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

[Nicolas Frattaroli]

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 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@collabora.com

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@collabora.com

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@collabora.com

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@collabora.com

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@collabora.com

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 at linux.intel.com>
To: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
To: Maxime Ripard <mripard at kernel.org>
To: Thomas Zimmermann <tzimmermann at suse.de>
To: David Airlie <airlied at gmail.com>
To: Simona Vetter <simona at ffwll.ch>
To: Andrzej Hajda <andrzej.hajda at intel.com>
To: Neil Armstrong <neil.armstrong at linaro.org>
To: Robert Foss <rfoss at kernel.org>
To: Laurent Pinchart <Laurent.pinchart at ideasonboard.com>
To: Jonas Karlman <jonas at kwiboo.se>
To: Jernej Skrabec <jernej.skrabec at gmail.com>
To: Luca Ceresoli <luca.ceresoli at bootlin.com>
To: Daniel Stone <daniel at fooishbar.org>
To: Hans Verkuil <hverkuil+cisco at kernel.org>
To: Chen-Yu Tsai <wens at kernel.org>
To: Samuel Holland <samuel at sholland.org>
To: Dave Stevenson <dave.stevenson at raspberrypi.com>
To: Maíra Canal <mcanal at igalia.com>
To: Raspberry Pi Kernel Maintenance <kernel-list at raspberrypi.com>
Cc: dri-devel at lists.freedesktop.org
Cc: linux-kernel at vger.kernel.org
Cc: kernel at collabora.com
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-sunxi at lists.linux.dev
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli at collabora.com>

---
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 at collabora.com>