Re: [PATCH v9 0/5] Add SCDC information to connector debugfs

[Jani Nikula]

On Fri, 24 Jul 2026, Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx> 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.

Is there a reason we can't add a character device node for this similar
to DP AUX? See config DRM_DISPLAY_DP_AUX_CHARDEV.

It's ultimately more generic than deciphering everything in kernel.


BR,
Jani.





>
> ---
> Changes in v9:
> - Unlock mode_config.mutex on disconnected connector early exit
> - Calculate and print Reed-Solomon error corrections counter on != 4
>   lane FRL as well. Technically a functional change to an already
>   reviewed patch, but I kept the R-b because it's so minor.
> - Link to v8: https://patch.msgid.link/20260722-scdc-link-health-v8-0-cd1dacbb85d9@xxxxxxxxxxxxx
>
> Changes in v8:
> - Acquire mode_config mutex in an interruptible way during
>   scdc_status_show to avoid racing connector state
> - Remove scdc_debugfs_priv, use connector as debugfs entry priv instead
> - Allocate and free state in scdc_status_show, since it doesn't need to
>   persist across debugfs reads
> - Check for NULL ddc in drm_scdc_read_state in the unlikely event an
>   EDID with the SCDC flag was forced on a connector with no DDC
> - Note: drm_connector_get() is omitted from drm_scdc_debugfs_init
>   because drm_scdc_debugfs_init is called on connector construction and
>   the debugfs is removed on connector destruction. Adding a redundant
>   reference increase here would necessitate introducing a destruction
>   callback to mirror the debugfs_init connector op for the sole purpose
>   of decreasing the counter again.
> - Link to v7: https://patch.msgid.link/20260721-scdc-link-health-v7-0-92df1b6ad5fc@xxxxxxxxxxxxx
>
> Changes in v7:
> - Move HDMI debugfs registration to new state helper, fix up all users
>   (sun4i, vc4, bridge)
> - Register scdc debugfs in new hdmi debugfs helper
> - Link to v6: https://patch.msgid.link/20260611-scdc-link-health-v6-0-6307875a6b5e@xxxxxxxxxxxxx
>
> 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>
> To: Chen-Yu Tsai <wens@xxxxxxxxxx>
> To: Samuel Holland <samuel@xxxxxxxxxxxx>
> To: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
> To: Maíra Canal <mcanal@xxxxxxxxxx>
> To: Raspberry Pi Kernel Maintenance <kernel-list@xxxxxxxxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel@xxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-sunxi@xxxxxxxxxxxxxxx
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx>
>
> ---
> Nicolas Frattaroli (5):
>       drm/scdc-helper: Don't use ssize_t return type for scdc_read/write
>       drm/scdc-helper: Add scdc_status debugfs entry
>       drm/scdc-helper: Implement parsing and printing HDMI 2.1 fields
>       drm/debugfs: Move HDMI debugfs registration to state helper
>       drm/display: hdmi-state-helper: Init SCDC debugfs for HDMI
>
>  drivers/gpu/drm/display/drm_bridge_connector.c  |   3 +
>  drivers/gpu/drm/display/drm_hdmi_state_helper.c | 156 +++++++++++++
>  drivers/gpu/drm/display/drm_scdc_helper.c       | 289 +++++++++++++++++++++++-
>  drivers/gpu/drm/drm_debugfs.c                   | 157 -------------
>  drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c          |   1 +
>  drivers/gpu/drm/vc4/vc4_hdmi.c                  |   1 +
>  include/drm/display/drm_hdmi_state_helper.h     |   3 +
>  include/drm/display/drm_scdc.h                  |  21 +-
>  include/drm/display/drm_scdc_helper.h           | 103 ++++++++-
>  9 files changed, 568 insertions(+), 166 deletions(-)
> ---
> base-commit: b747a63003130bef1a3d5d6ba97c94d0cee36f55
> change-id: 20260413-scdc-link-health-89326013d96c
>
> Best regards,
> --  
> Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx>
>

-- 
Jani Nikula, Intel