Re: Kernel .config and kconfig general question

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 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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