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

[Sebastien Bacher]

Upload to the bionic queue now

** Changed in: alsa-lib (Ubuntu Bionic)
       Status: In Progress => Fix Committed

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https://bugs.launchpad.net/bugs/1921452

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

Status in OEM Priority Project:
  Fix Released
Status in alsa-lib package in Ubuntu:
  Fix Released
Status in alsa-lib source package in Bionic:
  Fix Committed
Status in alsa-lib source package in Focal:
  Fix Released
Status in alsa-lib source package in Groovy:
  Fix Released
Status in alsa-lib source package in Hirsute:
  Fix Released

Bug description:
  [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.

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