drm/dp_mst: Protect drm_dp_mst_port members with locking

This is a complicated one. Essentially, there's currently a problem in the MST
core that hasn't really caused any issues that we're aware of (emphasis on "that
we're aware of"): locking.

When we go through and probe the link addresses and path resources in a
topology, we hold no locks when updating ports with said information. The
members I'm referring to in particular are:

- ldps
- ddps
- mcs
- pdt
- dpcd_rev
- num_sdp_streams
- num_sdp_stream_sinks
- available_pbn
- input
- connector

Now that we're handling UP requests asynchronously and will be using some of
the struct members mentioned above in atomic modesetting in the future for
features such as PBN validation, this is going to become a lot more important.
As well, the next few commits that prepare us for and introduce suspend/resume
reprobing will also need clear locking in order to prevent from additional
racing hilarities that we never could have hit in the past.

So, let's solve this issue by using &mgr->base.lock, the modesetting
lock which currently only protects &mgr->base.state. This works
perfectly because it allows us to avoid blocking connection_mutex
unnecessarily, and we can grab this in connector detection paths since
it's a ww mutex. We start by having drm_dp_mst_handle_up_req() hold this
when updating ports. For drm_dp_mst_handle_link_address_port() things
are a bit more complicated. As I've learned the hard way, we can grab
&mgr->lock.base for everything except for port->connector. See, our
normal driver probing paths end up generating this rather obvious
lockdep chain:

&drm->mode_config.mutex
  -> crtc_ww_class_mutex/crtc_ww_class_acquire
    -> &connector->mutex

However, sysfs grabs &drm->mode_config.mutex in order to protect itself
from connector state changing under it. Because this entails grabbing
kn->count, e.g. the lock that the kernel provides for protecting sysfs
contexts, we end up grabbing kn->count followed by
&drm->mode_config.mutex. This ends up creating an extremely rude chain:

&kn->count
  -> &drm->mode_config.mutex
    -> crtc_ww_class_mutex/crtc_ww_class_acquire
      -> &connector->mutex

I mean, look at that thing! It's just evil!!! This gross thing ends up
making any calls to drm_connector_register()/drm_connector_unregister()
impossible when holding any kind of modesetting lock. This is annoying
because ideally, we always want to ensure that
drm_dp_mst_port->connector never changes when doing an atomic commit or
check that would affect the atomic topology state so that it can
reliably and easily be used from future DRM DP MST helpers to assist
with tasks such as scanning through the current VCPI allocations and
adding connectors which need to have their allocations updated in
response to a bandwidth change or the like.

Being able to hold &mgr->base.lock throughout the entire link probe
process would have been _great_, since we could prevent userspace from
ever seeing any states in-between individual port changes and as a
result likely end up with a much faster probe and more consistent
results from said probes. But without some rework of how we handle
connector probing in sysfs it's not at all currently possible. In the
future, maybe we can try using the sysfs locks to protect updates to
connector probing state and fix this mess.

So for now, to protect everything other than port->connector under
&mgr->base.lock and ensure that we still have the guarantee that atomic
check/commit contexts will never see port->connector change we use a
silly trick. See: port->connector only needs to change in order to
ensure that input ports (see the MST spec) never have a ghost connector
associated with them. But, there's nothing stopping us from simply
throwing the entire port out and creating a new one in order to maintain
that requirement while still keeping port->connector consistent across
the lifetime of the port in atomic check/commit contexts. For all
intended purposes this works fine, as we validate ports in any contexts
we care about before using them and as such will end up reporting the
connector as disconnected until it's port's destruction finalizes. So,
we just do that in cases where we detect port->input has transitioned
from true->false. We don't need to worry about the other direction,
since a port without a connector isn't visible to userspace and as such
doesn't need to be protected by &mgr->base.lock until we finish
registering a connector for it.

For updating members of drm_dp_mst_port other than port->connector, we
simply grab &mgr->base.lock in drm_dp_mst_link_probe_work() for already
registered ports, update said members and drop the lock before
potentially registering a connector and probing the link address of it's
children.

