[systemd-devel] Preventing automatic seat assignments

[David Herrmann]

Hi

On Thu, Sep 11, 2014 at 9:16 AM, Tanu Kaskinen
<tanu.kaskinen at linux.intel.com> wrote:
> On Wed, 2014-09-10 at 12:44 +0200, David Herrmann wrote:
>> Hi
>>
>> On Tue, Sep 9, 2014 at 10:49 AM, Tanu Kaskinen
>> <tanu.kaskinen at linux.intel.com> wrote:
>> > On Wed, 2014-08-27 at 11:47 +0300, Tanu Kaskinen wrote:
>> >> On Tue, 2014-08-26 at 14:00 +0200, Lennart Poettering wrote:
>> >> > On Tue, 26.08.14 12:17, Tanu Kaskinen (tanu.kaskinen at linux.intel.com) wrote:
>> >> >
>> >> > > Hi,
>> >> > >
>> >> > > If I want to designate some sound card to be shared between seats,
>> >> > > then I suppose that sound card shouldn't be assigned to any seats.
>> >> > > However, currently /usr/lib/udev/rules.d/71-seat.rules
>> >> > > unconditionally tags all sound cards with the "seat" tag. How should
>> >> > > this be solved?
>> >> >
>> >> > What's the rationale here actually? PA doesn't really support "sharing"
>> >> > sound cards between multiple seats.
>> >> >
>> >> > I mean, If this is something generally useful we can see if we can
>> >> > support that in the default rules, but I am not seeing it?
>> >>
>> >> The use case is a car audio system. There are multiple seats, and each
>> >> seat can have dedicated audio hardware (e.g. headphones), but there's
>> >> also the speaker system that is shared by all seats.
>> >>
>> >> It's true that PA needs modifications too to support this. We haven't
>> >> yet decided how to implement this, but probably we will run PA in system
>> >> mode for the shared devices only, and user instances for the per-seat
>> >> hardware. The user instances will use the tunnel module to connect to
>> >> the hardware that is managed by the system instance.
>> >
>> > Ping?
>>
>> Sorry, a lot of people are on vacation..
>>
>> I think the right solution for this is to support "TAGS-=seat" in
>> udev. That is, the automatic seat-assignment will still be applied,
>> but you can revert it in your own udev rules. By dropping the "seat"
>> tag, logind will not treat it as 'seated' device.
>
> Thanks for the reply. The way you put it sounds like udev doesn't
> currently support "TAGS-=seat". Is that correct? If so, maybe I'll look
> into making a patch at some point, but no promises.

Exactly, it's currently not supported. I wrote a patch and sent it to the ML:
  [PATCH RFC] udev: allow removing tags via TAG-="foobar"

Maybe you can have a look whether this solves your issues.

Thanks
David