[PATCH 0/2] Add SCDC information to connector debugfs

[PATCH 0/2] 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.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
Nicolas Frattaroli (2):
      drm/scdc-helper: Add scdc_status debugfs entry
      drm/display: bridge_connector: init scdc debugfs for HDMI

 drivers/gpu/drm/display/drm_bridge_connector.c |   4 +
 drivers/gpu/drm/display/drm_scdc_helper.c      | 229 +++++++++++++++++++++++++
 include/drm/display/drm_scdc_helper.h          |  56 ++++++
 3 files changed, 289 insertions(+)
---
base-commit: 4c59525db84aca677fd9f052e662912ad9d88448
change-id: 20260413-scdc-link-health-89326013d96c

Best regards,
--  
Nicolas Frattaroli <nicolas.frattaroli@collabora.com>

[PATCH 1/2] drm/scdc-helper: Add scdc_status debugfs entry

[Nicolas Frattaroli]

SCDC provides status information on the current display link. At the
very least, it may be useful to expose this info through debugfs.

Add a debugfs entry for it under the connector, which displays a few
more details parsed out of the SCDC registers. A new
drm_scdc_debugfs_init function can be called by the connector
implementation to initialise the debugfs file.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
 drivers/gpu/drm/display/drm_scdc_helper.c | 229 ++++++++++++++++++++++++++++++
 include/drm/display/drm_scdc_helper.h     |  56 ++++++++
 2 files changed, 285 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_scdc_helper.c b/drivers/gpu/drm/display/drm_scdc_helper.c
index df878aad4a36..d582ba38a610 100644
--- a/drivers/gpu/drm/display/drm_scdc_helper.c
+++ b/drivers/gpu/drm/display/drm_scdc_helper.c
@@ -24,11 +24,14 @@
 #include <linux/export.h>
 #include <linux/i2c.h>
 #include <linux/slab.h>
+#include <linux/debugfs.h>
 #include <linux/delay.h>
+#include <linux/overflow.h>
 
 #include <drm/display/drm_scdc_helper.h>
 #include <drm/drm_connector.h>
 #include <drm/drm_device.h>
+#include <drm/drm_managed.h>
 #include <drm/drm_print.h>
 
