On 7/23/25 13:34, Demi Marie Obenour wrote:
I'm seriously wondering if Spectrum should switch from mdevd to udev. It's where all of the upstream device detection work happens, and it would avoid needing to statically configure sound devices for PipeWire.
The current PipeWire patch statically creates devices. This works for VMs because VMs are statically configured and only have virtio devices. This was also the most challenging part of the effort, and I still had to guess some of the parameters. Supporting hotplug would require either a custom PipeWire module, a custom daemon that talked to PipeWire, keeping pw-cli running in interactive mode, or patches to PipeWire to make device lingering work.
In the audio case, ALSA expects device paths of the form hw:card:device:subdevice. However, I don't believe card numbers are stable: they reflect the order in which the kernel enumerates devices, and that is not at all guaranteed. A VM that only has a virtio sound device will only ever have card 0, device 0, and subdevice 0, so hard-coding hw:0,0,0 is fine. This is not true if physical hardware is present. That requires a lot of device identification logic, and all of the upstream work on that is in udev or the systemd hardware database (which udev also uses).
In the future, there may be other projects Spectrum wants to use that rely on udev for enumerating devices. Some of them might *only* support PipeWire and have no other means of configuration. Even if other means of configuration *exist*, I expect them to be very rarely used by upstream and thus poorly tested.
tl;dr: you can get away without udev in VMs, but I expect problems with physical hardware, and I want to switch to udev as a preemptive measure to avoid those problems and to ease development.
From https://wiki.gentoo.org/wiki/Mdev: - udev is needed to load firmware for AMD Radeon GPUs. - libusb will not see USB devices unless deprecated kernel configuration options are set. From https://wayland.freedesktop.org/libinput/doc/latest/architecture.html: - Wayland compositors use udev (via libinput) to support device hotplugging. This is needed for USB keyboard and/or mouse support, which is a hard requirement for desktops that don't have PS/2 connectors (almost all of them I believe). - libinput relies on udev to determine what kind of device an input device is. FreeBSD also switched to udev for Mutter. https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271824#c4 states that for Wayland, udev is essentially required. Even Gentoo uses udev, though systemd is not the default init system. I think avoiding udev is a losing battle in the long run, except possibly for VMs that will never ever have physical hardware attached to them. That would require configuring VMs with and without physical hardware separately, which I would strongly prefer to avoid. If the memory consumption or disk space requirements of udev are excessive, I think this would be better addressed as part of a broader debloating effort. libudev-zero does exist, but it doesn't work with PipeWire (https://github.com/illiliti/libudev-zero/issues/26, still unfixed despite being closed) and there are other problems with it as shown by its issue tracker. Alpine's wiki doesn't recommend it for desktop environments, and while Spectrum VMs don't run full desktop environments they do run desktop software. Such software requires udev more and more nowadays. -- Sincerely, Demi Marie Obenour (she/her/hers)