Hi Nicolas,
On 11/06/2026 14:57, Nicolas Frattaroli wrote:
> HDMI uses the DDC I2C bus for communicating various bits of link status
> out of band with the actual HDMI video signal. This information can be
> useful for debugging issues like questionable cables sabotaged by feline
> teeth, Enthusiast Grade cables made of cow fencing wire, and other such
> problems that ruin one's media viewing plans.
>
> Consequently, this series exposes various bits of pertinent information
> from the SCDC protocol in an HDMI connector's debugfs. To continually
> poll the link status, userspace can poll the debugfs file.
Something is not quite right: I've been testing this series with my i915
based laptop with HDMI connector, and I never see the scdc_status file.
And that's because CONFIG_DRM_BRIDGE_CONNECTOR is not set for my configuration.
So I think you are creating the debugfs entry in the wrong place.
I can read the SCDC from the display using edid-decode with the right /dev/i2c-X
device, so it's definitely there.
Regards,
Hans
>
> ---
> Changes in v6:
> - Fix off-by-one error in drm_scdc_read_state
> - Link to v5:
> https://patch.msgid.link/[email protected]
>
> Changes in v5:
> - Read all SCDC data regardless of update flags
> - Dump SCDC data as hex before the human-readable output. It's separated
> with "\n----------------\n\n".
> - No longer write 0 to read-only registers
> - Add Reed-Solomon Corrections counter parsing
> - Parsing has been kept. A desire was expressed to get this data without
> any external userspace tooling, and the kernel will need to parse it
> eventually anyway to set the link status.
> - Functions have been made static as of right now, since external users
> may do another pass over the function signatures anyway.
> - Link to v4:
> https://patch.msgid.link/[email protected]
>
> Changes in v4:
> - Don't use C struct bitfields for parsing status flags. Switch to
> bitwise AND for boolean flags, and FIELD_GET for multi-bit values.
> - Drop the superfluous !! and parens
> - Drop the __pure attributes on static functions
> - Initialise stack local arrays with {}, not { 0 }.
> - I've kept the print macros and %-30s format. Reason being that I don't
> want to repeat the format specifier and str_yes_no(foo) a bunch, and I
> like the %-30s format because it means all values are aligned with the
> value of the longest field, which is 30 chars long.
> - Link to v3:
> https://patch.msgid.link/[email protected]
>
> Changes in v3:
> - Add patch to change return type of drm_scdc_read/write.
> - Rework error counter reading to duplicate less code.
> - Also check lane 3 counter valid flag when reading its error counter.
> - Use memset to clear buf for error counters, rather than doing it in
> the loop.
> - Make read_error_counters not accept 0 as num_lanes; fix it up in the
> caller instead.
> - Link to v2:
> https://patch.msgid.link/[email protected]
>
> Changes in v2:
> - Add HDMI 2.1 SCDC status reporting
> - Link to v1:
> https://patch.msgid.link/[email protected]
>
> To: Jani Nikula <[email protected]>
> To: Maarten Lankhorst <[email protected]>
> To: Maxime Ripard <[email protected]>
> To: Thomas Zimmermann <[email protected]>
> To: David Airlie <[email protected]>
> To: Simona Vetter <[email protected]>
> To: Andrzej Hajda <[email protected]>
> To: Neil Armstrong <[email protected]>
> To: Robert Foss <[email protected]>
> To: Laurent Pinchart <[email protected]>
> To: Jonas Karlman <[email protected]>
> To: Jernej Skrabec <[email protected]>
> To: Luca Ceresoli <[email protected]>
> To: Daniel Stone <[email protected]>
> To: Hans Verkuil <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Nicolas Frattaroli <[email protected]>
>
> ---
> Nicolas Frattaroli (4):
> drm/scdc-helper: Don't use ssize_t return type for scdc_read/write
> drm/scdc-helper: Add scdc_status debugfs entry
> drm/display: bridge_connector: init scdc debugfs for HDMI
> drm/scdc-helper: Implement parsing and printing HDMI 2.1 fields
>
> drivers/gpu/drm/display/drm_bridge_connector.c | 4 +
> drivers/gpu/drm/display/drm_scdc_helper.c | 285
> ++++++++++++++++++++++++-
> include/drm/display/drm_scdc.h | 21 +-
> include/drm/display/drm_scdc_helper.h | 103 ++++++++-
> 4 files changed, 404 insertions(+), 9 deletions(-)
> ---
> base-commit: 4fdfaadba04dc0f2f2490dbc91922caa290463a5
> change-id: 20260413-scdc-link-health-89326013d96c
>
> Best regards,
> --
> Nicolas Frattaroli <[email protected]>
>
