[Bug 1921452] [NEW] [SRU] alsa-lib: conf: USB - add "Cmedia Audio" to USB-Audio.pcm.iec958_device

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You have been subscribed to a public bug by Hui Wang (hui.wang):

[Impact]
On Cmedia Audio, unusable SPDIF can be selected as output from PulseAudio.

[Fix]
Disable IEC958 (SPDIF) through ALSA UCM.

[Test]
With the UCM applied, `pactl` and audio panel in gnome-control-center no longer have SPDIF option.

[Where problems will occur]
"Cmedia Audio" is the sound card name of this usb audio dongle, and this string is got from usb string descriptor, that is to say the "Cmedia Audio" is hard-coded in the firmware of that usb dongle.
Not all Cmedia usb audio dongle use "Cmedia Audio", most of the dongles don't set string descriptor, then the audio driver sets a generic name "USB Audio Device" for them, if a dongle has string descriptor, it may have different string like "C-Media USB Headphone Set", and all dongles I met before don't have SPDIF interface.
So If this SRU could introduce regression, it will happen on a Cmedia usb audio dongle which also hard-codes the "Cmedia Audio" in the string descriptor and it has SPDIF interface on it, after this SRU, users couldn't find the SPDIF playback device from the gnome-sound-setting. But this regression chance is very low since it is very rare a cmedia usb audio dongle uses "Cmedia Audio" in the string descriptor and it is very rare a usb audio dongle has SPDIF interface. So far we haven't met such a cmedia usb audio dongle.

** Affects: oem-priority
     Importance: Critical
     Assignee: Shengyao Xue (xueshengyao)
         Status: Fix Released

** Affects: alsa-lib (Ubuntu)
     Importance: Low
         Status: Fix Released

** Affects: alsa-lib (Ubuntu Bionic)
     Importance: Critical
     Assignee: Hui Wang (hui.wang)
         Status: In Progress

** Affects: alsa-lib (Ubuntu Focal)
     Importance: Low
         Status: Fix Released

** Affects: alsa-lib (Ubuntu Groovy)
     Importance: Low
         Status: Fix Released

** Affects: alsa-lib (Ubuntu Hirsute)
     Importance: Low
         Status: Fix Released


** Tags: oem-priority originate-from-1916751 originate-from-1916753 somerville verification-done verification-done-focal verification-done-groovy
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[SRU] alsa-lib: conf: USB - add "Cmedia Audio" to USB-Audio.pcm.iec958_device
https://bugs.launchpad.net/bugs/1921452
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