Re: [PATCH 1/2] drm/eld: Add drm_eld_set_port_id() helper

[Ville Syrjälä]

On Thu, Nov 05, 2026 at 06:02:39PM +0200, Kai Vehmanen wrote:
> The ELD baseline block reserves bytes 8..15 for a 64-bit Port_ID that
> identifies the source port of the audio endpoint. drm_edid_to_eld()
> leaves the field zero, and no driver fills it in, so it carries no
> information.
> 
> When two identical monitors are connected, their ELDs are then
> byte-identical, and userspace has no passive way to tell which HDMI/DP
> PCM belongs to which DRM connector. The only workaround is to toggle
> the connector audio property and watch which ELD goes away, which
> disrupts the endpoint.
> 
> Add a helper that drivers can use to fill in Port_ID with a common
> layout:
> 
>   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
> 
> Userspace can read the connector object ID directly from KMS, and the
> PCI address tells GPUs apart in multi-GPU systems. For non-PCI devices
> the upper half is left zero.
> 
> The split between a 16-bit PCI BDF in the upper half and a per-device
> display identifier in the lower half follows the Port_ID layout
> documented for the AMD Azalia sink info registers in dce_audio.c.
> 
> Signed-off-by: Kai Vehmanen <kai.vehmanen@xxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_eld.c | 42 +++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_eld.h     |  3 +++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_eld.c b/drivers/gpu/drm/drm_eld.c
> index c0428d07de53..baebece7b4ed 100644
> --- a/drivers/gpu/drm/drm_eld.c
> +++ b/drivers/gpu/drm/drm_eld.c
> @@ -4,7 +4,11 @@
>   */
>  
>  #include <linux/export.h>
> +#include <linux/pci.h>
> +#include <linux/unaligned.h>
>  
> +#include <drm/drm_connector.h>
> +#include <drm/drm_device.h>
>  #include <drm/drm_edid.h>
>  #include <drm/drm_eld.h>
>  
> @@ -55,3 +59,41 @@ int drm_eld_sad_set(u8 *eld, int sad_index, const struct cea_sad *cta_sad)
>  	return 0;
>  }
>  EXPORT_SYMBOL(drm_eld_sad_set);
> +
> +/**
> + * drm_eld_set_port_id - set the ELD Port_ID for a connector
> + * @eld: ELD buffer
> + * @connector: connector the ELD belongs to
> + *
> + * drm_edid_to_eld() leaves the 64-bit Port_ID field of the ELD baseline
> + * block zero. Drivers can use this helper to fill it in with a value
> + * that identifies the audio endpoint, so that userspace can associate an
> + * ELD, and the PCM device it belongs to, with a DRM connector, even when
> + * identical sinks are connected to multiple outputs.
> + *
> + * The Port_ID is stored in little-endian byte order, with the layout:
> + *
> + * - bits 63:48 - PCI domain of the DRM device, truncated to 16 bits
> + * - bits 47:32 - PCI bus/device/function of the DRM device
> + * - bits 31:0  - DRM connector object ID
> + *
> + * For non-PCI devices bits 63:32 are zero.
> + *
> + * The connector object ID is only stable for the lifetime of the
> + * connector, so userspace must look it up at runtime, and not store it.
> + */
> +void drm_eld_set_port_id(u8 *eld, const struct drm_connector *connector)
> +{
> +	struct device *dev = connector->dev->dev;
> +	u64 port_id = connector->base.id;
> +
> +	if (dev && dev_is_pci(dev)) {
> +		struct pci_dev *pdev = to_pci_dev(dev);
> +
> +		port_id |= (u64)(pci_domain_nr(pdev->bus) & 0xffff) << 48 |
> +			   (u64)pci_dev_id(pdev) << 32;

The numbers look correct.

> +	}
> +
> +	put_unaligned_le64(port_id, &eld[DRM_ELD_PORT_ID]);

Endianness agrees with the HDA spec.

The implementation looks good
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>

But I'll leave it to someone else to judge the actual
usefulness of the provided information.

> +}
> +EXPORT_SYMBOL(drm_eld_set_port_id);
> diff --git a/include/drm/drm_eld.h b/include/drm/drm_eld.h
> index 0a88d10b28b0..de0478720721 100644
> --- a/include/drm/drm_eld.h
> +++ b/include/drm/drm_eld.h
> @@ -9,6 +9,7 @@
>  #include <linux/types.h>
>  
>  struct cea_sad;
> +struct drm_connector;
>  
>  /* ELD Header Block */
>  #define DRM_ELD_HEADER_BLOCK_SIZE	4
> @@ -80,6 +81,8 @@ static inline int drm_eld_mnl(const u8 *eld)
>  int drm_eld_sad_get(const u8 *eld, int sad_index, struct cea_sad *cta_sad);
>  int drm_eld_sad_set(u8 *eld, int sad_index, const struct cea_sad *cta_sad);
>  
> +void drm_eld_set_port_id(u8 *eld, const struct drm_connector *connector);
> +
>  /**
>   * drm_eld_sad - Get ELD SAD structures.
>   * @eld: pointer to an eld memory structure with sad_count set
> -- 
> 2.43.0

-- 
Ville Syrjälä
Intel