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 <[email protected]> > --- > 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 <[email protected]> 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
