On (the fortunately rare) occasion, the need arises to modify a Linux kernel module/driver. I finally found a helpful blog-post that gave a walk-through on how to compile a single kernel module without necessitating recompiling the entire Linux kernel.
Following are the steps I took to modify the xpad driver (used by xbox controllers), along with a few clarifications of the original blog post I follgowed. Anything in bold, you'll want to tweak for your own system.
0. I'm assuming that you've already downloaded Linux source and modified the file(s) you need. In my case, I was tweaking "/usr/src/linux-source-2.6.32/drivers/input/joystick/xpad.c".
1. Make a working directory to hold your new module when it's compiled, along with overhead needed by the compilation process. I chose to name my directory "xpad" since I was specifically modifying the xpad driver.
$ sudo mkdir /usr/src/xpad
2. Copy in a few necessary files:
$ cd /usr/src/xpad
$ sudo cp /boot/config-`uname -r` .config
$ sudo cp /usr/src/linux-headers-`uname -r`/Module.symvers .
3. Move to your source directory and run some "make" commands to prepare your working directory.
Note that EXTRAVERSION should be whatever follows the first three numbers when you run "uname -r". For example, if running "uname -r" results in "2.6.32-43-generic", EXTRAVERSION would be "-43-generic".
$ cd /usr/src/linux-source-2.6.32
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad oldconfig
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad prepare
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad outputmakefile
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad archprepare
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad modules SUBDIRS=scripts
4. Make your specific module.
In my case, I tried first to make just the xpad module. That resulted in an error indicating that there wasn't a Makefile specifically for the xpad module.
First, unsuccessful attempt:
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad modules SUBDIRS=drivers/input/joystick/xpad
/usr/src/linux-source-2.6.32/scripts/Makefile.build:44: /usr/src/linux-source-2.6.32/drivers/input/joystick/xpad/Makefile: No such file or directory
make: *** No rule to make target `/usr/src/linux-source-2.6.32/drivers/input/joystick/xpad/Makefile'. Stop.
make: *** [_module_drivers/input/joystick/xpad] Error 2
make: *** [sub-make] Error 2
There WAS, however, a Makefile that built all the joystick drivers at once, so I fell back to using that.
Second, successful attempt:
$ sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad modules SUBDIRS=drivers/input/joystick
This generated, among other files, a .ko file, my modified kernel module. The new modules will be located in the working directory you created in step 1 (in my case, "/usr/src/xpad/drivers/input/joystick/xpad.ko").
5. Make a backup of the original kernel module, just in case (of course impossible) that there's a bug your new module.
$ sudo cp /lib/modules/`uname -r`/kernel/drivers/input/joystick/xpad.ko /lib/modules/`uname -r`/kernel/drivers/input/joystick/xpad.ko.bak
6. Copy in your new kernel module:
$ sudo cp /usr/src/xpad/drivers/input/joystick/xpad.ko /lib/modules/`uname -r`/kernel/drivers/input/joystick/xpad.ko
7. Reload module dependencies:
$ sudo depmod -a
8. Remove the old module, just in case it's currently loaded:
$ sudo modprobe -r xpad
9. (Unplug and) plug in your device to trigger a load of your new module!
To simplify repeated compiling of the code during development, I dumped the pertinent commands into a script I could run:
sudo make EXTRAVERSION=-43-generic O=/usr/src/xpad modules SUBDIRS=drivers/input/joystick
sudo cp /usr/src/xpad/drivers/input/joystick/xpad.ko /lib/modules/`uname -r`/kernel/drivers/input/joystick/xpad.ko
sudo depmod -a
sudo modprobe -r xpad