From http://newbiedoc.sourceforge.net/system/kernel-pkg.html

8.1. Using an existing configuration
When you installed your custom kernel, its configuration was copied to /boot/config-x.y.z. You may want to squirrel it away in another location (or several other locations) for safekeeping or later re-use.

Suppose that just last week you picked your way through the maze of options, and settled on a configuration that supports all the hardware and features you want. Today a new stable kernel is released and of course you want to upgrade right away. Is it necessary to start from scratch configuring the new kernel? Not at all.

Download and expand the new source tarball and make a new symlink. Then change directories to /usr/src/linux, copy your existing configuration there and do "make oldconfig":

bash:/usr/src/linux$ cp /boot/config-2.4.18.030320 .config
bash:/usr/src/linux$ make oldconfig


You'll be asked questions about new kernel options. It's generally safe to answer "no" to all the questions; make notes of the new options that interest you.

After you finish oldconfig you can run xconfig or menuconfig to review your selections, change your answers, or investigate the options you noted.

What is the difference if I just copy /boot/config-x.y.z. into /usr/src/linux/.config and do a make xconfig (skipping make oldconfig) ?

Thanks

Because the old config, will not match up. The new kernel will have options the old kernel won't also the settings will change sometimes, so what was value 2 in the old config maybe value 3 in the new, so if you don't make oldconfig, some of the values maybe inaccurate.

Also stuff moves about, say the networking card options could be under another section, and won't be parsed, so no network card for you.

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Sometimes, especially if you're moving an entirely new minor version (2.4-2.6), it's better to read the config file and use it as a guideline to choosing your kernel options. No method is perfect, so being able to look at the options and decide if you want to make a change or not is sometimes a better choice than a simple config copy or letting a parser do the work for you. Takes a little more time, but kernel compiling isn't something you should rush through. It's also quite educational, especially if your distro makes a lot of modifications to the base kernel (custom patches and experimental stuff).

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