How do I get the correct .config file for compiling the Linux kernel source specific to my hardware? - linux

Question

I tried using make defconfig to compile the kernel, but as expected, it failed to boot. I was wondering what .config file do kernel vendors like Canonical for Ubuntu use, that the kernel is able to boot right out-of-the-box. Of course, I am still a beginner and configuring the various parameters, is a little out of my league currently.
Specifically,I am looking to load a basic "hello, world!" module to my running kernel 2.6.32.41. For that, I would need to compile kernels source against the same .config file that was used for the running kernel.

Answer 1

If your running kernel was compiled with the CONFIG_IKCONFIG_PROC option, you can get the config in /proc/config.gz:
$ zcat /proc/config.gz >my_config
Copy my_config into your kernel build directory as .config and run make config to be prompted for configuration options that are absent from your config file (this will only happen if you are using a kernel source that is newer than your running kernel). You should then be able to compile a new kernel with the same features as your current one.
Distros typically use their own kernel configuration, where most of the drivers are compiled as modules to be dynamically loaded when the corresponding hardware is requested. Also the kernel needs to be booted with relevant boot options (like the one specifying the root filesystem). Your defconfig kernel probably failed to boot because of that.

Answer 2

I don't know about getting the one that's "correct for your hardware", but you can use the config that Ubuntu gives you by looking in /boot/ for a file starting with the name config. There may be more than one, in which case use the command uname -r to tell which kernel you're currently running, and then you can use the appropriate config.

Answer 3

option1:
source code of your booted system
cd /usr/src/linux-headers-3.2.0-29;
this will generate .config
sudo make oldconfig;
vi .config
option2:
zcat /proc/config.gz > my_config
option3:
echo /boot/config* > my_config

Answer 4

"defconfig" is usually pegged at the commonly used hardware - x86, or x86_64, and perhaps not so recent chipset or motherboard. Sometimes, like my Lenovo laptop, only the latest kernel source, and with enabling some config option, after googling through the bugzilla database, will it work.
Like what Jeff Welling said, to get the config in use, u can look under /boot directory. Same for my Fedora Core too. But if u want to compile a basic program as a "kernel module", and by that it simply means "loadable kernel module", u don't need to compile the kernel source. U just need the kernel headers for that current version. For example, "apt-cache search" in Ubuntu 10.04 returns several possible option:
linux-headers-2.6.38 - Header files related to Linux kernel, specifically,
linux-libc-dev - Linux Kernel Headers for development
Ubuntu normally patched the stock kernel (from kernel.org) to have their own kernel. If u have downloaded the stock kernel, and attempt to use the /boot's config file (or sometimes u can find the currently loaded config as /proc/config.gz, like the Backtrack's Ubuntu, which is based on 10.04 LTS), then u may need to do a "make oldconfig" with the current config file named as ".config". "make oldconfig" will then use the .config to generate a new .config that is compatible with the kernel source.