[Bug 1855893] Re: Properly let PCM leave suspended state when hardware doesn't support PCM resume

[Sebastien Bacher]

@Kai-Heng, you unassigned yourself, does it mean it's not important
anymore for bionic and the target should be dropped?

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Title:
  Properly let PCM leave suspended state when hardware doesn't support
  PCM resume

Status in HWE Next:
  New
Status in pulseaudio package in Ubuntu:
  Fix Released
Status in pulseaudio source package in Bionic:
  In Progress

Bug description:
  [Impact]
  Headset Mic plugged into Dell WD19TB dock stops working after suspend/resume.

  [Fix]
  According to alia-lib's document, to make PCM properly leave suspended state, we only use snd_pcm_resume() if hardware support PCM resume.
  For hardware that doesn't support PCM resume, like snd-usb-audio on WD19TB dock, we should use snd_pcm_prepare(), snd_pcm_drop() or snd_pcm_drain() to make device leave suspended state. This patch defaults to use snd_pcm_drop() to achieve the goal.

  [Test]
  1. See if microphone on snd-usb-audio works in gnome-control-center/cheese.
  2. Suspend/resume.
  3. See if microphone still works in those apps.
  I can the issue is fixed after each suspend/resume cycle.

  [Regression Potential]
  Low. This fix makes PCM suspend/resume more reliable. No functional change intended. I also don't find any regression during some smoke test.

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