[PATCH v7 12/17] drm/dp_mst: Add branch bandwidth validation to MST atomic check

[Leo]

I'm not well versed in MST bw validation, which might explain my confusion here - so bear with me :)
...

On 2019-11-16 5:01 p.m., mikita.lipski at amd.com wrote:
> From: Mikita Lipski <mikita.lipski at amd.com>
> 
> Adding PBN attribute to drm_dp_vcpi_allocation structure to
> keep track of how much bandwidth each Port requires.
> Adding drm_dp_mst_atomic_check_bw_limit to verify that
> state's bandwidth needs doesn't exceed available bandwidth.
> The funtion is called in drm_dp_mst_atomic_check after
> drm_dp_mst_atomic_check_topology_state to fully verify that
> the proposed topology is supported.
> 
> Cc: Jerry Zuo <Jerry.Zuo at amd.com>
> Cc: Harry Wentland <harry.wentland at amd.com>
> Cc: Lyude Paul <lyude at redhat.com>
> Signed-off-by: Mikita Lipski <mikita.lipski at amd.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 67 ++++++++++++++++++++++++++-
>  include/drm/drm_dp_mst_helper.h       |  1 +
>  2 files changed, 66 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 98cc93d5ddd7..5072c1e3dcfe 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -3243,7 +3243,7 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  {
>  	struct drm_dp_mst_topology_state *topology_state;
>  	struct drm_dp_vcpi_allocation *pos, *vcpi = NULL;
> -	int prev_slots, req_slots, ret;
> +	int prev_slots, prev_bw, req_slots, ret;
>  
>  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
>  	if (IS_ERR(topology_state))
> @@ -3254,6 +3254,7 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  		if (pos->port == port) {
>  			vcpi = pos;
>  			prev_slots = vcpi->vcpi;
> +			prev_bw = vcpi->pbn;
>  
>  			/*
>  			 * This should never happen, unless the driver tries
> @@ -3269,8 +3270,10 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  			break;
>  		}
>  	}
> -	if (!vcpi)
> +	if (!vcpi) {
>  		prev_slots = 0;
> +		prev_bw = 0;
> +	}
>  
>  	if (pbn_div <= 0)
>  		pbn_div = mgr->pbn_div;
> @@ -3280,6 +3283,9 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] VCPI %d -> %d\n",
>  			 port->connector->base.id, port->connector->name,
>  			 port, prev_slots, req_slots);
> +	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] [MST PORT:%p] PBN %d -> %d\n",
> +			 port->connector->base.id, port->connector->name,
> +			 port, prev_bw, pbn);
>  
>  	/* Add the new allocation to the state */
>  	if (!vcpi) {
> @@ -3292,6 +3298,7 @@ int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,
>  		list_add(&vcpi->next, &topology_state->vcpis);
>  	}
>  	vcpi->vcpi = req_slots;
> +	vcpi->pbn = pbn;
>  
>  	ret = req_slots;
>  	return ret;
> @@ -3837,6 +3844,59 @@ static void drm_dp_mst_destroy_state(struct drm_private_obj *obj,
>  	kfree(mst_state);
>  }
>  
> +static bool drm_dp_mst_port_downstream_of_branch(struct drm_dp_mst_port *port,
> +						 struct drm_dp_mst_branch *branch)
> +{
> +	while (port->parent) {
> +		if (port->parent == branch)
> +			return true;
> +
> +		if (port->parent->port_parent)
> +			port = port->parent->port_parent;
> +		else
> +			break;
> +	}
> +	return false;
> +}
> +
> +static inline
> +int drm_dp_mst_atomic_check_bw_limit(struct drm_dp_mst_branch *branch,
> +				     struct drm_dp_mst_topology_state *mst_state)
> +{
> +	struct drm_dp_mst_port *port;
> +	struct drm_dp_vcpi_allocation *vcpi;
> +	int pbn_limit = 0, pbn_used = 0;
> +
> +	list_for_each_entry(port, &branch->ports, next) {
> +		if (port->mstb) {
> +			if (drm_dp_mst_atomic_check_bw_limit(port->mstb, mst_state))
> +				return -EINVAL;
> +		}
> +		if (port->available_pbn > 0)
> +			pbn_limit = port->available_pbn;

Shouldn't the pbn_limit be the pbn limit of *this* mstb's upstream port (branch->port_parent)
and not the downstream ports? Otherwise, wouldn't this mean we're only considering the 'last'
downstream port in this mstb when saving the pbn_limit? (or perhapse you meant to use '+=' rather
than '=')

> +	}
> +	DRM_DEBUG_ATOMIC("[MST BRANCH:%p] branch has %d PBN available\n",
> +					 branch,
> +					 pbn_limit);
> +
> +	list_for_each_entry(vcpi, &mst_state->vcpis, next) {
> +		if (!vcpi->pbn)
> +			continue;
> +
> +		if (drm_dp_mst_port_downstream_of_branch(vcpi->port, branch))
> +			pbn_used += vcpi->pbn;
> +	}

It's not clear to me how this will behave when recursively called.

The above iteration over branch->ports looks to be checking that a downstream MSTB is
bandwidth-valid. Wouldn't this iteration here repeat the validation work again, since
drm_dp_mst_port_downstream_of_branch() will return true for *all* downstream ports
(including the ones we recursed on above)? It wouldn't be wrong exactly, but not sure if
that's intended.

Thanks,
Leo

> +	DRM_DEBUG_ATOMIC("[MST BRANCH:%p] branch used %d PBN\n",
> +			 branch,
> +			 pbn_used);
> +	if (pbn_used > pbn_limit) {
> +		DRM_DEBUG_ATOMIC("[MST BRANCH:%p] No available bandwidth\n",
> +				 branch);
> +		return -EINVAL;
> +	}
> +	return 0;
> +}
> +
>  static inline int
>  drm_dp_mst_atomic_check_topology_state(struct drm_dp_mst_topology_mgr *mgr,
>  				       struct drm_dp_mst_topology_state *mst_state)
> @@ -3968,6 +4028,9 @@ int drm_dp_mst_atomic_check(struct drm_atomic_state *state)
>  		ret = drm_dp_mst_atomic_check_topology_state(mgr, mst_state);
>  		if (ret)
>  			break;
> +		ret = drm_dp_mst_atomic_check_bw_limit(mgr->mst_primary, mst_state);
> +		if (ret)
> +			break;
>  	}
>  
>  	return ret;
> diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> index b1b00de3083b..5a119a40ed5a 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -431,6 +431,7 @@ struct drm_dp_payload {
>  struct drm_dp_vcpi_allocation {
>  	struct drm_dp_mst_port *port;
>  	int vcpi;
> +	int pbn;
>  	bool dsc_enabled;
>  	struct list_head next;
>  };
>