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List:       linux-newbie
Subject:    Re: Kernel .config  and kconfig general question
From:       Max <maxw_gcc () yahoo ! com>
Date:       2007-11-23 6:11:39
Message-ID: 495063.14257.qm () web33012 ! mail ! mud ! yahoo ! com
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First of all many thanks for your help. And very useful information.

> From: Kristof Provost
> To: Max
> Hi,
> I've moved this discussion to kernel newbies as Robert suggested.

Many thanks Kristof. Now I know where to post my newbie questions ;-) Although \
linux-newbie traffic seems very low...

> > On 2007-11-21 01:16:37 (-0800), Max <maxw_gcc@yahoo.com> wrote:
> > Is it the same to comment out a variable in .config than assigning
 'N' to it?

> No and yes. 
> I guess that doesn't help, so I'll try the long answer:
> The .config file is generated by kconfig and parsed by Make. That
 means
> there's a difference between "CONFIG_TEST = N" and "#CONFIG_TEST is
 not
> set". However, the CONFIG_ variables are usually used as 
> "obj-$(CONFIG_TEST)". The kernel makefiles don't add obj-n to the list
> of files to build, so nothing is done for those variables.

> > Or even more general: 
> > Could anybody please help me in understanding the main picture of
 how
 a .config variable gets #define'd or #undefine'd in the kernel
> > header files?

> That's actually done by kconfig. It generates the .config file (based
 on
> user choices or defaults) but it also generates
> include/linux/autoconf.h. That header file does "#define CONFIG_TEST".
> It will also touch include/config/test so the build system can be
 clever
> when rebuilding. It allows the system to avoid rebuilding everything
> which includes autoconf.h when the configuration is updated. The
 system
> will replace the dependency on autoconf.h with a dependency on
> config/test. Kconfig will only touch (update) the config/test file if
> the value of CONFIG_TEST changed. That way only files which actually
 use
> CONFIG_TEST will be rebuilt.
> Kristof


Let me see if I understand the whole picture:
When you do:

$ make defconfig (for example)
...
scripts/kconfig/conf -d arch/i386/Kconfig
...
Runs and the .config gets created (in this case with defaults)

Then when you do:
$ make
...
scripts/kconfig/conf -s arch/i386/Kconfig
...
Runs and the following 3 files are generated just before the whole
 shebang starts:

-include/config/auto.conf.cmd
-include/linux/autoconf.h
-include/config/auto.conf

Well I did this simple exercise

$ make defconfig

vi .config and assign 'n'  to CONFIG_HIGH_RES_TIMER (only for testing)

Then run

$ make oldconfig

And that make oldconfig automatically commented out this line. Which may suggest that \
 CONFIG_HIGH_RES_TIMER=n
#CONFIG_HIGH_RES_TIMER
Are treated in an equivalent way for the compilation building process....

I don't know if this holds true for all other CONFIG_ variables. But at least is a \
starting point. The clues could probably arise by reading and understanding \
scripts/kconfig/confdata.c


Thanks again for your help,

Max








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