LXC container and USB audio dongle

Piotr_Mackowiak · September 23, 2020, 4:33pm

I have an LXC container configured, which contains a Debian Buster 18. The container runs on a QNAP NAS (Intel 64 bit 8 GB RAM). The QNAP firmware is of course based on Linux.
I plugged a USB audio dongle to the USB port, which I would like to be a sound card for Debian installed in a container.
Unfortunately, neither ALSA nor PulseAudio can see this card, although the USB subsystem recognizes the dongle correctly. Below appropriate outputs. They are identical both when run from the NAS itself and from a container.

root@Debian:~# cat /proc/asound/cards 
 0 [PCH            ]: HDA-Intel - HDA Intel PCH HDA Intel PCH at 0x81810000 irq 130
 1 [Device         ]: USB-Audio - USB Audio Device GeneralPlus USB Audio Device at usb-0000:00:14.0-1, full speed
root@Debian:~# aplay -L
null
    Discard all samples (playback) or generate zero samples (capture)
jack
    JACK Audio Connection Kit
pulse
    PulseAudio Sound Server

What can I do to make USB audio dongle available for Debian in container?

simos · September 23, 2020, 4:40pm

ALSA is the audio subsystem in the Linux kernel. If you want to use this sound card, then the host’s Linux kernel (i.e. ALSA) should be able to make sense of it.

Can you run the following and post the URL here?
https://alsa-project.org/wiki/AlsaInfo

Piotr_Mackowiak · September 23, 2020, 5:04pm

I think its supported on host Linux (QTS):

[~] # lsmod 2>&1|grep snd
snd_usb_caiaq          53248  0 
snd_usb_audio         188416  2 
snd_usbmidi_lib        28672  1 snd_usb_audio
snd_seq_midi           16384  0 
snd_rawmidi            32768  3 snd_usb_caiaq,snd_usbmidi_lib,snd_seq_midi
[~] # aplay -l
**** List of PLAYBACK Hardware Devices ****
ALSA lib control.c:953:(snd_ctl_open_noupdate) Invalid CTL hw:0
aplay: device_list:277: control open (0): No such file or directory
ALSA lib control.c:953:(snd_ctl_open_noupdate) Invalid CTL hw:1
aplay: device_list:277: control open (1): No such file or directory

UPDATE
Using FFMPEG on host:

# ffmpeg -sources alsa 
Auto-detected sources for alsa:
  null [Discard all samples (playback) or generate zero samples (capture)]
  default:CARD=PCH [Default Audio Device]
  sysdefault:CARD=PCH [Default Audio Device]
  front:CARD=PCH,DEV=0 [Front speakers]
  default:CARD=Device [Default Audio Device]
  sysdefault:CARD=Device [Default Audio Device]
  front:CARD=Device,DEV=0 [Front speakers]
  surround21:CARD=Device,DEV=0 [2.1 Surround output to Front and Subwoofer speakers]
  surround40:CARD=Device,DEV=0 [4.0 Surround output to Front and Rear speakers]
  surround41:CARD=Device,DEV=0 [4.1 Surround output to Front, Rear and Subwoofer speakers]
  surround50:CARD=Device,DEV=0 [5.0 Surround output to Front, Center and Rear speakers]
  surround51:CARD=Device,DEV=0 [5.1 Surround output to Front, Center, Rear and Subwoofer speakers]
  surround71:CARD=Device,DEV=0 [7.1 Surround output to Front, Center, Side, Rear and Woofer speakers]
  iec958:CARD=Device,DEV=0 [IEC958 (S/PDIF) Digital Audio Output]

So, what the problem is?

simos · September 23, 2020, 5:31pm

There is no PulseAudio on the host (right?), therefore, you cannot share the PulseAudio socket to the container as we do with the GUI containers.

Instead, you can pass the USB device to the container, and figure out from the container on how to play audio. See https://linuxcontainers.org/lxd/docs/master/instances#type-usb for the usb LXD device. This only works if

you disable ALSA on the host
the USB audio card uses libusb
you use some userspace audio library like libmaru.

Piotr_Mackowiak · September 23, 2020, 5:42pm

I can not disable ALSA on host. It is always used by HDstation (native app from QNAP) for play sound via HDMI on TV. So as I can see, the only solution is to run my application (streaming radio station from internet) on host, not in container

simos · September 23, 2020, 6:12pm

So the device uses plain ALSA (ALSA + ALSA libraries) to play audio? If that is the case, you can try to share the relevant audio Unix character devices to the container, per https://linuxcontainers.org/lxd/docs/master/instances#type-unix-char

Piotr_Mackowiak · September 24, 2020, 4:14am

I not really sure that ALSA libs are complete on QNAP. Probably not. But I had similar problems with other libraries. Copying them from Debian has always helped.
Nevertheless ALSA itself works on host:

[~] # cat /proc/asound/version 
Advanced Linux Sound Architecture Driver Version k4.14.24-qnap.

Pulseaudio probably too. But it looks like an absence of lib module:

[/etc] # aplay  -L
ALSA lib control.c:953:(snd_ctl_open_noupdate) Invalid CTL hw:0
[/etc] # aplay  -l
**** List of PLAYBACK Hardware Devices ****
ALSA lib control.c:953:(snd_ctl_open_noupdate) Invalid CTL hw:0
aplay: device_list:277: control open (0): No such file or directory
ALSA lib control.c:953:(snd_ctl_open_noupdate) Invalid CTL hw:1
aplay: device_list:277: control open (1): No such file or directory

simos · September 24, 2020, 5:55am

Then this is primarily a QNAP question. As soon as you find how audio works on a QNAP, we can figure out how to pass the device into the container.

I mentioned earlier about alsa-info. This should give you a better insight.
If PulseAudio is working, you can log in into that account that can use PulseAudio and run pactl info.