linux-kernel - Re: [PATCH 2/2] Enable 'make CONFIG

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Date:	Wed, 10 Aug 2011 14:59:38 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 2/2] Enable 'make CONFIG_FOO=y oldconfig'

On 9.8.2011 17:26, Arnaud Lacombe wrote:
> On Tue, Aug 9, 2011 at 10:14 AM, Michal Marek <mmarek@...e.cz> wrote:
>> On 31.7.2011 02:05, Arnaud Lacombe wrote:
>>> Because kconfig might not be ran exclusively from a fully controlled
>>> and restricted environment ? Not to mention that it is used by other
>>> people than the linux kernel folks.
>>
>> Well, it has always been possible to trick kbuild (not kconfig) into
>> accepting CONFIG_* options from environment, because unset kconfig options
>> in auto.conf are not seen by make. Of course this is completely fragile,
>> because there is no dependency checking and such variables are only seen by
>> make and do not appear in autoconf.h. So a patch that teaches kconfig to
>> read options from the environment would actually make some (albeit currently
>> "illegal") use cases work correctly :).
>>
> kconfig can already set symbol value from the environment. The only
> limitation I can see is that it is not optional and require an
> explicit environment variable name.

I wasn't talking about the env= syntax, but about

make CONFIG_EXT2_FS=m all
which makes kbuild visit fs/ext2 even if CONFIG_EXT2_FS is disabled in
.config. With no update of the configuration or checking the dependencies.

Hm, actually this would be a problem even if kconfig does read the
CONFIG_* variables from the environment, because it could result in a
mismatch if kconfig determines that the variable cannot be set, but make
still sees it in the environment. So we would have to use 'undefine
CONFIG_FOO' instead of '# CONFIG_FOO is not set' in
include/config/auto.conf, to be able to properly support make CONFIG_FOO=y.

Michal
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