[PATCH v7 04/30] drm/display: scdc_helper: Add HDMI 2.0 scrambling management helpers
Cristian Ciocaltea
cristian.ciocaltea at collabora.com
Mon Jun 1 15:44:04 PDT 2026
Add helpers to manage the full lifecycle of HDMI 2.0 SCDC scrambling on
both source and sink sides:
- drm_scdc_start_scrambling(): configures SCDC scrambling and high TMDS
clock ratio and starts a periodic work item that monitors the sink's
SCDC scrambling status, retrying setup when the sink loses state
- drm_scdc_stop_scrambling(): tears down scrambling on both sides and
cancels the monitoring work
- drm_scdc_sync_status(): triggers a CRTC reset on reconnection to
restore SCDC state lost during sink disconnects within an active
display pipeline
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea at collabora.com>
---
drivers/gpu/drm/display/drm_scdc_helper.c | 245 +++++++++++++++++++++++++++++-
include/drm/display/drm_scdc_helper.h | 6 +-
2 files changed, 244 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/display/drm_scdc_helper.c b/drivers/gpu/drm/display/drm_scdc_helper.c
index cb6632346aad..f71728e5ca81 100644
--- a/drivers/gpu/drm/display/drm_scdc_helper.c
+++ b/drivers/gpu/drm/display/drm_scdc_helper.c
@@ -21,15 +21,22 @@
* DEALINGS IN THE SOFTWARE.
*/
+#include <linux/delay.h>
#include <linux/export.h>
#include <linux/i2c.h>
+#include <linux/minmax.h>
#include <linux/slab.h>
-#include <linux/delay.h>
+
+#include <drm/drm_atomic.h>
+#include <drm/drm_atomic_helper.h>
+#include <drm/drm_bridge_helper.h>
+#include <drm/drm_connector.h>
+#include <drm/drm_crtc.h>
+#include <drm/drm_device.h>
+#include <drm/drm_modeset_lock.h>
+#include <drm/drm_print.h>
#include <drm/display/drm_scdc_helper.h>
-#include <drm/drm_connector.h>
-#include <drm/drm_device.h>
-#include <drm/drm_print.h>
/**
* DOC: scdc helpers
@@ -50,10 +57,14 @@
* has to track the connector status changes using interrupts and
* restore the SCDC status. The typical solution for this is to trigger an
* empty modeset in drm_connector_helper_funcs.detect_ctx(), like what vc4 does
- * in vc4_hdmi_reset_link().
+ * in vc4_hdmi_reset_link(). Alternatively, use the HDMI connector framework
+ * which ensures drm_scdc_sync_status() is called in the context of
+ * drm_atomic_helper_connector_hdmi_hotplug_ctx().
*/
-#define SCDC_I2C_SLAVE_ADDRESS 0x54
+#define SCDC_I2C_SLAVE_ADDRESS 0x54
+#define SCDC_MAX_SOURCE_VERSION 0x1
+#define SCDC_STATUS_POLL_DELAY_MS 1000
#define drm_scdc_dbg(connector, fmt, ...) \
drm_dbg_kms((connector)->dev, "[CONNECTOR:%d:%s] " fmt, \
@@ -270,3 +281,225 @@ bool drm_scdc_set_high_tmds_clock_ratio(struct drm_connector *connector,
return true;
}
EXPORT_SYMBOL(drm_scdc_set_high_tmds_clock_ratio);
+
+static int drm_scdc_try_scrambling_setup(struct drm_connector *connector)
+{
+ bool done;
+
+ done = drm_scdc_set_high_tmds_clock_ratio(connector, true);
+ if (!done)
+ return -EIO;
+
+ done = drm_scdc_set_scrambling(connector, true);
+ if (!done)
+ return -EIO;
+
+ if (READ_ONCE(connector->hdmi.scrambler_enabled))
+ schedule_delayed_work(&connector->hdmi.scdc_work,
+ msecs_to_jiffies(SCDC_STATUS_POLL_DELAY_MS));
+
+ return 0;
+}
+
+static void drm_scdc_monitor_scrambler(struct drm_connector *connector)
+{
+ if (READ_ONCE(connector->hdmi.scrambler_enabled) &&
+ !drm_scdc_get_scrambling_status(connector))
+ drm_scdc_try_scrambling_setup(connector);
+}
+
+static int drm_scdc_reset_crtc(struct drm_connector *connector,
+ struct drm_modeset_acquire_ctx *ctx)
+{
+ struct drm_device *dev = connector->dev;
+ struct drm_crtc *crtc;
+ u8 config;
+ int ret;
+
+ if (!ctx)
+ return 0;
+
+ /*
+ * This is normally part of .detect_ctx() call path, which already holds
+ * connection_mutex through @ctx. However, re-acquiring it with the
+ * same context is a no-op and makes the helper safe under any caller.
