HDMI/DP audio: ELD not updated on hotplug event

When using HDMI or Displayport audio, the EDID-like Data (ELD) fails to update properly when hotplugging or hot-unplugging displays. This in turn prevents tools like the GNOME sound settings control panel from accurately presenting the system's current audio capabilities.

How reproducible:

Hot plug and hot unplug an audio-capable display and observe the state of the ELDs, either by directly reading the ELD files in /proc/asound/card*/eld* or by examining the available audio output devices in the GNOME sound settings control panel.

Steps to Reproduce:
1. Log into a desktop session with no audio-capable displays plugged in
2. Plug in an audio-capable display
3. Open the GNOME sound settings control panel

Actual results:

The audio-capable display is not selectable as an output device.

Expected results:

The audio-capable display should be selectable as an output device.

Additional info:

This bug has been fixed upstream with commit 37a3a98ef601f89100e3bb657fb0e190b857028c. Applying this change alone to the bionic 4.18 HWE kernel is sufficient to allow the expected behavior to occur. It is likely to be sufficient to fix the same bug, assuming it exists, in Cosmic, but neither the presence of the bug nor the sufficiency of the patch were tested. The bug is likely not present in Disco, but this was not tested.

This behavior was observed on a system with an NVIDIA GPU running the NVIDIA proprietary driver. The particular NVIDIA GPU on the system is not currently compatible with Nouveau. It is possible and likely that this problem is also reproducible on other systems which do have GPUs that are compatible with Nouveau, and even on systems with HDMI or Displayport connectors being driven by non-NVIDIA GPUs.

When the ELD fails to update in response to a hotplug event, an update can be forced by accessing the underlying ALSA codec directly. For example, playing an audio stream directly via ALSA or even just reading from the /proc/asound/cardX/codec#Y file will cause the ELD to become updated. This is of course unnecessary if the patch which fixes the issue is applied, but it does offer a somewhat simple workaround for affected users until the patch can be backported.