[PATCH RFC] drm/amd/display: Disable CRTC on MST connector unregister

[David Medina via B4 Relay <devnull+b2amedina.gmail.com@kernel.org>]

From: David Medina <b2amedina@gmail.com>

amdgpu_dm_mst_connector_early_unregister() releases the connector's
sink but leaves the CRTC it was driving active and its MST encoder
assigned. Because the connector is unregistered in the same step,
userspace has no chance to disable the CRTC before it is gone. When the
same display is re-plugged a new MST connector is created, and the
re-commit fails in drm_atomic_helper_check_modeset() because the stale
connector still owns the encoder (handle_conflicting_encoders), leaving
the display dark until a full modeset (e.g. a session restart) tears
everything down.

Disable the CRTC from the driver when the connector is torn down, so the
MST encoder is released and the DC stream dropped. Reuse the
drm_atomic_helper_disable_all() pattern: deactivate the CRTC, clear the
mode, and disconnect the connector.

Reproduced 100% of the time by unplugging and re-plugging the Thunderbolt
dock driving the LG ULTRAWIDE: on re-plug the display stays dark and
dmesg shows drm_atomic_helper_check_modeset() failing with an encoder
conflict (handle_conflicting_encoders). Verified over four unplug/replug
cycles with this patch applied.

I am not certain that forcing a modeset from early_unregister() is the
right approach, and the disable here is intentionally minimal: it
deactivates the CRTC and disconnects the connector but does not
add/detach the CRTC's affected planes the way
drm_atomic_helper_disable_all() does. The minimal version resolves the
failure in testing; I would welcome guidance on whether the full
affected-planes teardown (or a different location) is required.

Fixes: a1b27e99229a ("drm/amd/display: Implement MST Aux device registration")
Signed-off-by: David Medina <b2amedina@gmail.com>
---
 .../amd/display/amdgpu_dm/amdgpu_dm_mst_types.c    | 53 ++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
index 045a7f88b754..24b146068739 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
@@ -29,6 +29,7 @@
 #include <drm/display/drm_dp_mst_helper.h>
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
+#include <drm/drm_atomic_uapi.h>
 #include <drm/drm_fixed.h>
 #include <drm/drm_edid.h>
 #include "dm_services.h"
@@ -227,6 +228,7 @@ amdgpu_dm_mst_connector_early_unregister(struct drm_connector *connector)
 	struct amdgpu_dm_connector *root = aconnector->mst_root;
 	struct dc_link *dc_link = aconnector->dc_link;
 	struct dc_sink *dc_sink = aconnector->dc_sink;
+	struct drm_crtc *crtc = connector->state ? connector->state->crtc : NULL;
 
 	drm_dp_mst_connector_early_unregister(connector, port);
 
@@ -250,6 +252,57 @@ amdgpu_dm_mst_connector_early_unregister(struct drm_connector *connector)
 
 	aconnector->mst_status = MST_STATUS_DEFAULT;
 	drm_modeset_unlock(&root->mst_mgr.base.lock);
+
+	/*
+	 * The connector is being removed from the MST topology. If a CRTC is
+	 * still driving it, force a modeset that disables the CRTC so that the
+	 * MST encoder is released and the DC stream dropped. Otherwise, on a
+	 * subsequent re-plug the stale CRTC/encoder assignment triggers an
+	 * encoder conflict in drm_atomic_helper_check_modeset() and the display
+	 * stays dark.
+	 */
+	if (crtc) {
+		struct drm_atomic_commit *state;
+		struct drm_connector_state *conn_state;
+		struct drm_crtc_state *crtc_state;
+		struct drm_modeset_acquire_ctx ctx;
+		int ret;
+
+		drm_modeset_acquire_init(&ctx, 0);
+retry:
+		state = drm_atomic_commit_alloc(connector->dev);
+		if (!state)
+			goto out;
+		state->acquire_ctx = &ctx;
+
+		crtc_state = drm_atomic_get_crtc_state(state, crtc);
+		ret = PTR_ERR_OR_ZERO(crtc_state);
+		if (!ret)
+			crtc_state->active = false;
+		if (!ret)
+			ret = drm_atomic_set_mode_prop_for_crtc(crtc_state, NULL);
+		if (!ret) {
+			conn_state = drm_atomic_get_connector_state(state, connector);
+			ret = PTR_ERR_OR_ZERO(conn_state);
+		}
+		if (!ret)
+			ret = drm_atomic_set_crtc_for_connector(conn_state, NULL);
+		if (!ret)
+			ret = drm_atomic_commit(state);
+
+		drm_atomic_commit_put(state);
+		if (ret == -EDEADLK) {
+			drm_modeset_backoff(&ctx);
+			goto retry;
+		}
+		if (ret)
+			drm_err(connector->dev,
+				"DM_MST: failed to disable CRTC for removed connector %s (%d)\n",
+				connector->name, ret);
+out:
+		drm_modeset_drop_locks(&ctx);
+		drm_modeset_acquire_fini(&ctx);
+	}
 }
 
 static const struct drm_connector_funcs dm_dp_mst_connector_funcs = {

---
base-commit: 551c722f40809618230001baccf219193e22fc5a
change-id: 20260930-amd-mst-teardown-cab8a3731596

Best regards,
--  
David Medina <b2amedina@gmail.com>