+ */
+ ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
+ if (ret)
+ return ret;
+
+ if (!connector->state)
+ return 0;
+
+ crtc = connector->state->crtc;
+ if (!crtc)
+ return 0;
+
+ ret = drm_scdc_readb(connector->ddc, SCDC_TMDS_CONFIG, &config);
+ if (ret) {
+ drm_scdc_dbg(connector, "Failed to read TMDS config: %d\n", ret);
+ return ret;
+ }
+
+ if ((config & SCDC_SCRAMBLING_ENABLE) &&
+ (config & SCDC_TMDS_BIT_CLOCK_RATIO_BY_40))
+ return 0;
+
+ /*
+ * Reset the CRTC to suspend TMDS transmission, conforming to HDMI 2.0
+ * spec which requires scrambled data not to be sent before the sink is
+ * configured, and TMDS clock to be suspended while changing the clock
+ * ratio. The disable/re-enable cycle triggered by the reset should
+ * call drm_scdc_start_scrambling() during re-enable, properly
+ * configuring the sink before data transmission resumes.
+ */
+
+ drm_scdc_dbg(connector, "Resetting CRTC to restore SCDC status\n");
+
+ ret = drm_atomic_helper_reset_crtc(crtc, ctx);
+ if (ret && ret != -EDEADLK)
+ drm_scdc_dbg(connector, "Failed to reset CRTC: %d\n", ret);
+
+ return ret;
+}
+
+/**
+ * drm_scdc_start_scrambling - activate scrambling and monitor SCDC status
+ * @connector: connector
+ *
+ * Enables scrambling and high TMDS clock ratio on both source and sink sides.
+ * Additionally, use a delayed work item to monitor the scrambling status on
+ * the sink side and retry the operation, as some displays refuse to set the
+ * scrambling bit right away.
+ *
+ * Returns:
+ * Zero if scrambling is set successfully, an error code otherwise.
+ */
+int drm_scdc_start_scrambling(struct drm_connector *connector)
+{
+ struct drm_display_info *info = &connector->display_info;
+ struct drm_connector_hdmi *hdmi = &connector->hdmi;
+ int ret;
+ u8 ver;
+
+ if (!hdmi->scrambler_supported) {
+ drm_scdc_dbg(connector, "Scrambler not supported, bailing.\n");
+ return -EINVAL;
+ }
+
+ if (!info->is_hdmi ||
+ !info->hdmi.scdc.supported ||
+ !info->hdmi.scdc.scrambling.supported) {
+ drm_scdc_dbg(connector, "Sink doesn't support scrambling.\n");
+ return -EINVAL;
+ }
+
+ drm_scdc_dbg(connector, "Enabling scrambling\n");
+
+ ret = drm_scdc_readb(connector->ddc, SCDC_SINK_VERSION, &ver);
+ if (ret) {
+ drm_scdc_dbg(connector, "Failed to read SCDC_SINK_VERSION: %d\n", ret);
+ return ret;
+ }
+
+ ret = drm_scdc_writeb(connector->ddc, SCDC_SOURCE_VERSION,
+ min_t(u8, ver, SCDC_MAX_SOURCE_VERSION));
+ if (ret) {
+ drm_scdc_dbg(connector, "Failed to write SCDC_SOURCE_VERSION: %d\n", ret);
+ return ret;
+ }
+
+ hdmi->scdc_cb = drm_scdc_monitor_scrambler;
+ WRITE_ONCE(hdmi->scrambler_enabled, true);
+
+ ret = drm_scdc_try_scrambling_setup(connector);
+ if (!ret)
+ ret = hdmi->funcs->scrambler_enable(connector);
+
+ if (ret) {
+ WRITE_ONCE(hdmi->scrambler_enabled, false);
+ cancel_delayed_work_sync(&hdmi->scdc_work);
+ hdmi->scdc_cb = NULL;
+
+ drm_scdc_set_scrambling(connector, false);
+ drm_scdc_set_high_tmds_clock_ratio(connector, false);
+ }
+
+ return ret;
+}
+EXPORT_SYMBOL(drm_scdc_start_scrambling);
+
+/**
+ * drm_scdc_stop_scrambling - deactivate scrambling and SCDC status monitor
+ * @connector: connector
+ *
+ * Disables scrambling and high TMDS clock ratio on both source and sink sides.
