[PATCH v8 07/39] drm/display: hdmi: Add HDMI 2.0 scrambling management helpers
Cristian Ciocaltea
cristian.ciocaltea at collabora.com
Thu Jul 2 07:46:20 PDT 2026
Add helpers to manage the full lifecycle of HDMI 2.0 SCDC scrambling on
both source and sink sides:
- drm_hdmi_mode_needs_scrambling(): checks whether a mode, bits per
component and output format combination requires scrambling, which is
needed when the resulting TMDS character rate exceeds the HDMI 1.3
maximum of 340 MHz
- drm_connector_hdmi_enable_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_connector_hdmi_disable_scrambling(): tears down scrambling on both
sides and cancels the monitoring work
- drm_connector_hdmi_sync_scdc(): 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_hdmi_helper.c | 260 ++++++++++++++++++++++++++++++
include/drm/display/drm_hdmi_helper.h | 15 ++
2 files changed, 275 insertions(+)
diff --git a/drivers/gpu/drm/display/drm_hdmi_helper.c b/drivers/gpu/drm/display/drm_hdmi_helper.c
index 5cb0b033b171..7c96cccf683d 100644
--- a/drivers/gpu/drm/display/drm_hdmi_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_helper.c
@@ -4,12 +4,20 @@
#include <linux/module.h>
#include <drm/display/drm_hdmi_helper.h>
+#include <drm/display/drm_scdc_helper.h>
+
+#include <drm/drm_atomic.h>
+#include <drm/drm_atomic_helper.h>
#include <drm/drm_connector.h>
+#include <drm/drm_crtc.h>
#include <drm/drm_edid.h>
#include <drm/drm_modes.h>
+#include <drm/drm_modeset_lock.h>
#include <drm/drm_print.h>
#include <drm/drm_property.h>
+#define DRM_HDMI_SCDC_POLL_DELAY_MS 1000
+
static inline bool is_eotf_supported(u8 output_eotf, u8 sink_eotf)
{
return sink_eotf & BIT(output_eotf);
@@ -426,3 +434,255 @@ drm_hdmi_acr_get_n_cts(unsigned long long tmds_char_rate,
*out_cts = cts;
}
EXPORT_SYMBOL(drm_hdmi_acr_get_n_cts);
+
+/**
+ * drm_hdmi_mode_needs_scrambling() - Check if an HDMI mode requires scrambling
+ * @mode: Display mode to check
+ * @bpc: Bits per color component
+ * @fmt: Output pixel format
+ *
+ * Computes the TMDS character rate for the given mode, bits per component and
+ * output format, and checks whether it exceeds the HDMI 1.3 maximum TMDS
+ * character rate of 340 MHz.
+ *
+ * Returns:
+ * true if the mode requires scrambling, false otherwise.
+ */
+bool drm_hdmi_mode_needs_scrambling(const struct drm_display_mode *mode,
+ unsigned int bpc,
+ enum drm_output_color_format fmt)
+{
+ unsigned long long tmds_char_rate;
+
+ tmds_char_rate = drm_hdmi_compute_mode_clock(mode, bpc, fmt);
+
+ return tmds_char_rate > HDMI_1_3_TMDS_CHAR_RATE_MAX_HZ;
+}
+EXPORT_SYMBOL(drm_hdmi_mode_needs_scrambling);
+
+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(DRM_HDMI_SCDC_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 bool drm_scdc_sink_supports_scrambling(struct drm_connector *connector)
+{
+ const struct drm_display_info *info = &connector->display_info;
+
+ return info->is_hdmi &&
+ info->hdmi.scdc.supported &&
+ info->hdmi.scdc.scrambling.supported;
+}
+
+/**
+ * drm_connector_hdmi_enable_scrambling - enable scrambling and monitor SCDC status
+ * @connector: connector
+ * @conn_state: connector state
+ *
+ * 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_connector_hdmi_enable_scrambling(struct drm_connector *connector,
+ const struct drm_connector_state *conn_state)
+{
+ struct drm_connector_hdmi *hdmi = &connector->hdmi;
+ struct drm_device *dev = connector->dev;
+ int ret;
+
+ if (!conn_state)
+ return -EINVAL;
+
+ if (!conn_state->hdmi.scrambler_needed)
+ return 0;
+
+ if (!hdmi->scrambler_supported) {
+ drm_dbg_kms(dev, "Source doesn't support scrambling.\n");
+ return -EINVAL;
+ }
+
+ if (!drm_scdc_sink_supports_scrambling(connector)) {
+ drm_dbg_kms(dev, "Sink doesn't support scrambling.\n");
+ return -EINVAL;
+ }
+
+ drm_dbg_kms(dev, "Enabling scrambling\n");
+
+ 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_connector_hdmi_enable_scrambling);
+
+/**
+ * drm_connector_hdmi_disable_scrambling - disable scrambling and SCDC monitoring
+ * @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_connector_hdmi_disable_scrambling(struct drm_connector *connector)
+{
+ struct drm_connector_hdmi *hdmi = &connector->hdmi;
+ struct drm_device *dev = connector->dev;
+
+ if (!READ_ONCE(hdmi->scrambler_enabled))
+ return 0;
+
+ drm_dbg_kms(dev, "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 &&
+ drm_scdc_sink_supports_scrambling(connector)) {
+ drm_scdc_set_scrambling(connector, false);
+ drm_scdc_set_high_tmds_clock_ratio(connector, false);
+ }
+
+ return hdmi->funcs->scrambler_disable(connector);
+}
+EXPORT_SYMBOL(drm_connector_hdmi_disable_scrambling);
+
+/**
+ * drm_connector_hdmi_sync_scdc - 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().
