I find tracking down obscure bugs interesting, so I thought I would share another tale of one I just tracked down.

Ever since coming back from the holiday break this year, I found my bluetooth headsets had stopped working. (I have 2 very different models: a plantronics M50 that I have had for a long time, and a Bose QuietComfort 35 (yes, the headphones have a headset profile!)).

At first when I noticed the problem I tried the obvious things:

  • Downgraded pulseaudio
  • Downgraded bluez
  • Downgraded and tried various kernels.
  • Looked though tons of upstream pulseaudio and bluez bugs.

No change at all. I added myself to a existing bluez bug and provided the bluez maintainer a bunch of debugging output, but it all looked pretty normal, there was just no sound. So I gave up and moved on to more urgent things.

Fast forward to today: what better way to spend a sunday afternoon that more debugging. I repeated my former tests and had the same result. Then I decided to try some LiveUSB’s to isolate it. I downloaded and booted a F25 updated workstation iso and it still had the issue. So, I thought, Ah ha it has to be the updated pulseaudio since that landed in f25 updates. But to be sure, I downloaded and booted the stock f25 release iso. Still had the issue. Boggling. Then I went looking around in the kernel bugzilla and found a report that talked about issues with bluetooth and firmware. That was just the hint I was looking for. So, I downgraded the linux-firmware package and tried things and… it still didn’t work! There was one more trick: The firmware only reloads on cold boot. If it’s already loaded (or misloaded) it doesn’t even try again. So, cold boot and then everthing started working.

The key kernel boot message on the failing firmware: Bluetooth: hci0: Failed to send Intel_Write_DDC (-22) and the working firmware: Bluetooth: hci0: Applying Intel DDC parameters completed

For bonus points of course the failing firmware file is: ibt-11-5.ddc and the working one is: ibt-11-5.ddc (yes, thats right, they are changing the file around without changing the version any).

Next of course is to figure out where to report this to get it fixed, but some poking around and testing found that http://git.kernel.org/cgit/linux/kernel/git/firmware/linux-firmware.git/commit/?id=581f24500138f5e410d51ab63b205be9a52f4c77 already had a fix. I just need to wait for a update to the linux-firmware in Fedora and everything should be back to normal.

So, troubleshooting lessons for today: Sometimes cold boot matters. When you think you have downgraded everything that could cause a problem, remember firmware. Reading around on bug reports will sometimes give you ideas on how to solve yours.