Fix legacy DPMS changes with MST

With the exception of modesets which would switch the DPMS state of a connector from on to off, we want to make sure that we disallow all modesets which would result in enabling a new monitor or a new mode configuration on a monitor if the connector for the display in question is no longer registered. This allows us to stop userspace from trying to enable new displays on connectors for an MST topology that were just removed from the system, without preventing userspace from disabling DPMS on those connectors.

Signed-off-by Lyude Paul <[email protected]>

Cc [email protected]

---
 drivers/gpu/drm/drm_atomic_helper.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

drivers/gpu/drm/drm_atomic_helper.c +20 -1

index 80be74df7ba6..ce2decfc6826 100644
@@ -307,6 +307,26 @@ update_connector_routing(struct drm_atomic_state *state,
 		return 0;
 	}
 
+	crtc_state = drm_atomic_get_new_crtc_state(state,
+						   new_connector_state->crtc);
+	/*
+	 * For compatibility with legacy users, we want to make sure that
+	 * we allow DPMS On->Off modesets on unregistered connectors. Modesets
+	 * which would result in anything else must be considered invalid, to
+	 * avoid turning on new displays on dead connectors.
+	 *
+	 * Since the connector can be unregistered at any point during an
+	 * atomic check or commit, this is racy. But that's OK: all we care
+	 * about is ensuring that userspace can't do anything but shut off the
+	 * display on a connector that was destroyed after it's been notified,
+	 * not before.
+	 */
+	if (!READ_ONCE(connector->registered) && crtc_state->active) {
+		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] is not registered\n",
+				 connector->base.id, connector->name);
+		return -EINVAL;
+	}
+
 	funcs = connector->helper_private;
 
 	if (funcs->atomic_best_encoder)

@@ -351,7 +371,6 @@ update_connector_routing(struct drm_atomic_state *state,
 
 	set_best_encoder(state, new_connector_state, new_encoder);
 
-	crtc_state = drm_atomic_get_new_crtc_state(state, new_connector_state->crtc);
 	crtc_state->connectors_changed = true;
 
 	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] using [ENCODER:%d:%s] on [CRTC:%d:%s]\n",

Signature · 1 line

2.17.1

As mentioned in the previous commit, we currently prevent new modesets on recently-removed MST connectors by returning no encoder from our ->best_encoder() callback once the MST port has disappeared. This is wrong however, because it prevents legacy modesetting users from being able to disable CRTCs on MST connectors after the connector's respective topology has disappeared.

So, fix this by instead by just always returning a valid encoder.

Signed-off-by Lyude Paul <[email protected]>

Cc [email protected]

Changes since v2:
- Remove usage of atomic MST helper for now, since that got replaced with a much simpler solution

Signed-off-by Lyude Paul <[email protected]>

---
 drivers/gpu/drm/nouveau/dispnv50/disp.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

drivers/gpu/drm/nouveau/dispnv50/disp.c +4 -10

index 5691dfa1db6f..63a23a80f279 100644
@@ -843,22 +843,16 @@ nv50_mstc_atomic_best_encoder(struct drm_connector *connector,
 {
 	struct nv50_head *head = nv50_head(connector_state->crtc);
 	struct nv50_mstc *mstc = nv50_mstc(connector);
-	if (mstc->port) {
-		struct nv50_mstm *mstm = mstc->mstm;
-		return &mstm->msto[head->base.index]->encoder;
-	}
-	return NULL;
+
+	return &mstc->mstm->msto[head->base.index]->encoder;
 }
 
 static struct drm_encoder *
 nv50_mstc_best_encoder(struct drm_connector *connector)
 {
 	struct nv50_mstc *mstc = nv50_mstc(connector);
-	if (mstc->port) {
-		struct nv50_mstm *mstm = mstc->mstm;
-		return &mstm->msto[0]->encoder;
-	}
-	return NULL;
+
+	return &mstc->mstm->msto[0]->encoder;
 }
 
 static enum drm_mode_status

Signature · 1 line

2.17.1

Currently we set intel_connector->mst_port to NULL to signify that the MST port has been removed from the system so that we can prevent further action on the port such as connector probes, mode probing, etc. However, we're going to need access to intel_connector->mst_port in order to fixup ->best_encoder() so that it can always return the correct encoder for an MST port to prevent legacy DPMS prop changes from failing. This should be safe, so instead keep intel_connector->mst_port always set and instead add intel_connector->mst_port_gone in order to signify whether or not the connector has disappeared from the system.

Changes since v2:
- Add a comment to mst_port_gone (Jani Nikula)
- Change mst_port_gone to a u8 instead of a bool, per the kernel bot. Apparently bool is discouraged in structs these days

Signed-off-by Lyude Paul <[email protected]>

Cc [email protected]

---
 drivers/gpu/drm/i915/intel_dp_mst.c | 14 +++++++-------
 drivers/gpu/drm/i915/intel_drv.h    |  6 ++++++
 2 files changed, 13 insertions(+), 7 deletions(-)

drivers/gpu/drm/i915/intel_dp_mst.c +0 -0

index 4ecd65375603..fcb9b87b9339 100644

drivers/gpu/drm/i915/intel_dp_mst.c +3 -3

 		return intel_connector_update_modes(connector, NULL);
-	}
 
 	edid = drm_dp_mst_get_edid(connector, &intel_dp->mst_mgr, intel_connector->port);
 	ret = intel_connector_update_modes(connector, edid);

