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

[Harry Wentland <harry.wentland-5C7GfCeVMHo@public.gmane.org>]

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@redhat.com>
> Cc: stable@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@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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