+ * Also cancels the SCDC status monitoring work item, if it is still pending.
+ *
+ * Returns:
+ * Zero if scrambling is reset successfully, an error code otherwise.
+ */
+int drm_scdc_stop_scrambling(struct drm_connector *connector)
+{
+ struct drm_display_info *info = &connector->display_info;
+ struct drm_connector_hdmi *hdmi = &connector->hdmi;
+
+ if (!hdmi->scrambler_supported) {
+ drm_scdc_dbg(connector, "Scrambler not supported, bailing.\n");
+ return -EINVAL;
+ }
+
+ if (!READ_ONCE(hdmi->scrambler_enabled))
+ return 0;
+
+ drm_scdc_dbg(connector, "Disabling scrambling\n");
+
+ WRITE_ONCE(hdmi->scrambler_enabled, false);
+ cancel_delayed_work_sync(&hdmi->scdc_work);
+ hdmi->scdc_cb = NULL;
+
+ if (connector->status == connector_status_connected &&
+ info->is_hdmi && info->hdmi.scdc.supported &&
+ info->hdmi.scdc.scrambling.supported) {
+ drm_scdc_set_scrambling(connector, false);
+ drm_scdc_set_high_tmds_clock_ratio(connector, false);
+ }
+
+ return hdmi->funcs->scrambler_disable(connector);
+}
+EXPORT_SYMBOL(drm_scdc_stop_scrambling);
+
+/**
+ * drm_scdc_sync_status - resync the sink-side SCDC upon reconnect
+ * @connector: connector
+ * @plugged: connector plugged status event
+ * @ctx: initialized lock acquisition context
+ *
+ * When receiving hotplug disconnect/reconnect event, while the display is
+ * still active (CRTC enabled), the SCDC status on the sink side is reset
+ * and must be explicitly restored.
+ *
+ * The typical solution for this is to trigger an empty modeset in
+ * drm_connector_helper_funcs.detect_ctx(), which is what this helper does
+ * by triggering a CRTC reset on reconnection.
+ *
+ * When making use of the HDMI connector framework, this is automatically
+ * triggered via drm_atomic_helper_connector_hdmi_hotplug_ctx().
+ *
+ * Returns:
+ * Zero on success, an error code otherwise, including -EDEADLK.
+ */
+int drm_scdc_sync_status(struct drm_connector *connector, bool plugged,
+ struct drm_modeset_acquire_ctx *ctx)
+{
+ struct drm_connector_hdmi *hdmi = &connector->hdmi;
+
+ if (hdmi->scrambler_supported && plugged &&
+ READ_ONCE(hdmi->scrambler_enabled))
+ return drm_scdc_reset_crtc(connector, ctx);
+
+ /* TODO: Also handle HDMI 2.1 FRL link training */
+
+ return 0;
+}
+EXPORT_SYMBOL(drm_scdc_sync_status);
diff --git a/include/drm/display/drm_scdc_helper.h b/include/drm/display/drm_scdc_helper.h
index 34600476a1b9..5d9a37bbb362 100644
--- a/include/drm/display/drm_scdc_helper.h
+++ b/include/drm/display/drm_scdc_helper.h
@@ -29,6 +29,7 @@
#include <drm/display/drm_scdc.h>
struct drm_connector;
+struct drm_modeset_acquire_ctx;
struct i2c_adapter;
ssize_t drm_scdc_read(struct i2c_adapter *adapter, u8 offset, void *buffer,
@@ -76,5 +77,8 @@ 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_start_scrambling(struct drm_connector *connector);
+int drm_scdc_stop_scrambling(struct drm_connector *connector);
+int drm_scdc_sync_status(struct drm_connector *connector, bool plugged,
+ struct drm_modeset_acquire_ctx *ctx);
#endif
--
2.54.0
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