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

[Jani Nikula]

On Thu, 05 Nov 2026, Kai Vehmanen <kai.vehmanen@xxxxxxxxxxxxxxx> wrote:
> 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.

We might want drm_edid_to_eld() to do this automatically for everyone in
the future, but I think this is a good, cautious first step.

Acked-by: Jani Nikula <jani.nikula@xxxxxxxxx>

>
> 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(+)

-- 
Jani Nikula, Intel