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Subject[PATCH 1/6] drm/dp_mst: Introduce drm_dp_mst_connector_atomic_check()
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Currently the way that we prevent userspace from performing new modesets
on MST connectors that have just been destroyed is rather broken.
There's nothing in the actual DRM DP MST topology helpers that checks
whether or not a connector still exists, instead each DRM driver does
this on it's own, usually by returning NULL from the best_encoder
callback which in turn, causes the atomic commit to fail.

However, this is wrong in a rather subtle way. If ->best_encoder()
returns NULL, this makes ALL modesets involving the connector fail. This
includes modesets from userspace that would shut off the CRTCs being
used by the connector. Since this results in blocking any changes to a
connector's DPMS prop, it has the sideaffect of preventing legacy
modesetting users from ever disabling a CRTC that was previously enabled
for use in an MST topology. An example of this, where X tries to
change the DPMS property of an MST connector that was just detached from
the system:

[ 2908.320131] [drm:drm_helper_probe_single_connector_modes [drm_kms_helper]] [CONNECTOR:82:DP-6]
[ 2908.320148] [drm:drm_helper_probe_single_connector_modes [drm_kms_helper]] [CONNECTOR:82:DP-6] status updated from connected to disconnected
[ 2908.320166] [drm:drm_helper_probe_single_connector_modes [drm_kms_helper]] [CONNECTOR:82:DP-6] disconnected
[ 2908.320193] [drm:drm_mode_object_put.part.2 [drm]] OBJ ID: 111 (1)
[ 2908.320230] [drm:drm_sysfs_hotplug_event [drm]] generating hotplug event
...
[ 2908.638539] [drm:drm_ioctl [drm]] pid=12928, dev=0xe201, auth=1, DRM_IOCTL_MODE_SETPROPERTY
[ 2908.638546] [drm:drm_atomic_state_init [drm]] Allocated atomic state 000000007155ba49
[ 2908.638553] [drm:drm_mode_object_get [drm]] OBJ ID: 114 (1)
[ 2908.638560] [drm:drm_mode_object_get [drm]] OBJ ID: 108 (1)
[ 2908.638568] [drm:drm_atomic_get_crtc_state [drm]] Added [CRTC:41:head-0] 0000000097a6396e state to 000000007155ba49
[ 2908.638575] [drm:drm_atomic_add_affected_connectors [drm]] Adding all current connectors for [CRTC:41:head-0] to 000000007155ba49
[ 2908.638582] [drm:drm_mode_object_get [drm]] OBJ ID: 82 (3)
[ 2908.638589] [drm:drm_mode_object_get [drm]] OBJ ID: 82 (4)
[ 2908.638596] [drm:drm_atomic_get_connector_state [drm]] Added [CONNECTOR:82:DP-6] 0000000087427144 state to 000000007155ba49
[ 2908.638603] [drm:drm_atomic_check_only [drm]] checking 000000007155ba49
[ 2908.638609] [drm:drm_atomic_helper_check_modeset [drm_kms_helper]] [CRTC:41:head-0] active changed
[ 2908.638613] [drm:drm_atomic_helper_check_modeset [drm_kms_helper]] Updating routing for [CONNECTOR:82:DP-6]
[ 2908.638616] [drm:drm_atomic_helper_check_modeset [drm_kms_helper]] No suitable encoder found for [CONNECTOR:82:DP-6]
[ 2908.638623] [drm:drm_atomic_check_only [drm]] atomic driver check for 000000007155ba49 failed: -22
[ 2908.638630] [drm:drm_atomic_state_default_clear [drm]] Clearing atomic state 000000007155ba49
[ 2908.638637] [drm:drm_mode_object_put.part.2 [drm]] OBJ ID: 82 (4)
[ 2908.638643] [drm:drm_mode_object_put.part.2 [drm]] OBJ ID: 82 (3)
[ 2908.638650] [drm:drm_mode_object_put.part.2 [drm]] OBJ ID: 114 (2)
[ 2908.638656] [drm:drm_mode_object_put.part.2 [drm]] OBJ ID: 108 (2)
[ 2908.638663] [drm:__drm_atomic_state_free [drm]] Freeing atomic state 000000007155ba49
[ 2908.638669] [drm:drm_mode_object_put.part.2 [drm]] OBJ ID: 82 (2)
[ 2908.638676] [drm:drm_ioctl [drm]] pid=12928, ret = -22

While this doesn't usually result in any errors that would be obvious to
the user, it does result in us leaving display resources on. This in
turn leads to unwanted sideaffects like inactive GPUs being left on
(usually from the resulting leaked runtime PM ref).