 /**
@@ -55,6 +58,11 @@
 
 #define SCDC_I2C_SLAVE_ADDRESS 0x54
 
+struct scdc_debugfs_priv {
+	struct drm_connector *connector;
+	struct drm_scdc_state state;
+};
+
 /**
  * drm_scdc_read - read a block of data from SCDC
  * @adapter: I2C controller
@@ -276,3 +284,224 @@ bool drm_scdc_set_high_tmds_clock_ratio(struct drm_connector *connector,
 	return true;
 }
 EXPORT_SYMBOL(drm_scdc_set_high_tmds_clock_ratio);
+
+/**
+ * drm_scdc_read_status0_flags - Read SCDC "Status Flags" Register
+ * @connector: pointer to &struct drm_connector to issue the scdc request on
+ * @flags: pointer to the caller's &struct drm_scdc_status_flags to output to
+ *
+ * Reads the SCDC Status Flags 0 register, and outputs its contents to the
+ * destination @flags. Contents of @flags.status0 is only valid if function
+ * returns 0.
+ *
+ * Returns: %0 on success, negative errno on error.
+ */
+int drm_scdc_read_status0_flags(struct drm_connector *connector,
+				struct drm_scdc_status_flags *flags)
+{
+	int ret;
+
+	ret = drm_scdc_writeb(connector->ddc, SCDC_UPDATE_0, SCDC_STATUS_UPDATE);
+	if (ret)
+		return ret;
+
+	return drm_scdc_readb(connector->ddc, SCDC_STATUS_FLAGS_0, &flags->status0.data);
+}
+EXPORT_SYMBOL(drm_scdc_read_status0_flags);
+
+/**
+ * drm_scdc_read_error_counters - Read and clear SCDC error counters
+ * @connector: pointer to &struct drm_connector to issue the scdc request on
+ * @counter: Caller's u16 array with 3 elements to write the counter values into
+ *
+ * Read the SCDC channel error counters. If the count of channel *n* is valid,
+ * write it into counter[n]. Otherwise, set counter[n] to 0. Reads all counters
+ * in one read chunk, then clears every counter, as is mandated.
+ *
+ * Returns: %0 on success, negative errno on error.
+ */
+int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[3])
+{
+	u8 buf[7] = { 0 };
+	ssize_t ret;
+	u8 sum = 0;
+	int i;
+
+	ret = drm_scdc_writeb(connector->ddc, SCDC_UPDATE_0, SCDC_CED_UPDATE);
+	if (ret)
+		return ret;
+
+	/* why in the fucking fuck does this return a ssize_t */
+	ret = drm_scdc_read(connector->ddc, SCDC_ERR_DET_0_L, buf, ARRAY_SIZE(buf));
+	if (ret)
+		return ret;
+
+	/*
+	 * Verify the "checksum", i.e. sum up everything including the checksum
+	 * register as a wrapping unsigned 8-bit addition and verify it's 0.
+	 */
+	for (i = 0; i < ARRAY_SIZE(buf); i++)
+		sum = wrapping_add(u8, sum, buf[i]);
+
+	if (sum)
+		return -EPROTO;
+
+	for (i = 0; i < ARRAY_SIZE(buf) - 1; i += 2) {
+		if (buf[i + 1] & SCDC_CHANNEL_VALID)
+			counter[i / 2] = buf[i] | (buf[i + 1] & ~SCDC_CHANNEL_VALID) << 8;
+		else
+			counter[i / 2] = 0;
+
+		buf[i] = 0;
+		buf[i + 1] = 0;
+	}
+	buf[ARRAY_SIZE(buf) - 1] = 0;
+
+	return drm_scdc_write(connector->ddc, SCDC_ERR_DET_0_L, buf, ARRAY_SIZE(buf));
+}
+EXPORT_SYMBOL(drm_scdc_read_error_counters);
+
+/**
+ * drm_scdc_read_state - Update state from SCDC
+ * @connector: pointer to a &struct drm_connector on which to operate on
+ * @state: pointer to a &struct drm_scdc_state to fill
+ *
+ * Reads update flags from SCDC, and updates the parts of @state that SCDC
+ * claims have changed, as well as populating those where such a distinction
+ * can't be made.
+ *
+ * Returns: %0 on success, negative errno on failure.
+ */
+int drm_scdc_read_state(struct drm_connector *connector, struct drm_scdc_state *state)
+{
+	u8 upd_flags[2] = { 0 };
+	struct i2c_adapter *ddc;
+	struct drm_scdc *scdc;
+	int ret;
+	u8 val;
+
+	if (!state || !connector)
+		return -ENODEV;
+
+	scdc = &connector->display_info.hdmi.scdc;
+	ddc = connector->ddc;
+
+	if (!scdc->supported)
+		return -EOPNOTSUPP;
+
+	ret = drm_scdc_readb(ddc, SCDC_TMDS_CONFIG, &val);
+	if (ret)
+		return ret;
+
+	state->scrambling_enabled = !!(val & SCDC_SCRAMBLING_ENABLE);
+	state->tmds_bclk_x40 = !!(val & SCDC_TMDS_BIT_CLOCK_RATIO_BY_40);
+
+	state->scrambling_detected = drm_scdc_get_scrambling_status(connector);
+
+	ret = drm_scdc_read(ddc, SCDC_UPDATE_0, &upd_flags, sizeof(upd_flags));
+	if (ret)
+		return ret;
+
+	if (upd_flags[0] & SCDC_STATUS_UPDATE) {
+		ret = drm_scdc_read_status0_flags(connector, &state->stf);
+		if (ret)
+			return ret;
+	}
+
+	if (upd_flags[0] & SCDC_CED_UPDATE) {
+		ret = drm_scdc_read_error_counters(connector, state->error_count);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_scdc_read_state);
+
+#define scdc_print_str(_f, key, s) \
+	(seq_printf((_f), "%-30s: %s\n", (key), (s)))
+#define scdc_print_flag(_f, key, val) \
+	(scdc_print_str((_f), (key), str_yes_no((val))))
+#define scdc_print_dec(_f, key, val) \
+	(seq_printf((_f), "%-30s: %d\n", (key), (val)))
+
+static int scdc_status_show(struct seq_file *m, void *data)
+{
+	struct scdc_debugfs_priv *priv = m->private;
+	struct drm_scdc_state *st = &priv->state;
+	struct drm_connector *connector = priv->connector;
+	struct drm_scdc *scdc = &connector->display_info.hdmi.scdc;
+	int ret;
+
+	drm_connector_get(connector);
+
+	if (connector->status != connector_status_connected) {
+		ret = -ENODEV;
+		goto err_conn_put;
+	}
+
+	scdc_print_flag(m, "SCDC Supported", scdc->supported);
+	if (!scdc->supported) {
+		ret = 0;
+		goto err_conn_put;
+	}
+
+	scdc_print_flag(m, "Sink Read Request Capable", scdc->read_request);
+	scdc_print_flag(m, "Scrambling Supported", scdc->scrambling.supported);
+	scdc_print_flag(m, "Low Rate Scrambling Supported", scdc->scrambling.low_rates);
+
+	ret = drm_scdc_read_state(connector, st);
+	drm_connector_put(connector);
+	if (ret)
+		return ret;
+
+	scdc_print_flag(m, "Scrambling Enabled", st->scrambling_enabled);
+	scdc_print_flag(m, "Scrambling Detected", st->scrambling_detected);
+
+	if (st->tmds_bclk_x40)
+		scdc_print_str(m, "TMDS Bit Clock Ratio", "1/40");
+	else
+		scdc_print_str(m, "TMDS Bit Clock Ratio", "1/10");
+
+	scdc_print_flag(m, "Clock Detected", st->stf.status0.flags.clock_detected);
+	scdc_print_flag(m, "Channel 0 Locked", st->stf.status0.flags.ch0_locked);
+	scdc_print_flag(m, "Channel 1 Locked", st->stf.status0.flags.ch1_locked);
+	scdc_print_flag(m, "Channel 2 Locked", st->stf.status0.flags.ch2_locked);
+
+	scdc_print_dec(m, "Channel 0 Errors", st->error_count[0]);
+	scdc_print_dec(m, "Channel 1 Errors", st->error_count[1]);
+	scdc_print_dec(m, "Channel 2 Errors", st->error_count[2]);
+
+	return 0;
+
+err_conn_put:
+	drm_connector_put(connector);
+
+	return ret;
+}
+DEFINE_SHOW_ATTRIBUTE(scdc_status);
+
+/**
+ * drm_scdc_debugfs_init - Initialize scdc files in connector debugfs
+ * @connector: pointer to &struct drm_connector to operate on
+ * @root: debugfs &struct dentry for the debugfs root of @connector
+ *
+ * Creates SCDC-related debugfs files for @connector. Must be called after
+ * @root is already created.
+ */
+void drm_scdc_debugfs_init(struct drm_connector *connector, struct dentry *root)
+{
+	struct scdc_debugfs_priv *priv;
+
+	if (!root || !connector)
+		return;
+
+	priv = drmm_kzalloc(connector->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return;
+
+	priv->connector = connector;
+
+	debugfs_create_file("scdc_status", 0444, root, priv, &scdc_status_fops);
+}
+EXPORT_SYMBOL(drm_scdc_debugfs_init);
diff --git a/include/drm/display/drm_scdc_helper.h b/include/drm/display/drm_scdc_helper.h
index 34600476a1b9..b597e24a2976 100644
--- a/include/drm/display/drm_scdc_helper.h
+++ b/include/drm/display/drm_scdc_helper.h
@@ -30,6 +30,55 @@
 