@@ -328,9 +327,10 @@ intel_dp_mst_detect(struct drm_connector *connector, bool force)
 	struct intel_connector *intel_connector = to_intel_connector(connector);
 	struct intel_dp *intel_dp = intel_connector->mst_port;
 
-	if (!intel_dp)
+	if (intel_connector->mst_port_gone)
 		return connector_status_disconnected;
-	return drm_dp_mst_detect_port(connector, &intel_dp->mst_mgr, intel_connector->port);
+	return drm_dp_mst_detect_port(connector, &intel_dp->mst_mgr,
+				      intel_connector->port);
 }
 
 static void

From a functional pov the patch lgtm. With mst_port_gone removed and the

drivers/gpu/drm/i915/intel_dp_mst.c +2 -2

 	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)

@@ -402,7 +402,7 @@ static struct drm_encoder *intel_mst_atomic_best_encoder(struct drm_connector *c
 	struct intel_dp *intel_dp = intel_connector->mst_port;
 	struct intel_crtc *crtc = to_intel_crtc(state->crtc);
 
-	if (!intel_dp)
+	if (intel_connector->mst_port_gone)
 		return NULL;
 	return &intel_dp->mst_encoders[crtc->pipe]->base.base;
 }

@@ -514,7 +514,7 @@ static void intel_dp_destroy_mst_connector(struct drm_dp_mst_topology_mgr *mgr,
 						   connector);
 	/* prevent race with the check in ->detect */
 	drm_modeset_lock(&connector->dev->mode_config.connection_mutex, NULL);
-	intel_connector->mst_port = NULL;
+	intel_connector->mst_port_gone = true;
 	drm_modeset_unlock(&connector->dev->mode_config.connection_mutex);
 
 	drm_connector_put(connector);

drivers/gpu/drm/i915/intel_drv.h +0 -0

index 8fc61e96754f..8261d4579452 100644

drivers/gpu/drm/i915/intel_drv.h +1 -0

+
 	/* Work struct to schedule a uevent on link train failure */
 	struct work_struct modeset_retry_work;
 

Signature · 1 line

2.17.1

Since we need to be able to allow DPMS on->off prop changes after an MST port has disappeared from the system, we need to be able to make sure we can compute a config for the resulting atomic commit. Currently this is impossible when the port has disappeared, since the VCPI slot searching we try to do in intel_dp_mst_compute_config() will fail with -EINVAL.

Since the only commits we want to allow on no-longer-present MST ports are ones that shut off display hardware, we already know that no VCPI allocations are needed. So, hardcode the VCPI slot count to 0 when intel_dp_mst_compute_config() is called on an MST port that's gone.

Signed-off-by Lyude Paul <[email protected]>

Cc [email protected]

---
 drivers/gpu/drm/i915/intel_dp_mst.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

drivers/gpu/drm/i915/intel_dp_mst.c +1 -1

index fcb9b87b9339..a366f32b048a 100644
@@ -42,7 +42,7 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 		to_intel_connector(conn_state->connector);
 	struct drm_atomic_state *state = pipe_config->base.state;
 	int bpp;
-	int lane_count, slots;
+	int lane_count, slots = 0;
 	const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
 	int mst_pbn;
 	bool reduce_m_n = drm_dp_has_quirk(&intel_dp->desc,

drivers/gpu/drm/i915/intel_dp_mst.c +9 -0

+		slots = drm_dp_atomic_find_vcpi_slots(state,
+						      &intel_dp->mst_mgr,
+						      connector->port,
+						      mst_pbn);
+		if (slots < 0) {
+			DRM_DEBUG_KMS("failed finding vcpi slots:%d\n",
+				      slots);
+			return false;
+		}
 	}
 
 	intel_link_compute_m_n(bpp, lane_count,

Signature · 1 line

2.17.1

Currently, i915 appears to rely on blocking modesets on no-longer-present MSTB ports by simply returning NULL for ->best_encoder(), which in turn causes any new atomic commits that don't disable the CRTC to fail. This is wrong however, since we still want to allow userspace to disable CRTCs on no-longer-present MSTB ports by changing the DPMS state to off and this still requires that we retrieve an encoder.

So, fix this by always returning a valid encoder regardless of the state of the MST port.

Signed-off-by Lyude Paul <[email protected]>

Cc [email protected]

Changes since v1:
- Remove mst atomic helper, since this got replaced with a much simpler solution

Signed-off-by Lyude Paul <[email protected]>

---
 drivers/gpu/drm/i915/intel_dp_mst.c | 2 --
 1 file changed, 2 deletions(-)

drivers/gpu/drm/i915/intel_dp_mst.c +0 -2

index a366f32b048a..daade60c5714 100644
@@ -407,8 +407,6 @@ static struct drm_encoder *intel_mst_atomic_best_encoder(struct drm_connector *c
 	struct intel_dp *intel_dp = intel_connector->mst_port;
 	struct intel_crtc *crtc = to_intel_crtc(state->crtc);
 
-	if (intel_connector->mst_port_gone)
-		return NULL;
 	return &intel_dp->mst_encoders[crtc->pipe]->base.base;
 }
 

Signature · 1 line

2.17.1