Re: [PATCH RFC 00/12] Add support for DisplayPort link training information report

Ville Syrjälä
2026-04-09T20:36:36Z

On Thu, Apr 09, 2026 at 07:08:16PM +0200, Kory Maincent wrote:
> DisplayPort link training negotiates the physical-layer parameters needed
> for a reliable connection: lane count, link rate, voltage swing,
> pre-emphasis, and optionally Display Stream Compression (DSC). Currently,
> each driver exposes this state in its own way, often through
> driver-specific debugfs entries, with no standard interface for userspace
> diagnostic and monitoring tools.
>
> This series introduces a generic, DRM-managed framework for exposing DP
> link training state as standard connector properties, modeled after the
> existing HDMI helper drmm_connector_hdmi_init().
>
> The new drmm_connector_dp_init() helper initializes a DP connector and
> registers the following connector properties to expose the negotiated link
> state to userspace:
>
> - num_lanes: negotiated lane count (1, 2 or 4)
> - link_rate: negotiated link rate
> - dsc_en: whether Display Stream Compression is active
> - voltage_swingN: per-lane voltage swing level (lanes 0-3)
> - pre_emphasisN: per-lane pre-emphasis level (lanes 0-3)

I don't see why any real userspace would be interested in those (apart
from maybe DSC). If this is just for diagnostics and whatnot then I
think sysfs/debugfs could be a better fit.

>
> Two runtime helpers update and clear these properties when link training
> completes or the link goes down:
> - drm_connector_dp_set_link_train_properties()
> - drm_connector_dp_reset_link_train_properties()
>
> Two drivers are updated as reference implementations: i915 (direct
> connector path) and MediaTek (via the bridge connector framework using a
> new DRM_BRIDGE_OP_DP flag). The i915 patches are preceded by a series of
> conversions to DRM managed resources, which are required before adopting
> drmm_connector_dp_init().
>
> The MST case in i915 driver is not supported yet.
>
> Patches 1-3: Fix two error-path cleanup bugs in i915 sdvo and lvds
> [Will probably be sent standalone]
> Patches 4-8: Convert i915 display resources to DRM managed lifetime
> Patch 9: Introduce the core drmm_connector_dp_init() framework
> Patch 10: Wire the i915 DP connector to use the new helpers
> Patch 11: Introduce DRM_BRIDGE_OP_DP and wire bridge connectors
> Patch 12: Wire the MediaTek DP bridge to the new helpers [untested]
>
> Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
> ---
> Kory Maincent (12):
> drm/i915/display/intel_sdvo: Fix double connector destroy in error paths
> drm/i915/display/intel_lvds: Drop redundant manual cleanup on init failure
> drm/i915/display/intel_dp: Drop redundant intel_dp_aux_fini() on init failure
> drm/i915/display: Switch to drmm_mode_config_init() and drop manual cleanup
> drm/i915/display: Switch to managed for crtc
> drm/i915/display: Switch to managed for plane
> drm/i915/display: Switch to managed for encoder
> drm/i915/display: Switch to managed for connector
> drm: Introduce drmm_connector_dp_init() with link training state properties
> drm/i915/display/dp: Adopt dp_connector helpers to expose link training state
> drm/bridge: Wire drmm_connector_dp_init() via new DRM_BRIDGE_OP_DP flag
> drm/mediatek: Use dp_connector helpers to report link training state
>
> drivers/gpu/drm/Makefile | 1 +
> drivers/gpu/drm/display/drm_bridge_connector.c | 26 +-
> drivers/gpu/drm/drm_dp_connector.c | 344 +++++++++++++++++++++
> drivers/gpu/drm/i915/display/g4x_dp.c | 39 +--
> drivers/gpu/drm/i915/display/g4x_hdmi.c | 27 +-
> drivers/gpu/drm/i915/display/i9xx_plane.c | 97 +++---
> drivers/gpu/drm/i915/display/icl_dsi.c | 50 ++-
> drivers/gpu/drm/i915/display/intel_connector.c | 26 +-
> drivers/gpu/drm/i915/display/intel_connector.h | 5 +-
> drivers/gpu/drm/i915/display/intel_crt.c | 28 +-
> drivers/gpu/drm/i915/display/intel_crtc.c | 102 +++---
> drivers/gpu/drm/i915/display/intel_cursor.c | 41 ++-
> drivers/gpu/drm/i915/display/intel_ddi.c | 64 ++--
> drivers/gpu/drm/i915/display/intel_display.c | 8 -
> drivers/gpu/drm/i915/display/intel_display.h | 1 -
> .../gpu/drm/i915/display/intel_display_driver.c | 37 ++-
> drivers/gpu/drm/i915/display/intel_dp.c | 43 ++-
> .../gpu/drm/i915/display/intel_dp_link_training.c | 25 ++
> drivers/gpu/drm/i915/display/intel_dp_mst.c | 33 +-
> drivers/gpu/drm/i915/display/intel_dvo.c | 43 +--
> drivers/gpu/drm/i915/display/intel_encoder.c | 6 +-
> drivers/gpu/drm/i915/display/intel_encoder.h | 3 +-
> drivers/gpu/drm/i915/display/intel_hdmi.c | 15 +-
> drivers/gpu/drm/i915/display/intel_lvds.c | 45 ++-
> drivers/gpu/drm/i915/display/intel_plane.c | 45 +--
> drivers/gpu/drm/i915/display/intel_plane.h | 5 +-
> drivers/gpu/drm/i915/display/intel_sdvo.c | 134 +++-----
> drivers/gpu/drm/i915/display/intel_sprite.c | 119 ++++---
> drivers/gpu/drm/i915/display/intel_tv.c | 26 +-
> drivers/gpu/drm/i915/display/skl_universal_plane.c | 102 +++---
> drivers/gpu/drm/i915/display/vlv_dsi.c | 42 +--
> drivers/gpu/drm/mediatek/mtk_dp.c | 34 +-
> include/drm/drm_bridge.h | 13 +
> include/drm/drm_connector.h | 38 +++
> include/drm/drm_dp_connector.h | 109 +++++++
> 35 files changed, 1125 insertions(+), 651 deletions(-)
> ---
> base-commit: db5a75cfd29766536be62aece9f19c6e7a858fa6
> change-id: 20260226-feat_link_cap-20cbb6f31d40
>
> Best regards,
> --
> Köry Maincent, Bootlin
> Embedded Linux and kernel engineering
> https://bootlin.com

--
Ville Syrjälä
Intel