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

[Jani Nikula]

On Mon, 27 Jul 2026, Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx> wrote:
> On Monday, 27 July 2026 10:35:15 Central European Summer Time Jani Nikula wrote:
>> 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.
>
> You already can just point tools at the ddc symlink to the i2c dev.
>
> The idea here is to be able to read this out of debugfs without
> any additional external tooling needed.

Fair enough, thanks.

BR,
Jani.

>
>> It's ultimately more generic than deciphering everything in kernel.
>
> The kernel will need to decipher this in the future anyway for HDMI 2.x
> support, including so that it can set link status to bad when there's a
> glut of errors.
>
>> 
>> 
>> 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