that are completely
> > useless on your hardware. The idea of COMPILE_TEST is to have as much
> > compile test exposure as possible to the code that is enabled by
> > COMPILE_TEST. Stubbing out ioremap() and friends when HAS_IOMEM is not set
> > and COMPILE_TEST is set makes it easier to get there.
> 
> I run my kernels with COMPILE_TEST enabled as I need to build test
> things that I don't happen to use.
> 
> I like the 'return NULL' option for this, it hits us all the time, might
> as well fix it properly like this so that we don't have to deal with
> Kconfig changes everywhere.

I agree. One nit, though: devm_ioremap_resource() returns an ERR_PTR()-
encoded error code, so the dummy should probably be returning something
like ERR_PTR(-ENOSYS) instead of NULL.

> Also put a big "This platform does not support IOMEM" error printk in
> there, so that people have a chance to figure out what is going on if
> they happen to run such a driver on a platform that can't support it.

Yes, that sounds like a very good idea and should be indication enough
of what exactly has gone wrong.

Thierry

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