[alsa-devel] multiseat and alsa audio systems (usb headsets and usb audio adapters)

Jelle de Jong jelledejong at powercraft.nl
Thu Oct 23 09:43:34 CEST 2008

Travis Place wrote:
> On Thursday 23 October 2008 6:48:18 am Sean McNamara wrote:
>> Hi,
>> On Wed, Oct 22, 2008 at 4:37 PM, Jelle de Jong
>> <jelledejong at powercraft.nl> wrote:
>>> Sean McNamara wrote:
>>>> On Wed, Oct 22, 2008 at 1:48 PM, Jelle de Jong
>>>> <jelledejong at powercraft.nl> wrote:
>>>>> Hello everybody,
>>>>> I have a four to eighth muliseat debian linux system here and it is
>>>>> missing sound for the users. I was hoping the alsa team can help me
>>>>> with this issue.
>>>> All sound issues on Linux are extremely application-specific. Getting
>>>> sound to work without any specific application in mind is asking for
>>>> trouble, because there is no single configuration of the sound stack
>>>> (which, if you expand into historical sound APIs and servers, contains
>>>> about 20 different ways to play sound) that will satisfy any
>>>> application without further configuration.
>>>> Since this is not a really well-formed question, I will proceed with
>>>> the assumption that you want to be able to run, for example,
>>>> aplay /usr/share/sounds/pop.wav
>>>> in the console, and have it produce sound in the "appropriate"
>>>> headset. Here's how you proceed:
>>>> 1. Each person on the multiseat box must have their own UNIX user
>>>> account. 2. Each user account must have an ~/.asoundrc file.
>>>> 3. Each ~/.asoundrc file must redefine the "default" device. How you
>>>> redefine it depends on a lot of things:
>>>> (a) Do you want _direct_ hardware access to the sound card, i.e. one
>>>> application at a time?
>>>> (b) Do you want to use ALSA's built-in software mixing, i.e. dmix?
>>>> (c) How many channels?
>>>> (d) Do you want to use PulseAudio?
>>>> (e) Do you want to use JACK?
>>>> (f) For each USB headset, what is its corresponding "raw device
>>>> string"? This will be something like hw:n, where n is a number
>>>> starting from 0, depending on the order in which the USB drivers
>>>> initialize each USB controller.
>>>> All of the above questions, and others, will determine exactly how
>>>> each user configures their ~/.asoundrc file.
>>>> The *extremely naive* (not recommended) example of an ~/.asoundrc is
>>>> like:
>>>> pcm.!default {
>>>> card 3
>>>> }
> Would this not be:
> pcm.!default {
> type hw
> card 3
> }
> Also, depending on the order in which they are plugged in, depends on what 
> card number they get assigned. If all USB headsets are the same, you run into 
> more problems telling them apart.
> For example, if you plug them in a some random order, card3 might infact be 
> going to the 5th seat of the system..
> You will need some way to uniquely identify EACH headset, and force its slot 
> or index parameter to the required number.

OK here we go, every usb device has an unique ID in his usb tree, this
is the benefit of using usb devices. I am going to provide an example
how this works with identical usb mouse and keyboards.

ls -hal /dev/input/by-path/
pci-0000:00:13.5-usb-0:10.1:1.0-event-kbd -> ../event13
pci-0000:00:13.5-usb-0:10.2:1.0-event-mouse -> ../event15
pci-0000:00:13.5-usb-0:3.1:1.0-event-kbd -> ../event4
pci-0000:00:13.5-usb-0:3.2:1.0-event-mouse -> ../event6
pci-0000:00:13.5-usb-0:4.1:1.0-event-kbd -> ../event7
pci-0000:00:13.5-usb-0:4.2:1.0-event-mouse -> ../event9

and then we can attached each input device to an specific task:
Driver "evdev"
Option "Path" "/././by-path/pci-0000:00:13.5-usb-0:10.1:1.0-event-kbd"

current system:
cat /proc/asound/card

needed wanted requested system:

and then in the users,


pcm.!default {
   type hw
   card-bypath "/.../.../by-path/pci-0000:00:13.5-usb-0:4.3:1.0-card"

So hereby my feature request, is this possible and well achievable, what
would it take?

