Update
CyanogenMod has effectively been replaced by LineageOS. The links in this article originally all pointed to CyanogenMod but since it no longer exists, I've updated them to use the LineageOS equivilents where possible. The instructions will also need to be slightly modified for use with LineageOS builds.
Introduction
I finally just bought a new phone (a Nexus 5), and after flashing CyanogenMod onto it, I've been messing around with its OTG support. My old phone (a Nexus 4) didn't have the capability so being able to plug in keyboards, mice, external hard drives, and various other peripherals was (embarrassingly enough) a thrill.
Keyboards worked, mice worked, but when an Xbox 360 controller was plugged in, nothing happened. Turns out that while the kernel didn't have support for it enabled out of the box, getting it enabled and pushing the change upstream into the CyanogenMod project was easier than I thought.
Beware that this post is going to be more of a record of what I did so that I can do it again and less of a coherent tutorial.
Building CyanogenMod
First step was to download and compile CyanogenMod for my phone. I'm not going to go into this in any detail on this because there's already a good overview on the CyanogenMod wiki that covers the entire process, as well as a customized guide for the Nexus 5. If those articles are followed correctly, you should be able to build a fresh CyanogenMod image with:
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Enabling the kernel module
Once the image was flashed to the phone and verified to be working, the next step was to enable the xpad kernel module.
Copy current kernel config to the kernel directory for editing:
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Edit the config:
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At this point, a menu will come up. Search for xpad (type /xpad), and take note of the paths.
ESC out of the search, navigate the tree and enable the modules found with the search. Save and
exit.
Copy the config back and clean up any stray *.o files:
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After enabling the module, rebuild CyanogenMod using the normal brunch hammerhead and flash it to
the phone.
Submitting the change upstream
Since Xbox controller support had already been requested in a JIRA ticket(dead link), I went
through the process to submit the change on the CyanogenMod Gerrit instance for inclusion in the
next release. There's a guide for this here. As well as the guide, I found the people in
#cyanogenmod-dev on Freenode to be very helpful.
A short while after submitting the change to Gerrit, it was merged into the cm-11.0 branch as
commit i7ef4f6a and pushed out in the next nightly build (changelog screenshot).
Wrap-up
It feels great to be able to contribute to an amazing open source project that had at least a million people using it at one point or another. I recognize that the change by itself was just changing a config file to enable functionality that already existed, but it's helped me get through some of the non-coding hurdles of contributing to an Android fork like CyanogenMod (adb, fastboot, unlocking and rooting phones, compiling AOSP, flashing images, the contribution process, etc). This time the actual contribution itself was minor, but it's gotten me more familier with the process, hopefully making it easier to contribute something more substantial in the future.
In the end, I learned a lot about Android development and how all the different pieces that I had read about at one point or another actually fit together. During the whole process I also lost a lot of the fear I had about messing with phones and other more expensive locked down devices. Going forward, I'm going to attempt to compile and flash open source operating systems like CyanogenMod onto more devices that I own. I like being able to tweak things and feel like I really own them, hardware and software.