Re: [PATCH v6 0/4] Add SCDC information to connector debugfs

[Hans Verkuil]

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/20260604-scdc-link-health-v5-0-11173b0ac3de@xxxxxxxxxxxxx
> 
> 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/20260527-scdc-link-health-v4-0-622ea40a1f59@xxxxxxxxxxxxx
> 
> 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/20260526-scdc-link-health-v3-0-59e4a4aaead1@xxxxxxxxxxxxx
> 
> 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/20260520-scdc-link-health-v2-0-511af18cd64b@xxxxxxxxxxxxx
> 
> Changes in v2:
> - Add HDMI 2.1 SCDC status reporting
> - Link to v1: https://patch.msgid.link/20260415-scdc-link-health-v1-0-8e731e88eaf0@xxxxxxxxxxxxx
> 
> To: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> To: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> To: Maxime Ripard <mripard@xxxxxxxxxx>
> To: Thomas Zimmermann <tzimmermann@xxxxxxx>
> To: David Airlie <airlied@xxxxxxxxx>
> To: Simona Vetter <simona@xxxxxxxx>
> To: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
> To: Neil Armstrong <neil.armstrong@xxxxxxxxxx>
> To: Robert Foss <rfoss@xxxxxxxxxx>
> To: Laurent Pinchart <Laurent.pinchart@xxxxxxxxxxxxxxxx>
> To: Jonas Karlman <jonas@xxxxxxxxx>
> To: Jernej Skrabec <jernej.skrabec@xxxxxxxxx>
> To: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> To: Daniel Stone <daniel@xxxxxxxxxxxxx>
> To: Hans Verkuil <hverkuil+cisco@xxxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel@xxxxxxxxxxxxx
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx>
> 
> ---
> 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 <nicolas.frattaroli@xxxxxxxxxxxxx>
>