Best regards,


>>>> to have ALSA clients play sound, by default, to the fourth sound
>>>> device that happened to be initialized.
>>>>> I have usb headsets and usb audio devices and all users have there own
>>>>> usb hub. There are no internal audio cards in the multiseat server.
>>>>> How can we configure an system that the user can listen to audio only
>>>>> to the devices connected to his usb hub.
>>>> Well, this is guaranteed by design, isn't it? Without physical access
>>>> to someone else's headset, how would I get access to the audio streams
>>>> being played to that headset? I don't think this really expresses what
>>>> your problem is.
>>>> If we flip your question around, it might be a little more
>>>> interesting: "How can we configure a system so that each user's
>>>> applications will only play audio to that user's corresponding
>>>> headset?"
>>>> You can interpret this question in two ways:
>>>> 1. "How can we _prevent_ users from playing sound to each other's
>>>> sound cards?" [mutually untrust[ed/ing] users]
>>>> or
>>>> 2. "How can we configure the default settings so that, if untampered
>>>> with, a user's applications will play sound to that user's own headset
>>>> and not anyone else's?" [mutually trust[ed/ing] users]
>>>> The lack of hardware mixing actually comes back to being a security
>>>> benefit to user space control in this kind of situation. Recall that
>>>> if I run
>>>> aplay --device=hw:0 /usr/share/sounds/pop.wav
> Couldn't we allow dmix, but just append to the users UID to the ipc key, with:
> ipc_key 123456
> ipc_key_add_uid true
> in the dmix config stanza in .asoundrc for each user ?
>>>> then as long as the aplay program is playing sound to hw:0, it is
>>>> *impossible* (short of some really devious hacking in alsa-lib or the
>>>> kernel) for another application -- from this user, or another -- to
>>>> also play sound to hw:0 at the same time. I'll call this claim "No
>>>> Native SW Mixing".
>>>> Now, let's look at what is required to satisfy the question in each of
>>>> the two above cases.
>>>> Trusted:
>>>> 1. We know that users aren't going to maliciously try and hurt one
>>>> another's ears by setting someone else's mixer really loud and playing
>>>> static to them.
>>>> 2. We know that users won't try to tamper with eachother's processes
>>>> or configuration files; once things are configured correctly, it'll
>>>> stay that way.
>>>> 3. By 'No Native SW Mixing', we know that users can *theoretically*
>>>> hog another user's sound device whenever that user has zero
>>>> applications currently using it.
>>>> 4. Since users trust each other, they won't do this.
>>>> 5. We can use something like dmix, or just use hw:0 if users don't
>>>> need simultaneous app output, and things will be fine.
>>>> 6. dmix is a viable option to implement software mixing.
>>>> 7. PulseAudio is also a viable option to implement software mixing.
>>>> Untrusted:
>>>> 1. Users may want to maliciously try and hurt one another's ears by
>>>> setting someone else's mixer really loud and playing static to them.
>>>> 2. Users may try to tamper with eachother's processes or configuration
>>>> files; once things are configured correctly, it could get messed up
>>>> again. [There's not much you can do to prevent this ... at least not
>>>> based on existing open source tools.]
>>>> 3. By 'No Native SW Mixing', we know that users can *theoretically*
>>>> hog another user's sound device whenever that user has zero
>>>> applications currently using it.
>>>> 4. Since users do not trust each other, there is potential for 3 to
>>>> happen. 5. We must use 'No Native SW Mixing' to our advantage, to create
>>>> a walled garden around each user's sound experience. We need a process
>>>> that, once started by a user, will permanently "hog" the sound card that
>>>> user wants to use. This process should, ideally, allow other processes
>>>> run by that user (and by that user only) to access their USB headset for
>>>> output or input. We need a "gatekeeper" that is sensitive to the context
>>>> in which an app is being run.
>>>> 6. dmix is a NOT viable option to implement software mixing, because:
>>>> (a) Once all dmix applications close their streams, the audio device
>>>> hw:n is available, and some malicious user can decide to acquire it,
>>>> abuse it, and prevent its rightful user from accessing it with his own
>>>> applications.
>>>> (b) dmix does not restrict users from outputting sound based on their
>>>> UID or session. It is NOT a gatekeeper, just an open gate.
>>>> 7. PulseAudio is [perhaps the best] viable solution to implement
>>>> software mixing for each respective user. It is also a viable solution
>>>> to preventing users from taking control of one another's sound
>>>> devices.
>>>> Configuring pulseaudio for a multi-soundcard multi-user system is out
>>>> of the scope of this mailing list, but suffice it to say that it can
>>>> be done. You will want to run one PulseAudio daemon per user, and each
