[CentOS] wiki howto on custom kernel

Mon Mar 12 19:53:50 UTC 2007
Alfred von Campe <alfred at 110.net>

On Mar 12, 2007, at 10:15, Jim Perrin wrote:

>>    3. After copying the config file an making one small
>>       change with "make xconfig", why are there so many
>>       differences in the config file?
> Are they actual changes, or is the diff now simply off by a few lines?

I went back and started the process of building a kernel from scratch  
on a different system.  This time I paid closer attention to what was  
going on.  After the "rpmbuild -bp --target=`uname -m`  
kernel-2.6.spec" step, the ~/rpmbuild/BUILD/kernel-2.6.9/linux-2.6.9  
directory contains among other things these two files:

   -rw-r--r--  1 rpmbuild rpmbuild 50332 Mar 12 15:32 .config
   -rw-r--r--  1 rpmbuild rpmbuild 61600 Mar 12 15:31 .config.old

Notice the size difference of these two files.  After copying the  
config file from the ~/rpmbuild/SOURCES directory, these two files  
now look like this:

   -rw-rw-r--  1 rpmbuild rpmbuild 61600 Mar 12 15:42 .config
   -rw-r--r--  1 rpmbuild rpmbuild 61600 Mar 12 15:35 .config.old

And a diff confirms the the two files are now identical.  However,  
after doing a "make xconfig", changing two settings in the Serial  
Port section and saving the config file, the directory looks like this:

   -rw-rw-r--  1 rpmbuild rpmbuild 50322 Mar 12 15:44 .config
   -rw-rw-r--  1 rpmbuild rpmbuild 61600 Mar 12 15:42 .config.old

Finally, comparing the new .config file with the original .config  
file only shows the expected changes:

   # diff /tmp/saved_configs/.config .config
   < # Mon Mar 12 15:32:01 2007
   > # Mon Mar 12 15:44:32 2007
   < # CONFIG_SERIAL_8250_MANY_PORTS is not set

While I don't quite understand what is going on, it appears that  
xconfig is doing the right thing.  I'm rebuilding the kernel again as  
I type this.  BTW, there are lots of compiler warnings: is this  
normal/expected for a kernel build?