Finally, we modify drm_dp_mst_detect_port() to take a modesetting lock
acquisition context in order to acquire &mgr->base.lock under
&connection_mutex and convert all it's users over to using the
.detect_ctx probe hooks.

With that, we finally have well defined locking.

Changes since v4:
* Get rid of port->mutex, stop using connection_mutex and just use our own
  modesetting lock - mgr->base.lock. Also, add a probe_lock that comes
  before this patch.
* Just throw out ports that get changed from an output to an input, and
  replace them with new ports. This lets us ensure that modesetting
  contexts never see port->connector go from having a connector to being
  NULL.
* Write an extremely detailed explanation of what problems this is
  trying to fix, since there's a _lot_ of context here and I honestly
  forgot some of it myself a couple times.
* Don't grab mgr->lock when reading port->mstb in
  drm_dp_mst_handle_link_address_port(). It's not needed.

Cc: Juston Li <juston.li@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022023641.8026-7-lyude@redhat.com

1 files changed, 157 insertions, 73 deletions

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 2f87dec31c3c..55f289eebea7 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1906,41 +1906,118 @@ void drm_dp_mst_connector_early_unregister(struct drm_connector *connector,
 EXPORT_SYMBOL(drm_dp_mst_connector_early_unregister);
 
 static void
+drm_dp_mst_port_add_connector(struct drm_dp_mst_branch *mstb,
+			      struct drm_dp_mst_port *port)
+{
+	struct drm_dp_mst_topology_mgr *mgr = port->mgr;
+	char proppath[255];
+	int ret;
+
+	build_mst_prop_path(mstb, port->port_num, proppath, sizeof(proppath));
+	port->connector = mgr->cbs->add_connector(mgr, port, proppath);
+	if (!port->connector) {
+		ret = -ENOMEM;
+		goto error;
+	}
+
+	if ((port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV ||
+	     port->pdt == DP_PEER_DEVICE_SST_SINK) &&
+	    port->port_num >= DP_MST_LOGICAL_PORT_0) {
+		port->cached_edid = drm_get_edid(port->connector,
+						 &port->aux.ddc);
+		drm_connector_set_tile_property(port->connector);
+	}
+
+	mgr->cbs->register_connector(port->connector);
+	return;
+
+error:
+	DRM_ERROR("Failed to create connector for port %p: %d\n", port, ret);
+}
+
+/*
+ * Drop a topology reference, and unlink the port from the in-memory topology
+ * layout
+ */
+static void
+drm_dp_mst_topology_unlink_port(struct drm_dp_mst_topology_mgr *mgr,
+				struct drm_dp_mst_port *port)
+{
+	mutex_lock(&mgr->lock);
+	list_del(&port->next);
+	mutex_unlock(&mgr->lock);
+	drm_dp_mst_topology_put_port(port);
+}
+
+static struct drm_dp_mst_port *
+drm_dp_mst_add_port(struct drm_device *dev,
+		    struct drm_dp_mst_topology_mgr *mgr,
+		    struct drm_dp_mst_branch *mstb, u8 port_number)
+{
+	struct drm_dp_mst_port *port = kzalloc(sizeof(*port), GFP_KERNEL);
+
+	if (!port)
+		return NULL;
+
+	kref_init(&port->topology_kref);
+	kref_init(&port->malloc_kref);
+	port->parent = mstb;
+	port->port_num = port_number;
+	port->mgr = mgr;
+	port->aux.name = "DPMST";
+	port->aux.dev = dev->dev;
+	port->aux.is_remote = true;
+
+	/*
+	 * Make sure the memory allocation for our parent branch stays
+	 * around until our own memory allocation is released
+	 */
+	drm_dp_mst_get_mstb_malloc(mstb);
+
+	return port;
+}
+
+static void
 drm_dp_mst_handle_link_address_port(struct drm_dp_mst_branch *mstb,
 				    struct drm_device *dev,
 				    struct drm_dp_link_addr_reply_port *port_msg)
 {
 	struct drm_dp_mst_topology_mgr *mgr = mstb->mgr;
 	struct drm_dp_mst_port *port;
-	bool created = false;
-	int old_ddps = 0;
+	int old_ddps = 0, ret;
+	u8 new_pdt = DP_PEER_DEVICE_NONE;
+	bool created = false, send_link_addr = false;
 
