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Lyude Paul

2018-Sep-18 23:06 UTC

[Nouveau] [PATCH 1/6] drm/dp_mst: Introduce drm_dp_mst_connector_atomic_check()

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 at redhat.com>
Cc: stable at 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

Sasha Levin

2018-Sep-19 18:58 UTC

head link

[Nouveau] [PATCH 1/6] drm/dp_mst: Introduce drm_dp_mst_connector_atomic_check()

Hi,

[This is an automated email]

This commit has been processed because it contains a -stable tag.
The stable tag indicates that it's relevant for the following trees: all

The bot has tested the following trees: v4.18.8, v4.14.70, v4.9.127, v4.4.156,
v3.18.122,

v4.18.8: Build OK!
v4.14.70: Build OK!
v4.9.127: Failed to apply! Possible dependencies:
    3f3353b7e121 ("drm/dp: Introduce MST topology state to track available
link bandwidth")
    edb1ed1ab7d3 ("drm/dp: Add DP MST helpers to atomically find and
release vcpi slots")

v4.4.156: Failed to apply! Possible dependencies:
    3f3353b7e121 ("drm/dp: Introduce MST topology state to track available
link bandwidth")
    edb1ed1ab7d3 ("drm/dp: Add DP MST helpers to atomically find and
release vcpi slots")

v3.18.122: Failed to apply! Possible dependencies:
    3f3353b7e121 ("drm/dp: Introduce MST topology state to track available
link bandwidth")
    edb1ed1ab7d3 ("drm/dp: Add DP MST helpers to atomically find and
release vcpi slots")


Please let us know how to resolve this.

--
Thanks,
Sasha

Lyude Paul

2018-Sep-19 22:00 UTC

head link

[Nouveau] [PATCH 1/6] drm/dp_mst: Introduce drm_dp_mst_connector_atomic_check()

On Wed, 2018-09-19 at 18:58 +0000, Sasha Levin wrote:> Hi,
> 
> [This is an automated email]
> 
> This commit has been processed because it contains a -stable tag.
> The stable tag indicates that it's relevant for the following trees:
all
> 
> The bot has tested the following trees: v4.18.8, v4.14.70, v4.9.127,
v4.4.156,
> v3.18.122, 
> 
> v4.18.8: Build OK!
> v4.14.70: Build OK!
> v4.9.127: Failed to apply! Possible dependencies:
>     3f3353b7e121 ("drm/dp: Introduce MST topology state to track
available
> link bandwidth")
>     edb1ed1ab7d3 ("drm/dp: Add DP MST helpers to atomically find and
release
> vcpi slots")
> 
> v4.4.156: Failed to apply! Possible dependencies:
>     3f3353b7e121 ("drm/dp: Introduce MST topology state to track
available
> link bandwidth")
>     edb1ed1ab7d3 ("drm/dp: Add DP MST helpers to atomically find and
release
> vcpi slots")
> 
> v3.18.122: Failed to apply! Possible dependencies:
>     3f3353b7e121 ("drm/dp: Introduce MST topology state to track
available
> link bandwidth")
>     edb1ed1ab7d3 ("drm/dp: Add DP MST helpers to atomically find and
release
> vcpi slots")
> 
> 
> Please let us know how to resolve this....is this the email address I'm supposed to "let you know how to
resolve this"
with? if that is the case, it's 100% OK to simply ignore all of the versions
that don't apply with this patch.
> 
> --
> Thanks,
> Sasha

Dan Carpenter

2018-Sep-20 09:16 UTC

head link

[Nouveau] [PATCH 1/6] drm/dp_mst: Introduce drm_dp_mst_connector_atomic_check()

Hi Lyude,

Thank you for the patch! Perhaps something to improve:

url:   
https://github.com/0day-ci/linux/commits/Lyude-Paul/Fix-legacy-DPMS-changes-with-MST/20180919-203434
base:   git://anongit.freedesktop.org/drm-intel for-linux-next

smatch warnings:
drivers/gpu/drm/drm_dp_mst_topology.c:3144 drm_dp_mst_connector_still_exists()
error: we previously assumed 'port' could be null (see line 3146)

