How to use persistent ALSA devices?

Question

I followed this guide to set the default device like this:

create /etc/asound.conf with following:

defaults.pcm.card 1
defaults.ctl.card 1

But I have the problem that the ALSA card numbers are not deterministic across reboots.

So I followed this guide to change the name of my USB sound cards to something predictable. Essentially it says:

Create a file in /lib/udev/rules.d/ with a name like 85-my-usb-audio.rules and contents like the following:

DEVPATH=="/devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card?", ATTR{id}="MyDev_A"

Using the devices Use the ID string where you would otherwise use the card number, such as

aplay -D default:MyDev_A something.wav

And then I changed my /etc/asound.conf to:

defaults.pcm.card MyDev_A
defaults.ctl.card MyDev_A

I also tried

defaults.pcm.card "MyDev_A"
defaults.ctl.card "MyDev_A"

But when running for example aplay, I get errors:

ALSA lib conf.c:1207:(parse_value) card is not a string
ALSA lib conf.c:1887:(_snd_config_load_with_include) _toplevel_:2:0:Invalid argument
ALSA lib conf.c:3650:(config_file_open) /etc/asound.conf may be old or corrupted: consider to remove or fix it
ALSA lib conf.c:3572:(snd_config_hooks_call) function snd_config_hook_load returned error: Invalid argument
ALSA lib conf.c:4026:(snd_config_update_r) hooks failed, removing configuration
aplay: device_list:281: control open (0): Invalid argument

Answer 1

This is not a direct solution for the problem, but it's the way I went forward. Instead of assigning textual IDs, I set a fixed order of indexes for the sound cards:

I created a file /etc/modprobe.d/alsa.conf and put into it:

blacklist snd_bcm2835
blacklist snd_soc_hdmi_codec
options snd-usb-audio index=0,1 vid=0x1415,0x0d8c pid=0x2000,0x000c

The options line sets the indexes of all sound cards handled by the snd_usb_audio module.

Those indexes have to be consecutive starting at 0 and I couldn't figure out how options snd slots and indexes work together, so I just disabled all the other two audio modules with the blacklist lines so they don't take up any indexes.