 struct drm_connector;
 struct i2c_adapter;
+struct dentry;
+
+struct drm_scdc_status_flags {
+	/* Status Register 0 */
+	union {
+		struct {
+			bool clock_detected : 1;
+			bool ch0_locked : 1;
+			bool ch1_locked : 1;
+			bool ch2_locked : 1;
+			bool rsvd_4 : 1;
+			bool rsvd_5 : 1;
+			bool rsvd_6 : 1;
+			bool rsvd_7 : 1;
+		} flags __packed;
+		u8 data;
+	} status0;
+
+	/* Status Register 1 */
+	union {
+		struct {
+			bool rsvd_0 : 1;
+			bool rsvd_1 : 1;
+			bool rsvd_2 : 1;
+			bool rsvd_3 : 1;
+			bool rsvd_4 : 1;
+			bool rsvd_5 : 1;
+			bool rsvd_6 : 1;
+			bool rsvd_7 : 1;
+		} flags __packed;
+		u8 data;
+	} status1;
+};
+
+struct drm_scdc_state {
+	/** @stf: contents of the status flag registers */
+	struct drm_scdc_status_flags stf;
+	/** @scramling_enabled: true if TMDS scrambling is on */
+	bool scrambling_enabled;
+	/** @scrambling_detected: true if the sink actually detected scrambling */
+	bool scrambling_detected;
+	/**
+	 * @tmds_bclk_x40: true if TMDS bit period is 1/40th of the TMDS
+	 * clock period, false if it's 1/10th of the clock period.
+	 */
+	bool tmds_bclk_x40;
+	/** @error_count: character error counts for each channel */
+	u16 error_count[3];
+};
 