 	port = drm_dp_get_port(mstb, port_msg->port_number);
 	if (!port) {
-		port = kzalloc(sizeof(*port), GFP_KERNEL);
+		port = drm_dp_mst_add_port(dev, mgr, mstb,
+					   port_msg->port_number);
 		if (!port)
 			return;
-		kref_init(&port->topology_kref);
-		kref_init(&port->malloc_kref);
-		port->parent = mstb;
-		port->port_num = port_msg->port_number;
-		port->mgr = mgr;
-		port->aux.name = "DPMST";
-		port->aux.dev = dev->dev;
-		port->aux.is_remote = true;
-
-		/*
-		 * Make sure the memory allocation for our parent branch stays
-		 * around until our own memory allocation is released
+		created = true;
+	} else if (port_msg->input_port && !port->input && port->connector) {
+		/* Destroying the connector is impossible in this context, so
+		 * replace the port with a new one
 		 */
-		drm_dp_mst_get_mstb_malloc(mstb);
+		drm_dp_mst_topology_unlink_port(mgr, port);
+		drm_dp_mst_topology_put_port(port);
 
+		port = drm_dp_mst_add_port(dev, mgr, mstb,
+					   port_msg->port_number);
+		if (!port)
+			return;
 		created = true;
 	} else {
+		/* Locking is only needed when the port has a connector
+		 * exposed to userspace
+		 */
+		drm_modeset_lock(&mgr->base.lock, NULL);
 		old_ddps = port->ddps;
 	}
 
 	port->input = port_msg->input_port;
+	if (!port->input)
+		new_pdt = port_msg->peer_device_type;
 	port->mcs = port_msg->mcs;
 	port->ddps = port_msg->ddps;
 	port->ldps = port_msg->legacy_device_plug_status;
@@ -1968,59 +2045,39 @@ drm_dp_mst_handle_link_address_port(struct drm_dp_mst_branch *mstb,
 		}
 	}
 
-	if (!port->input) {
-		int ret = drm_dp_port_set_pdt(port,
-					      port_msg->peer_device_type);
-		if (ret == 1) {
-			drm_dp_send_link_address(mgr, port->mstb);
-		} else if (ret < 0) {
-			DRM_ERROR("Failed to change PDT on port %p: %d\n",
-				  port, ret);
-			goto fail;
-		}
+	ret = drm_dp_port_set_pdt(port, new_pdt);
+	if (ret == 1) {
+		send_link_addr = true;
+	} else if (ret < 0) {
+		DRM_ERROR("Failed to change PDT on port %p: %d\n",
+			  port, ret);
+		goto fail;
 	}
 
-	if (created && !port->input) {
-		char proppath[255];
-
-		build_mst_prop_path(mstb, port->port_num, proppath,
-				    sizeof(proppath));
-		port->connector = (*mgr->cbs->add_connector)(mgr, port,
-							     proppath);
-		if (!port->connector)
-			goto fail;
-
-		if ((port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV ||
-		     port->pdt == DP_PEER_DEVICE_SST_SINK) &&
-		    port->port_num >= DP_MST_LOGICAL_PORT_0) {
-			port->cached_edid = drm_get_edid(port->connector,
-							 &port->aux.ddc);
-			drm_connector_set_tile_property(port->connector);
-		}
+	if (!created)
+		drm_modeset_unlock(&mgr->base.lock);
+	else if (!port->connector && !port->input)
+		drm_dp_mst_port_add_connector(mstb, port);
 
-		(*mgr->cbs->register_connector)(port->connector);
-	}
+	if (send_link_addr && port->mstb)
+		drm_dp_send_link_address(mgr, port->mstb);
 
