Installing a Release Candidate Kernel in Arch Linux

I recently received a shiny new Thinkpad for my shiny new job. I had the option to select what I wanted from Lenovo (or Apple, but I dislike MacOS) and chose a machine that wouldn't have the same issues my other workstation had. I made it a point to avoid Intel chipsets (due to webcam issues) and stayed far away from discrete Nvidia graphics. I settled on a nice AMD Ryzen-based system. It had a very new Qualcomm radio, but I saw people online saying that Wi-Fi and Bluetooth worked fine. I took the plunge and selected that as my workstation.

It arrived, I popped in my live USB, and a few hours later, I had a fully provisioned Arch Linux system. Wi-Fi worked fine, and I was able to scan for Bluetooth devices. Success! Or was it?

The next morning, I sat down at my desk, clocked in, and grabbed my Bluetooth headphones to listen to some morning tunes. I initiated pairing from the headphones, selected them from my laptop's device list, and... nothing. It timed out. I tried again, and again, and again, messing with timeouts and using bluetoothctl to manually negotiate things, but no luck.

I continued my digging. First, I validated that the computer could not pair with another set of Bluetooth headphones. This let me isolate the problem to the system. I then started comparing the list of Bluetooth-related packages against a computer with which the headphones worked — no differences. I did the same with configs; no differences again. Curious. Worryingly, this was starting to look like a hardware support issue.

I pulled up my Thinkpad's spec sheet and saw it uses a Qualcomm NCM825 WLAN + BT chip. Some strategic Googling (pro-tip: use double quotes to ensure your search matches specific terms) revealed other Linux users across various distributions and hardware configurations having similar issues with this chip. While sad to see my hypothesis reinforced, it was reassuring that it wasn't just me. This meant it was more likely to be solved!

I continued my search and stumbled upon this post in the Framework computer forum, where someone had installed it in their computer. They, too, had issues connecting audio devices via Bluetooth. However, the author was knowledgeable enough to submit a patch to the Linux kernel's Bluetooth subsystem that allowlisted the chip for certain wideband audio capabilities. The patch was approved and merged. Exciting! Unfortunately, it was only in the bluetooth-next repository, meaning it was not in a stable kernel.

When would it be out? I pulled the Git history of the latest kernel (6.13-rc3) and was pleased to see the commit was included. Unfortunately, the 6.13 branch is not scheduled to be stable until late January 2025. I didn't want to wait that long to validate this was the issue.

Time to install a new kernel.

Arch doesn't make release candidate kernels (referred to as mainline) available for installation via Pacman (likely for good reason). Instead, one can try building it themselves (which takes a long time and can be finicky to get right for my distribution), installing an AUR-maintained version (which would also take a long time), or... installing someone else's prebuilt kernel (which requires trusting an internet rando). Given the feedback I saw regarding the prebuilt kernels, I felt okay moving forward with that.

To make those available, open /etc/pacman.conf and add a section:

[miffe]
SigLevel = Optional TrustAll
Server = https://arch.miffe.org/$arch/

I then saved it and updated my package databases (pacman -Syy), accepting the prompt to import a new PGP key. I then installed the release candidate kernel (pacman -S linux-mainline linux-mainline-headers) and rebooted into it. I placed my headphones in pairing mode, selected them from the device list, and... success! They paired, and sure enough, I was able to play some music.

Now, I could have stopped there and run a pre-release kernel for a month until a stable version was released, but this is my work system. I'd prefer to avoid bleeding-edge instability where possible. Since the problem appeared to be with pairing, now that the headphones were paired with the system, I wondered if I could just boot into my old kernel. Nothing to lose. I rebooted into the old version, hit connect, opened Tidal, hit play, and was greeted with my music. EDIT (2024-12-20): This is incorrect. It still suffered from the same issue, so I needed to remain on the RC kernel.

Moral of the story: never give up.