Friday, February 27, 2009

Fainting and the Linux audio stack, or Know Your Enemy

I've been working off and on to draft a Linux audio overview in the vein of Bryce's excellent X Window System architecture docs. Lennart, the brains behind Avahi and PulseAudio, already has a quite-good writeup on the different APIs with well thought-out reasons for choosing whichever one(s) for a task. Given that I've been knee-deep in triaging audio bugs, it has been amusing to watch various (developer) parties argue the merits of their wheels.

Nonetheless, in Linux, it's still a jungle [PNG].

Any user, power or novice, generally approaches bugs in an identical fashion: stuff needs to work now. Without further ado, here is part one of rationale behind guidelines for troubleshooting audio anomalies in Ubuntu (some of which is documented).

My sound is broken. Well, not quite. Generally when one's sound is inaudible, it can be caused by numerous culprits. Prior to Intrepid, one's user had to be in the "audio" group to use sound devices. PulseAudio's use of PolicyKit obviates that requirement, so one's user no longer has to be in that group. However, users experimenting with multiple desktop environments like Xfce, KDE, and GNOME - some of which don't use PulseAudio with a PolicyKit backend - need to be in that group. What am I driving at?
  • The user has to know too much about traditional discretionary access control.
Thanks to PulseAudio's integration with PolicyKit - and here, classic abstraction at work - we no longer have to concern ourselves with questions regarding incorrect ownership and/or permissions on /dev/dsp*, /dev/snd/*, etc. Yet that is a common culprit for inaudible applications that we catch repeatedly using a community-maintained troubleshooting script.

Next time, we look at how Linux speaks to our audio hardware.

Wednesday, February 25, 2009


If it seems fashionable to post about PulseAudio experiences, I'll refrain from spouting my own mishaps and instead focus on fixing and maintaining the audio stack in Ubuntu (a rather thankless job in any Linux distribution).

Firstly, people claim that PulseAudio brings a lot of fail to the Linux desktop environment. As with many maturing architectures, there are massive growing pains. David F is not the first to note that Jaunty's packages have thrown many desktop users for a loop. It should come as no surprise that there is more breakage on the horizon, and in fact, I documented it twice to, well, near silence on the ubuntu-devel-discuss mailing list. Much more importantly, however, PulseAudio has been instrumental in exposing numerous alsa-kernel (which in Ubuntu affects the linux source package, not the alsa-driver source package) and alsa-lib bugs. We all owe Takashi, Jaroslav, Lennart, and myriad others a round of $beverage for making things bearable^Waudible.

I'll take this opportunity to state again [ Impress file] (as during my 2008 Ohio LinuxFest presentation) that Linux audio can be hairy, and if it has worked for us directly via ALSA, OSS (including v4.x), etc., we should be pleased but mindful that there is always room for improvement.

Secondly, Jaunty is in development. While it is reasonable to expect that filesystem-munching bugs don't hit us daily, it is also reasonable to expect that running a development branch brings experimental configurations that could munch our filesystems. The PulseAudio configuration currently in Jaunty was delivered precisely to trigger alsa-kernel and alsa-lib bugs so that they can be fixed. Surely we don't want another instance of Hardy's PulseAudio configuration to bite us!

Finally, it is much more productive to Ubuntu developers and maintainers for the Jaunty testing community to file bugs. In the case of Jaunty's PulseAudio, there already exists one "master" bug tracking the most common complaints of:
  • my sound is inaudible on GNOME session login
  • my sound disappears randomly while using GNOME
  • music and/or video applications seem to hang or crash while using GNOME
Rest assured that we are working diligently to resolve the bug (which affects linux, alsa-lib, and pulseaudio).