 	/* put reference to this port */
 	drm_dp_mst_topology_put_port(port);
 	return;
 
 fail:
-	/* Remove it from the port list */
-	mutex_lock(&mgr->lock);
-	list_del(&port->next);
-	mutex_unlock(&mgr->lock);
-
-	/* Drop the port list reference */
-	drm_dp_mst_topology_put_port(port);
-	/* And now drop our reference */
+	drm_dp_mst_topology_unlink_port(mgr, port);
 	drm_dp_mst_topology_put_port(port);
+	if (!created)
+		drm_modeset_unlock(&mgr->base.lock);
 }
 
 static void
 drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch *mstb,
 			    struct drm_dp_connection_status_notify *conn_stat)
 {
+	struct drm_dp_mst_topology_mgr *mgr = mstb->mgr;
 	struct drm_dp_mst_port *port;
 	int old_ddps;
 	bool dowork = false;
@@ -2029,6 +2086,10 @@ drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch *mstb,
 	if (!port)
 		return;
 
+	/* Locking is only needed if the port's exposed to userspace */
+	if (port->connector)
+		drm_modeset_lock(&mgr->base.lock, NULL);
+
 	old_ddps = port->ddps;
 	port->mcs = conn_stat->message_capability_status;
 	port->ldps = conn_stat->legacy_device_plug_status;
@@ -2054,10 +2115,12 @@ drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch *mstb,
 		}
 	}
 
+	if (port->connector)
+		drm_modeset_unlock(&mgr->base.lock);
+
 	drm_dp_mst_topology_put_port(port);
 	if (dowork)
 		queue_work(system_long_wq, &mstb->mgr->work);
-
 }
 
 static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_topology_mgr *mgr,
@@ -2157,8 +2220,11 @@ static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *m
 		if (port->input || !port->ddps)
 			continue;
 
-		if (!port->available_pbn)
+		if (!port->available_pbn) {
+			drm_modeset_lock(&mgr->base.lock, NULL);
 			drm_dp_send_enum_path_resources(mgr, mstb, port);
+			drm_modeset_unlock(&mgr->base.lock);
+		}
 
 		if (port->mstb)
 			mstb_child = drm_dp_mst_topology_get_mstb_validated(
@@ -2189,11 +2255,16 @@ static void drm_dp_mst_link_probe_work(struct work_struct *work)
 			mstb = NULL;
 	}
 	mutex_unlock(&mgr->lock);
-	if (mstb) {
-		drm_dp_check_and_send_link_address(mgr, mstb);
-		drm_dp_mst_topology_put_mstb(mstb);
+	if (!mstb) {
+		mutex_unlock(&mgr->probe_lock);
+		return;
 	}
+
+	drm_dp_check_and_send_link_address(mgr, mstb);
+	drm_dp_mst_topology_put_mstb(mstb);
+
 	mutex_unlock(&mgr->probe_lock);
+	drm_kms_helper_hotplug_event(dev);
 }
 
 static bool drm_dp_validate_guid(struct drm_dp_mst_topology_mgr *mgr,
@@ -2478,8 +2549,6 @@ static void drm_dp_send_link_address(struct drm_dp_mst_topology_mgr *mgr,
 		drm_dp_mst_handle_link_address_port(mstb, mgr->dev,
 						    &reply->ports[i]);
 
-	drm_kms_helper_hotplug_event(mgr->dev);
-
 out:
 	if (ret <= 0)
 		mstb->link_address_sent = false;
@@ -3274,13 +3343,14 @@ clear_down_rep_recv:
 	return 0;
 }
 
-static inline void
+static inline bool
 drm_dp_mst_process_up_req(struct drm_dp_mst_topology_mgr *mgr,
 			  struct drm_dp_pending_up_req *up_req)
 {
 	struct drm_dp_mst_branch *mstb = NULL;
 	struct drm_dp_sideband_msg_req_body *msg = &up_req->msg;
 	struct drm_dp_sideband_msg_hdr *hdr = &up_req->hdr;
+	bool hotplug = false;
 
 	if (hdr->broadcast) {
 		const u8 *guid = NULL;
@@ -3298,16 +3368,17 @@ drm_dp_mst_process_up_req(struct drm_dp_mst_topology_mgr *mgr,
 	if (!mstb) {
 		DRM_DEBUG_KMS("Got MST reply from unknown device %d\n",
 			      hdr->lct);
-		return;
+		return false;
 	}
 