>>>> daemon will hold exclusive control over exactly one USB headset. Each
>>>> daemon will only accept clients' requests for audio I/O from the user
>>>> who started the daemon. Do make sure that you disable
>>>> module-suspend-on-idle, or PulseAudio will actually give up the sound
>>>> device after a period of inactivity, making it insecure in an
>>>> untrusted environment.
>>>> BTW, if this project is for a commercial interest, you usually pay
>>>> someone for this kind of in-depth analysis. If you're that someone,
>>>> you might want to read up on some of the underlying technologies so
>>>> that you can develop this sort of reasoning on your own. I can
>>>> envision a very real situation for both the trusted and the untrusted
>>>> environment, so there is definitely a demand for this kind of
>>>> specialist. Trusted environments are popular at workplaces, and
>>>> untrusted multi-seat boxes are popular at computer labs, public
>>>> libraries, etc.
>>>>> Any help is really appreciated.
>>>> Please let me know if this information was helpful -- it will help me
>>>> gauge, in turn, whether I should spend my time writing up these sorts
>>>> of answers on the ML. FWIW, I searched around for existing articles
>>>> touching this subject, but I couldn't find anyone who addresses this
>>>> specific issue accurately or in enough detail.
>>>> Sean
>>>> P.S. - I once spoke to a user on IRC who was having intermittent
>>>> problems with multiple USB headsets. It turns out that the USB
>>>> controllers in the computer didn't have enough bandwidth, or power (or
>>>> both) to allow all of the headsets to play at once. If, after
>>>> configuring, you run into issues where only so many users can play
>>>> sound at once, then you should invest in additional PCI or
>>>> (preferably) PCI-Express USB controllers. Note that adding hubs
>>>> upstream of a USB controller does absolutely nothing to lessen the
>>>> load on the USB controllers that are integrated into your motherboard.
>>>>> Thank in advance,
>>>>> Jelle
>>> Thank you Sean, this is by far the most extensive response i received in
>>> months with an "open" question on an mailinglist, this really is a good
>>> character, that I will reward if you sent an paypal address :-p.
>> Ha ha, I appreciate the offer :) Yes, it was a very open question;
>> that's why I didn't want to assume too much about your knowledge, and
>> to answer you with something that would point you in the right
>> direction if you were, in fact, ignorant of the basic situation here.
>>> I have extensive experience configuring alsa and usb headset, little new
>>> info was delivered for me but i think others will really like your
>>> response too.
>>> My main issue is how to start alsa when there are no audio cards on the
>>> system using debian sid? so i can use bluetooth or hotplug usb audio
>>> devices...anybody?
>> A "quick hack" would be to add 'snd' and 'soundcore' (and maybe other)
>> kernel modules to /etc/modules. That way, on boot, ALSA will be in the
>> kernel even if the initial probing routines don't find any sound
>> hardware. Then, as long as your bluetooth and USB modules are loading
>> correctly, the hotplug support should take over from there if your
>> ALSA is recent enough.
>>> Then I think it will become an udev issue to regulate a fixed group and
>>> permissions for usb audio devices. I was kind of hoping somebody had
>>> done this kind of udev ruling for usb audio devices and can provide some
>>> examples this will save me time...anybody?
>> Yes, definitely a udev issue if you want to restrict things at the
>> ALSA level. Otherwise the permissions on the raw character devices
>> /dev/snd/pcm* will be liberal by default.
>> But if you're willing to move up into application space, I'm pretty
>> sure one of PulseAudio's advertised features is to handle just this
>> kind of ruling among different users and/or groups. You could
>> certainly set this up so that the PulseAudio daemons for all the users
>> are launched at boot time so that all of the audio devices are
>> [permanently] "hogged" by their appropriate users before anyone has a
>> chance to log in.
>> And of course, PulseAudio is excellent with hotplug nowadays.
>>> PS. I have also noticed usb audio problems (also posted them to irc
>>> maybe 1 year ago) in the past with usb hubs and other kinds of things,
>>> these things can brake an design or make it:-p
>> Yes, the Win32 device manager has a nice little applet that shows the
>> current bandwidth usage/availability for USB controllers... I'm not
>> sure how accurate it is... but having something like this for Linux
>> would be immensely useful as you are setting up the topology of your
>> USB headsets. It would be nice not to have to empirically try and
>> break the sound by playing all of the headsets at once...
>>> So thank you very much for this great response, and keep up doing this
>>> kind of work.
>> Thanks for the encouragement!
>> Sean
>>> Kind regards,
>>> Jelle

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