[SRU][Bionic][PATCH 0/2] HDMI/DP audio can't work on the laptop of Dell Latitude 5495

[Hui Wang]

BugLink: https://bugs.launchpad.net/bugs/1782689

This bug comes from the OEM project, and the engineer Jim Qu from AMD took
almost 2 weeks to investigate this problem, then he worte these two patches to
fix this problem and now the patches are merged to sound repository.

According to Jim Qu's investigation, the root cause of this problem is:
1. there is two GPU on the system. iGPU has a audio codec for HDMI output,
   and dGPU is without audio codec it is only for offload rendering.
2. under runtime pm, when dGPU suspend, amdgpu will also call vgaswitchroo
   driver to power off audio which vgaswitchroo client is VGA_SWITCHEROO_DIS.
3. In current HDA audio driver, the iGPU's audio will register to vgaswitchroo
   as VGA_SWITCHEROO_DIS, therefore, the audio will be selected and powered off
   by runtime pm.

[Impact]
On the Dell Latitude 5495, If we plug a monitor with audio capability to the
HDMI/DP ports, we can't find HDMI/DP audio sinks as expected, then we can't
play any sound through HDMI/DP audio.

[Fix]
With these two patches, the driver will not always set vgaswicheroo clients of
HDA audio as VGA_SWITCHEROO_DIS, it will set it to _DIS or _IGD with the help
of callback function of DRM drivers. So on this machine, the vgaswicheroo client
will be set _IGD, then it will not be powered off when discrete gpu is powered off. 

[Test Case]
We tested plug/unplug detection and playback through HDMI/DP audio, everything works
well.

[Regression Potential]
Very low, without these two patches, the vgaswitchroo client of audio will be
set to _DIS unconditionally, it did not expose any problem because in the past,
all the HDMI/DP audio codecs are in the discreate GPU. But on Latitude 5495, the
HDMI/DP audio codec is in the integrated GPU, so we need to change the driver to
let DRM driver decide if it is _DIS or _IGD, it will not introduce regression for
old mahcines on old machines, the client will be set to _DIS as before.

And we have tested these two patches on some old machines with two gpus like A+A
, I+A and I+N, all of them worked well as before.


Jim Qu (2):
  ALSA: hda: use PCI_BASE_CLASS_DISPLAY to replace PCI_CLASS_DISPLAY_VGA
  vga_switcheroo: set audio client id according to bound GPU id

 drivers/gpu/vga/vga_switcheroo.c | 62 +++++++++++++++++++++++++++++++++-------
 include/linux/vga_switcheroo.h   |  8 +++---
 sound/pci/hda/hda_intel.c        | 13 +++++----
 3 files changed, 62 insertions(+), 21 deletions(-)

-- 
2.7.4