 ssize_t drm_scdc_read(struct i2c_adapter *adapter, u8 offset, void *buffer,
 		      size_t size);
@@ -77,4 +126,11 @@ bool drm_scdc_get_scrambling_status(struct drm_connector *connector);
 bool drm_scdc_set_scrambling(struct drm_connector *connector, bool enable);
 bool drm_scdc_set_high_tmds_clock_ratio(struct drm_connector *connector, bool set);
 
+int drm_scdc_read_status0_flags(struct drm_connector *connector,
+				struct drm_scdc_status_flags *flags);
+int drm_scdc_read_state(struct drm_connector *connector,
+			struct drm_scdc_state *state);
+int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[3]);
+void drm_scdc_debugfs_init(struct drm_connector *connector, struct dentry *root);
+
 #endif

-- 
2.53.0

[PATCH 2/2] drm/display: bridge_connector: init scdc debugfs for HDMI

[Nicolas Frattaroli]

On drm_bridge_connectors that contain an HDMI bridge, initialise the
SCDC debugfs entry under the connector's debugfs root.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
 drivers/gpu/drm/display/drm_bridge_connector.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_bridge_connector.c b/drivers/gpu/drm/display/drm_bridge_connector.c
index 39cc18f78eda..3e4e6bead321 100644
--- a/drivers/gpu/drm/display/drm_bridge_connector.c
+++ b/drivers/gpu/drm/display/drm_bridge_connector.c
@@ -25,6 +25,7 @@
 #include <drm/display/drm_hdmi_cec_helper.h>
 #include <drm/display/drm_hdmi_helper.h>
 #include <drm/display/drm_hdmi_state_helper.h>
+#include <drm/display/drm_scdc_helper.h>
 
 /**
  * DOC: overview
@@ -263,6 +264,9 @@ static void drm_bridge_connector_debugfs_init(struct drm_connector *connector,
 		if (bridge->funcs->debugfs_init)
 			bridge->funcs->debugfs_init(bridge, root);
 	}
+
+	if (bridge_connector->bridge_hdmi)
+		drm_scdc_debugfs_init(connector, root);
 }
 
 static void drm_bridge_connector_reset(struct drm_connector *connector)

-- 
2.53.0

Re: [PATCH 0/2] Add SCDC information to connector debugfs

[Nicolas Frattaroli]

On Wednesday, 15 April 2026 17:56:41 Central European Summer Time 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.
> 
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> ---
> Nicolas Frattaroli (2):
>       drm/scdc-helper: Add scdc_status debugfs entry
>       drm/display: bridge_connector: init scdc debugfs for HDMI
> 
>  drivers/gpu/drm/display/drm_bridge_connector.c |   4 +
>  drivers/gpu/drm/display/drm_scdc_helper.c      | 229 +++++++++++++++++++++++++
>  include/drm/display/drm_scdc_helper.h          |  56 ++++++
>  3 files changed, 289 insertions(+)
> ---
> base-commit: 4c59525db84aca677fd9f052e662912ad9d88448
> change-id: 20260413-scdc-link-health-89326013d96c
> 
> Best regards,
> --  
> Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> 
> 

Is anyone interested in picking this up for review? It has been over
three weeks with no response so far, and I'm getting worried this
will just be forgotten about.

Thanks,
Nicolas Frattaroli