On 04/06/2026 17:52, Nicolas Frattaroli wrote:
> 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 <[email protected]>
> ---
> drivers/gpu/drm/display/drm_scdc_helper.c | 184
> ++++++++++++++++++++++++++++++
> include/drm/display/drm_scdc_helper.h | 32 ++++++
> 2 files changed, 216 insertions(+)
>
> diff --git a/drivers/gpu/drm/display/drm_scdc_helper.c
> b/drivers/gpu/drm/display/drm_scdc_helper.c
> index 8403f2390ab6..d98bcb8ce748 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 +284,179 @@ bool drm_scdc_set_high_tmds_clock_ratio(struct
> drm_connector *connector,
> return true;
> }
> EXPORT_SYMBOL(drm_scdc_set_high_tmds_clock_ratio);
> +
> +static void
> +drm_scdc_parse_status0_flags(u8 val, struct drm_scdc_status_flags *flags)
> +{
> + flags->clock_detected = val & SCDC_CLOCK_DETECT;
> + flags->ch0_locked = val & SCDC_CH0_LOCK;
> + flags->ch1_locked = val & SCDC_CH1_LOCK;
> + flags->ch2_locked = val & SCDC_CH2_LOCK;
> +}
> +
> +static int drm_scdc_parse_error_counters(const u8 scdc[256], u16 counter[3])
> +{
> + u8 sum = 0;
> + int i;
> +
> + for (i = SCDC_ERR_DET_0_L; i <= SCDC_ERR_DET_CHECKSUM ; i++)
> + sum = wrapping_add(u8, sum, scdc[i]);
> +
> + if (sum)
> + return -EPROTO;
> +
> + for (i = 0; i < 3; i++) {
> + if (scdc[SCDC_ERR_DET_0_H + i * 2] & SCDC_CHANNEL_VALID)
> + counter[i] = (scdc[SCDC_ERR_DET_0_H + i * 2] &
> + ~SCDC_CHANNEL_VALID) << 8 |
> + scdc[SCDC_ERR_DET_0_L + i * 2];
> + else
> + counter[i] = 0;
> + }
> +
> + return 0;
> +}
> +
> +/**
> + * 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 the entire 256 byte SCDC state and parses it.
> + *
> + * Returns: %0 on success, negative errno on failure.
> + */
> +int drm_scdc_read_state(struct drm_connector *connector, struct
> drm_scdc_state *state)
> +{
> + struct i2c_adapter *ddc;
> + struct drm_scdc *scdc;
> + u8 *buf = state->scdc;
> + int ret;
> +
> + if (!state || !connector)
> + return -ENODEV;
> +
> + scdc = &connector->display_info.hdmi.scdc;
> + ddc = connector->ddc;
> +
> + if (!scdc->supported)
> + return -EOPNOTSUPP;
> +
> + /* Read in 128-byte chunks, to work around DP<->HDMI converters with
> issues. */
> + ret = drm_scdc_read(ddc, 0, buf, 128);
> + if (ret)
> + return ret;
> +
> + ret = drm_scdc_read(ddc, 127, &buf[127], 128);
Huh? Shouldn't this be: ret = drm_scdc_read(ddc, 128, &buf[128], 128);
I'm unable to test this series at the moment: I'm pretty sure I have
suitable hardware, but I don't have access to that for another week.
Regards,
Hans
> + if (ret)
> + return ret;
> +
> + state->scrambling_enabled = buf[SCDC_TMDS_CONFIG] &
> SCDC_SCRAMBLING_ENABLE;
> + state->tmds_bclk_x40 = buf[SCDC_TMDS_CONFIG] &
> SCDC_TMDS_BIT_CLOCK_RATIO_BY_40;
> +
> + state->scrambling_detected = buf[SCDC_SCRAMBLER_STATUS] &
> SCDC_SCRAMBLING_STATUS;
> +
> + drm_scdc_parse_status0_flags(buf[SCDC_STATUS_FLAGS_0], &state->stf);
> + ret = drm_scdc_parse_error_counters(buf, 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 i, ret;
> +
> + drm_connector_get(connector);
> +
> + if (connector->status != connector_status_connected) {
> + ret = -ENODEV;
> + goto err_conn_put;
> + }
> +
> + if (scdc->supported) {
> + ret = drm_scdc_read_state(connector, st);
> + if (ret)
> + goto err_conn_put;
> +
> + for (i = 0; i < ARRAY_SIZE(st->scdc); i += 16)
> + seq_printf(m, "%*ph\n", 16, &st->scdc[i]);
> +
> + seq_puts(m, "\n----------------\n\n");
> + }
> +
> + 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);
> +
> + drm_connector_put(connector);
> +
> + 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.clock_detected);
> + scdc_print_flag(m, "Channel 0 Locked", st->stf.ch0_locked);
> + scdc_print_flag(m, "Channel 1 Locked", st->stf.ch1_locked);
> + scdc_print_flag(m, "Channel 2 Locked", st->stf.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 e9ccaeba56dd..e0b79d79e1ff 100644
> --- a/include/drm/display/drm_scdc_helper.h
> +++ b/include/drm/display/drm_scdc_helper.h
> @@ -30,6 +30,34 @@
>
> struct drm_connector;
> struct i2c_adapter;
> +struct dentry;
> +
> +struct drm_scdc_status_flags {
> + /* Status Register 0 */
> + bool clock_detected;
> + bool ch0_locked;
> + bool ch1_locked;
> + bool ch2_locked;
> +};
> +
> +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];
> +
> + /** @scdc: raw SCDC data buffer */
> + u8 scdc[256];
> +};
>
> int drm_scdc_read(struct i2c_adapter *adapter, u8 offset, void *buffer,
> size_t size);
> @@ -77,4 +105,8 @@ 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_state(struct drm_connector *connector,
> + struct drm_scdc_state *state);
> +void drm_scdc_debugfs_init(struct drm_connector *connector, struct dentry
> *root);
> +
> #endif
>
