We have a system which has the HDMI audio capability internally,
but pulseaudio is not giving it a very high priority compared
to e.g. USB sound cards.
The sound device appears on the platform bus and it is not
currently tagged with any form factor information.
It seems safe to assume that any sound card that is directly on the
platform bus is of internal form factor, but we must be careful because
udev rules will match all parent devices, not just the immediate parent,
and you will frequently encounter setups such as:
Platform bus -> USB host controller -> USB sound card
In that case, SUBSYSTEMS==platform would match even though we're
clearly working with an external USB sound card.
In order to detect true platform devices here, we rely on the observation
that if any parent devices of the sound card are PCI, USB or firewire
devices, then this sound card cannot directly connected to the platform
bus. Otherwise, if we find a parent device on the platform bus, we assume
this is an internal sound card connected directly to the platform bus.
USB and PCI soundcards have a nice set of ID_* properties. It would
be handy for firewire soundcards to have the same.
Note that this removes the explicit setting of ID_ID in the firewire
conditional. Because we are now setting ID_SERIAL, ID_ID will come
from later in the file.
The ALSA id sysattr is generated by the sound subsystem and is not
a stable identifier. It is generated though some string manipulation
then made unique if there is a conflict. This means that it is
enumeration-dependent and shouldn't be used for ID_ID.
If ID_ID is supposed to be system-unique, it is not already since
for firewire it is generated from the guid and there are broken
firewire devices that have duplicate guids across devices.
This is tracked for PulseAudio at
https://bugs.freedesktop.org/show_bug.cgi?id=90129.
This is essentially a revert of systemd
ed1b2d9fc7.
Daniel Drake