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

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Date:	Thu, 17 Jul 2014 12:40:25 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Arnd Bergmann <arnd@...db.de>, Chen Gang <gang.chen.5i5j@...il.com>
CC:	Guenter Roeck <linux@...ck-us.net>,
	Richard Weinberger <richard@....at>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	dmitry.torokhov@...il.com, linux-iio@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>, teg@...m.no,
	Thierry Reding <thierry.reding@...il.com>,
	Lennox Wu <lennox.wu@...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,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] drivers: Let several drivers depends on HAS_IOMEM for
 'devm_ioremap_resource'

On 07/17/2014 11:20 AM, Arnd Bergmann wrote:
> On Thursday 17 July 2014 09:27:58 Chen Gang wrote:
>>                                           gfp_t gfp_mask, unsigned int order);
>>   extern void devm_free_pages(struct device *dev, unsigned long addr);
>>
>> +#ifdef CONFIG_HAS_IOMEM
>>   void __iomem *devm_ioremap_resource(struct device *dev, struct resource *res);
>> +#elif defined(CONFIG_COMPILE_TEST)
>> +static inline void __iomem *devm_ioremap_resource(struct device *dev,
>> +                                               struct resource *res)
>> +{
>> +       pr_warn("no hardware io memory, only for COMPILE_TEST\n");
>> +       return (__force void __iomem *)ERR_PTR(-ENXIO);
>> +}
>> +#endif /* CONFIG_HAS_IOMEM || CONFIG_COMPILE_TEST */
>>
>>   /* allows to add/remove a custom action to devres stack */
>
> To be honest, I think it's a bad idea to introduce wrappers functions
> that are only available when CONFIG_COMPILE_TEST is set.
>
> COMPILE_TEST is a great tool in general, but it has its limits.
> In particular, the case for !CONFIG_IOMEM is completely obscure
> and we won't find any bugs by allowing more drivers to be built
> in those configurations, but attempting to do it would cause
> endless churn by changing each instance of 'depends on HAS_IOMEM'
> to 'depends on HAS_IOMEM || COMPILE_TEST'.

The point of this exercise is that we do not have to replace a good chunk of 
'depends on COMPILE_TEST' with 'depends on COMPILE_TEST && HAS_IOMEM'

E.g. the typical Kconfig entry for your random SoC peripheral driver looks like

config ARCH_FOOBAR_DRIVER
	depends on ARCH_FOOBAR || COMPILE_TEST
	...

Now when COMPILE_TEST is not set there is a implicit dependency on HAS_IOMEM 
since the architecture will provide it. If COMPILE_TEST is selected the 
driver will also be build-able on architectures that do no have HAS_IOMEM 
and hence linking the driver fails. One way to fix this is of course to 
replace the COMPILE_TEST with (COMPILE_TEST && HAS_IOMEM). But this is very 
often overlooked and only noticed later on when somebody actually builds a 
allyesconfig on an architecture that does not provide HAS_IOMEM. To avoid 
these kinds of build errors and tedious fixup patches the idea is to provide 
a stub function when HAS_IOMEM is not enabled, but COMPILE_TEST is enabled.

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