#
https://github.com/0day-ci/linux/commit/f8df31d5221b9a6da6698d4a37e622253bb17cdc
git remote add linux-review https://github.com/0day-ci/linux
git remote update linux-review
git checkout f8df31d5221b9a6da6698d4a37e622253bb17cdc
vim +/port +3144 drivers/gpu/drm/drm_dp_mst_topology.c

3f3353b7 Pandiyan, Dhinakaran 2017-04-20  3131  
f8df31d5 Lyude Paul           2018-09-18  3132  static bool
f8df31d5 Lyude Paul           2018-09-18  3133 
drm_dp_mst_connector_still_exists(struct drm_connector *connector,
f8df31d5 Lyude Paul           2018-09-18  3134  				  struct
drm_dp_mst_topology_mgr *mgr,
f8df31d5 Lyude Paul           2018-09-18  3135  				  struct drm_dp_mst_branch
*mstb)
f8df31d5 Lyude Paul           2018-09-18  3136  {
f8df31d5 Lyude Paul           2018-09-18  3137  	struct drm_dp_mst_port *port;
f8df31d5 Lyude Paul           2018-09-18  3138  	bool exists = false;
f8df31d5 Lyude Paul           2018-09-18  3139  
f8df31d5 Lyude Paul           2018-09-18  3140  	mstb =
drm_dp_get_validated_mstb_ref(mgr, mstb);
f8df31d5 Lyude Paul           2018-09-18  3141  	if (!mstb)
f8df31d5 Lyude Paul           2018-09-18  3142  		return false;
f8df31d5 Lyude Paul           2018-09-18  3143  
f8df31d5 Lyude Paul           2018-09-18 @3144  	list_for_each_entry(port,
&mstb->ports, next) {
                                                                            ^^^^
We need to use a different loop iterator, or possibly
list_for_each_entry_safe() because it looks like we're freeing
something.  Maybe it's cleanest to do both.

f8df31d5 Lyude Paul           2018-09-18  3145  		port =
drm_dp_get_validated_port_ref(mgr, port);
f8df31d5 Lyude Paul           2018-09-18 @3146  		if (!port)
f8df31d5 Lyude Paul           2018-09-18  3147  			continue;
f8df31d5 Lyude Paul           2018-09-18  3148  
f8df31d5 Lyude Paul           2018-09-18  3149  		exists = (port->connector
== connector ||
f8df31d5 Lyude Paul           2018-09-18  3150  			  (port->mstb &&
f8df31d5 Lyude Paul           2018-09-18  3151  			  
drm_dp_mst_connector_still_exists(connector, mgr,
f8df31d5 Lyude Paul           2018-09-18  3152  							     port->mstb)));
f8df31d5 Lyude Paul           2018-09-18  3153  
f8df31d5 Lyude Paul           2018-09-18  3154  		drm_dp_put_port(port);
f8df31d5 Lyude Paul           2018-09-18  3155  		if (exists)
f8df31d5 Lyude Paul           2018-09-18  3156  			break;
f8df31d5 Lyude Paul           2018-09-18  3157  	}
f8df31d5 Lyude Paul           2018-09-18  3158  
f8df31d5 Lyude Paul           2018-09-18  3159  
drm_dp_put_mst_branch_device(mstb);
f8df31d5 Lyude Paul           2018-09-18  3160  	return exists;
f8df31d5 Lyude Paul           2018-09-18  3161  }
f8df31d5 Lyude Paul           2018-09-18  3162  

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

Harry Wentland

2018-Sep-20 23:35 UTC

head link

[Nouveau] [PATCH 1/6] drm/dp_mst: Introduce drm_dp_mst_connector_atomic_check()

On 2018-09-18 07:06 PM, Lyude Paul wrote:> 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 at redhat.com>
> Cc: stable at vger.kernel.org
This does seem like a saner way to handle the case when the MST connector is
gone. As this doesn't currently seem to affect amdgpu directly and I
therefore might miss something I'll leave the RB to someone else, but you
have my
Acked-by: Harry Wentland <harry.wentland at amd.com>

Harry
> ---
>  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);
>

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