+ *
+ * Returns:
+ * Zero on success, an error code otherwise, including -EDEADLK.
+ */
+int drm_connector_hdmi_sync_scdc(struct drm_connector *connector, bool plugged,
+ struct drm_modeset_acquire_ctx *ctx)
+{
+ struct drm_connector_hdmi *hdmi = &connector->hdmi;
+ struct drm_device *dev = connector->dev;
+ struct drm_connector_state *conn_state;
+ struct drm_crtc *crtc;
+ u8 config;
+ int ret;
+
+ if (!connector || !plugged || !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(&connector->dev->mode_config.connection_mutex, ctx);
+ if (ret)
+ return ret;
+
+ /* TODO: Also handle HDMI 2.1 FRL link training */
+
+ if (!hdmi->scrambler_supported || !READ_ONCE(hdmi->scrambler_enabled))
+ return 0;
+
+ conn_state = connector->state;
+ if (!conn_state)
+ return 0;
+
+ crtc = connector->state->crtc;
+ if (!crtc)
+ return 0;
+
+ ret = drm_modeset_lock(&crtc->mutex, ctx);
+ if (ret)
+ return ret;
+
+ if (!crtc->state || !crtc->state->active)
+ return 0;
+
+ if (!drm_scdc_sink_supports_scrambling(connector))
+ return 0;
+
+ if (conn_state->commit &&
+ !try_wait_for_completion(&conn_state->commit->hw_done))
+ return 0;
+
+ ret = drm_scdc_readb(connector->ddc, SCDC_TMDS_CONFIG, &config);
+ if (ret) {
+ drm_dbg_kms(dev, "Failed to read TMDS config: %d\n", ret);
+ return ret;
+ }
+
+ if (config & SCDC_SCRAMBLING_ENABLE)
+ return 0;
+
+ /*
+ * HDMI 2.0 requires that one should not send scrambled data prior
+ * to configuring the sink scrambling, and that TMDS clock/data
+ * transmission should be suspended when changing the TMDS clock rate
+ * in the sink. So let's just do a full modeset here, even though some
+ * sinks would be perfectly happy if were to just reconfigure the SCDC
+ * settings on the fly.
+ *
+ * The disable/re-enable cycle triggered by the reset should call
+ * drm_connector_hdmi_enable_scrambling() during re-enable, thus
+ * properly configuring the sink before data transmission resumes.
+ */
+ drm_dbg_kms(dev, "Resetting CRTC to restore SCDC status\n");
+
+ ret = drm_atomic_helper_reset_crtc(crtc, ctx);
+ if (ret && ret != -EDEADLK)
+ drm_dbg_kms(dev, "Failed to reset CRTC: %d\n", ret);
+
+ return ret;
+}
+EXPORT_SYMBOL(drm_connector_hdmi_sync_scdc);
diff --git a/include/drm/display/drm_hdmi_helper.h b/include/drm/display/drm_hdmi_helper.h
index 9c31ed90516b..2cdd6e3bc090 100644
--- a/include/drm/display/drm_hdmi_helper.h
+++ b/include/drm/display/drm_hdmi_helper.h
@@ -8,6 +8,7 @@
struct drm_connector;
struct drm_connector_state;
struct drm_display_mode;
+struct drm_modeset_acquire_ctx;
enum drm_output_color_format;
void
@@ -35,4 +36,18 @@ drm_hdmi_acr_get_n_cts(unsigned long long tmds_char_rate,
unsigned int *out_n,
unsigned int *out_cts);
+bool
+drm_hdmi_mode_needs_scrambling(const struct drm_display_mode *mode,
+ unsigned int bpc, enum drm_output_color_format fmt);
+
+int
+drm_connector_hdmi_enable_scrambling(struct drm_connector *connector,
+ const struct drm_connector_state *conn_state);
+int
+drm_connector_hdmi_disable_scrambling(struct drm_connector *connector);
+
+int
+drm_connector_hdmi_sync_scdc(struct drm_connector *connector, bool plugged,
+ struct drm_modeset_acquire_ctx *ctx);
+
#endif
--
2.54.0
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