[PATCH 0/2] drm: Populate ELD Port_ID to identify audio endpoints

[Kai Vehmanen]

The ELD Port_ID field (bytes 8..15 of the baseline block) is typically
zero on Linux today: drm_edid_to_eld() leaves it empty and no in-tree driver
fills it in. amdgpu, radeon and nouveau pass the raw connector->eld to
the audio component. On the amdgpu DC Azalia register path the value
is a hardcoded constant, the same for every port.
Only closed display drivers fill the value.

Pipewire added support for "Port_ID" in merge request:
https://gitlab.freedesktop.org/pipewire/pipewire/-/merge_requests/3025

As a result, when two identical monitors are connected their ELDs are
byte-identical, and userspace (e.g. PipeWire/WirePlumber) cannot
passively tell which HDMI/DP PCM belongs to which DRM connector.

This series adds a DRM helper that fills Port_ID with a common layout,
and uses it in i915:

  bits 63:48 - PCI domain of the DRM device
  bits 47:32 - PCI bus/device/function of the DRM device
  bits 31:0  - DRM connector object ID

The upper half follows the layout documented for the AMD Azalia sink
info registers (dce_audio.c: "PORT_ID1 = 16bit BDF"). The aim is for
this to become a convention other drivers can opt into. For drivers
that copy connector->eld to the audio component (amdgpu DM, radeon,
nouveau), adopting it would be a one-line change. Comments from the
AMD and nouveau folks on whether the layout works for them are very
welcome.

An alternative would be to fill Port_ID directly in drm_edid_to_eld(),
so every driver gets it without changes. I went with an opt-in helper
to avoid changing the ELD contents seen by all drivers and existing
userspace at once.

The value is exposed to userspace via the ALSA ELD control and
/proc/asound/cardX/eld#Y.

Kai Vehmanen (2):
  drm/eld: Add drm_eld_set_port_id() helper
  drm/i915/audio: Populate ELD Port_ID

 drivers/gpu/drm/drm_eld.c                  | 42 ++++++++++++++++++++++
 drivers/gpu/drm/i915/display/intel_audio.c |  2 ++
 include/drm/drm_eld.h                      |  3 ++
 3 files changed, 47 insertions(+)

-- 
2.43.0