So, provide an easier way of doing this that doesn't require breaking
->best_encoder(): add a common drm_dp_mst_connector_atomic_check()
function that DRM drivers can call in order to have CRTC enabling
commits fail automatically if the MST port driving the connector no
longer exists. We'll also be able to expand upon this later as well once
we add MST fallback retraining support.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
---
drivers/gpu/drm/drm_dp_mst_topology.c | 76 +++++++++++++++++++++++++++
include/drm/drm_dp_mst_helper.h | 3 ++
2 files changed, 79 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 7780567aa669..0162d4bf2549 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3129,6 +3129,82 @@ static const struct drm_private_state_funcs mst_state_funcs = {
.atomic_destroy_state = drm_dp_mst_destroy_state,
};

+static bool
+drm_dp_mst_connector_still_exists(struct drm_connector *connector,
+ struct drm_dp_mst_topology_mgr *mgr,
+ struct drm_dp_mst_branch *mstb)
+{
+ struct drm_dp_mst_port *port;
+ bool exists = false;
+
+ mstb = drm_dp_get_validated_mstb_ref(mgr, mstb);
+ if (!mstb)
+ return false;
+
+ list_for_each_entry(port, &mstb->ports, next) {
+ port = drm_dp_get_validated_port_ref(mgr, port);
+ if (!port)
+ continue;
+
+ exists = (port->connector == connector ||
+ (port->mstb &&
+ drm_dp_mst_connector_still_exists(connector, mgr,
+ port->mstb)));
+
+ drm_dp_put_port(port);
+ if (exists)
+ break;
+ }
+
+ drm_dp_put_mst_branch_device(mstb);
+ return exists;
+}
+
+/**
+ * drm_dp_mst_connector_atomic_check - Helper for validating a new atomic
+ * state on an MST connector
+ * @connector: drm connector
+ * @connector_state: the new atomic state of @connector
+ * @mgr: the MST topology mgr for @connector
+ *
+ * This function performs various atomic checks that apply to all drivers
+ * using the DRM DP MST helpers. This should be called by all drivers at the
+ * start of the atomic_check function for their MST connectors.
+ *
+ * Return 0 for success, or negative error code on failure.
+ */
+int
+drm_dp_mst_connector_atomic_check(struct drm_connector *connector,
+ struct drm_connector_state *connector_state,
+ struct drm_dp_mst_topology_mgr *mgr)
+{
+ struct drm_atomic_state *state = connector_state->state;
+ struct drm_crtc *crtc = connector_state->crtc;
+ struct drm_crtc_state *new_crtc_state;
+
+ if (!crtc)
+ return 0;
+
+ new_crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
+ if (!new_crtc_state)
+ return 0;
+
+ if (!drm_atomic_crtc_needs_modeset(new_crtc_state) ||
+ !new_crtc_state->active)
+ return 0;
+
+ /* Make sure that the port for this MST connector still exists */
+ if (!drm_dp_mst_connector_still_exists(connector, mgr,
+ mgr->mst_primary)) {
+ DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] has disappeared from the MST topology\n",
+ connector->base.id, connector->name);
+ return -EINVAL;
+ }
+
+ return 0;
+}
+EXPORT_SYMBOL(drm_dp_mst_connector_atomic_check);
+
/**
* drm_atomic_get_mst_topology_state: get MST topology state
*
diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
index 7f78d26a0766..8e33c2c85d1e 100644
--- a/include/drm/drm_dp_mst_helper.h
+++ b/include/drm/drm_dp_mst_helper.h
@@ -625,6 +625,9 @@ void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr);
int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
struct drm_dp_mst_topology_mgr *mgr);
+int drm_dp_mst_connector_atomic_check(struct drm_connector *connector,
+ struct drm_connector_state *connector_state,
+ struct drm_dp_mst_topology_mgr *mgr);
int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
struct drm_dp_mst_topology_mgr *mgr,
struct drm_dp_mst_port *port, int pbn);
--
2.17.1
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