 	/* TODO: Add missing handler for DP_RESOURCE_STATUS_NOTIFY events */
 	if (msg->req_type == DP_CONNECTION_STATUS_NOTIFY) {
 		drm_dp_mst_handle_conn_stat(mstb, &msg->u.conn_stat);
-		drm_kms_helper_hotplug_event(mgr->dev);
+		hotplug = true;
 	}
 
 	drm_dp_mst_topology_put_mstb(mstb);
+	return hotplug;
 }
 
 static void drm_dp_mst_up_req_work(struct work_struct *work)
@@ -3316,6 +3387,7 @@ static void drm_dp_mst_up_req_work(struct work_struct *work)
 		container_of(work, struct drm_dp_mst_topology_mgr,
 			     up_req_work);
 	struct drm_dp_pending_up_req *up_req;
+	bool send_hotplug = false;
 
 	mutex_lock(&mgr->probe_lock);
 	while (true) {
@@ -3330,10 +3402,13 @@ static void drm_dp_mst_up_req_work(struct work_struct *work)
 		if (!up_req)
 			break;
 
-		drm_dp_mst_process_up_req(mgr, up_req);
+		send_hotplug |= drm_dp_mst_process_up_req(mgr, up_req);
 		kfree(up_req);
 	}
 	mutex_unlock(&mgr->probe_lock);
+
+	if (send_hotplug)
+		drm_kms_helper_hotplug_event(mgr->dev);
 }
 
 static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
@@ -3441,22 +3516,31 @@ EXPORT_SYMBOL(drm_dp_mst_hpd_irq);
 /**
  * drm_dp_mst_detect_port() - get connection status for an MST port
  * @connector: DRM connector for this port
+ * @ctx: The acquisition context to use for grabbing locks
  * @mgr: manager for this port
- * @port: unverified pointer to a port
+ * @port: pointer to a port
  *
- * This returns the current connection state for a port. It validates the
- * port pointer still exists so the caller doesn't require a reference
+ * This returns the current connection state for a port.
  */
-enum drm_connector_status drm_dp_mst_detect_port(struct drm_connector *connector,
-						 struct drm_dp_mst_topology_mgr *mgr, struct drm_dp_mst_port *port)
+int
+drm_dp_mst_detect_port(struct drm_connector *connector,
+		       struct drm_modeset_acquire_ctx *ctx,
+		       struct drm_dp_mst_topology_mgr *mgr,
+		       struct drm_dp_mst_port *port)
 {
-	enum drm_connector_status status = connector_status_disconnected;
+	int ret;
 
 	/* we need to search for the port in the mgr in case it's gone */
 	port = drm_dp_mst_topology_get_port_validated(mgr, port);
 	if (!port)
 		return connector_status_disconnected;
 
+	ret = drm_modeset_lock(&mgr->base.lock, ctx);
+	if (ret)
+		goto out;
+
+	ret = connector_status_disconnected;
+
 	if (!port->ddps)
 		goto out;
 
@@ -3466,7 +3550,7 @@ enum drm_connector_status drm_dp_mst_detect_port(struct drm_connector *connector
 		break;
 
 	case DP_PEER_DEVICE_SST_SINK:
-		status = connector_status_connected;
+		ret = connector_status_connected;
 		/* for logical ports - cache the EDID */
 		if (port->port_num >= 8 && !port->cached_edid) {
 			port->cached_edid = drm_get_edid(connector, &port->aux.ddc);
@@ -3474,12 +3558,12 @@ enum drm_connector_status drm_dp_mst_detect_port(struct drm_connector *connector
 		break;
 	case DP_PEER_DEVICE_DP_LEGACY_CONV:
 		if (port->ldps)
-			status = connector_status_connected;
+			ret = connector_status_connected;
 		break;
 	}
 out:
 	drm_dp_mst_topology_put_port(port);
-	return status;
+	return ret;
 }
 EXPORT_SYMBOL(drm_dp_mst_detect_port);