drivers/gpu/drm/display/drm_bridge_connector.c | 4 + drivers/gpu/drm/display/drm_scdc_helper.c | 377 ++++++++++++++++++++++++- include/drm/display/drm_scdc.h | 16 +- include/drm/display/drm_scdc_helper.h | 105 ++++++- 4 files changed, 493 insertions(+), 9 deletions(-)
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.
---
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@collabora.com
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@collabora.com
Changes in v2:
- Add HDMI 2.1 SCDC status reporting
- Link to v1: https://patch.msgid.link/20260415-scdc-link-health-v1-0-8e731e88eaf0@collabora.com
To: Jani Nikula <jani.nikula@linux.intel.com>
To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: Maxime Ripard <mripard@kernel.org>
To: Thomas Zimmermann <tzimmermann@suse.de>
To: David Airlie <airlied@gmail.com>
To: Simona Vetter <simona@ffwll.ch>
To: Andrzej Hajda <andrzej.hajda@intel.com>
To: Neil Armstrong <neil.armstrong@linaro.org>
To: Robert Foss <rfoss@kernel.org>
To: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
To: Jonas Karlman <jonas@kwiboo.se>
To: Jernej Skrabec <jernej.skrabec@gmail.com>
To: Luca Ceresoli <luca.ceresoli@bootlin.com>
To: Daniel Stone <daniel@fooishbar.org>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel@collabora.com
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
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 | 377 ++++++++++++++++++++++++-
include/drm/display/drm_scdc.h | 16 +-
include/drm/display/drm_scdc_helper.h | 105 ++++++-
4 files changed, 493 insertions(+), 9 deletions(-)
---
base-commit: 971288d0e77f7a0d356287121ce549ab5dad570a
change-id: 20260413-scdc-link-health-89326013d96c
Best regards,
--
Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Hi Nicolas,
On 27/05/2026 16:03, 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.
Daniel pointed out this series to me when we met earlier this week at the
Embedded Recipes conference.
I very recently added SCDC parsing (up to HDMI 2.2) to the edid-decode utility
(git://linuxtv.org/v4l-utils.git).
Just as with the InfoFrames that are exposed in debugfs, it would be good to
ensure that the debugfs output of SCDC can be parsed by edid-decode.
Next week I'll review this series.
One option is to just start the debugfs output with the hexdump of the SCDC
data, and then either leave the parsing to edid-decode, or add the parsing
yourself, which edid-decode will just skip.
Regards,
Hans
>
> ---
> 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@collabora.com
>
> 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@collabora.com
>
> Changes in v2:
> - Add HDMI 2.1 SCDC status reporting
> - Link to v1: https://patch.msgid.link/20260415-scdc-link-health-v1-0-8e731e88eaf0@collabora.com
>
> To: Jani Nikula <jani.nikula@linux.intel.com>
> To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> To: Maxime Ripard <mripard@kernel.org>
> To: Thomas Zimmermann <tzimmermann@suse.de>
> To: David Airlie <airlied@gmail.com>
> To: Simona Vetter <simona@ffwll.ch>
> To: Andrzej Hajda <andrzej.hajda@intel.com>
> To: Neil Armstrong <neil.armstrong@linaro.org>
> To: Robert Foss <rfoss@kernel.org>
> To: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
> To: Jonas Karlman <jonas@kwiboo.se>
> To: Jernej Skrabec <jernej.skrabec@gmail.com>
> To: Luca Ceresoli <luca.ceresoli@bootlin.com>
> To: Daniel Stone <daniel@fooishbar.org>
> Cc: dri-devel@lists.freedesktop.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel@collabora.com
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
>
> ---
> 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 | 377 ++++++++++++++++++++++++-
> include/drm/display/drm_scdc.h | 16 +-
> include/drm/display/drm_scdc_helper.h | 105 ++++++-
> 4 files changed, 493 insertions(+), 9 deletions(-)
> ---
> base-commit: 971288d0e77f7a0d356287121ce549ab5dad570a
> change-id: 20260413-scdc-link-health-89326013d96c
>
> Best regards,
> --
> Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
>
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