Re: [PATCH v3 3/5] drm/dp_mst: Add drm_dp_mst_topology_mgr_lower_link_rate()

[Manasi Navare]

On Fri, Mar 09, 2018 at 04:32:29PM -0500, Lyude Paul wrote:
> Unlike SST, MST can have far more then a single display hooked up on a
> single port. What this also means however, is that if the DisplayPort
> link to the top-level MST branch device becomes unstable then every
> single branch device also has an unstable link. Additionally, MST has a
> few more steps that must be taken in order to properly retrain the
> entire topology under a lower link rate. Since the VCID allocations for
> each mstb is determined based off the link rate for the top of the
> topology, we also have to recalculate all of the VCID allocations for
> the downstream ports as well to ensure that we still have enough link
> bandwidth to drive each mstb.
> 
> Additionally, since we have multiple downstream connectors per port,
> setting the link status of the parent mstb's port to bad isn't enough:
> all of the downstream mstb ports have to have their link status set to
> bad.
> 
> This basically follows the same paradigm that our DP link status logic
> in DRM does, in that we simply tell userspace that all of the mstb ports
> need retraining and additionally applying the new lower bandwidth
> constraints to all of the atomic commits on the topology that follow.
> 
> Since the work of figuring out which connectors live downstream on an
> MST topology and updating their link status is something that any driver
> supporting MST is going to need to do in order to retrain MST links
> properly, we add the drm_dp_mst_topology_mgr_lower_link_rate() helper
> which simply recalculates the pbn_div for a given mst topology, then
> marks the link status of all connectors living in that topology as bad.
> We'll make use of it in i915 later in this series.
> 
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Manasi Navare <manasi.d.navare@xxxxxxxxx>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 73 ++++++++++++++++++++++++++++++++++-
>  include/drm/drm_dp_mst_helper.h       |  3 ++
>  2 files changed, 75 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 6fac4129e6a2..0d6604500b29 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -2076,7 +2076,7 @@ static bool drm_dp_get_vc_payload_bw(int dp_link_bw,
>  {
>  	switch (dp_link_bw) {
>  	default:
> -		DRM_DEBUG_KMS("invalid link bandwidth in DPCD: %x (link count: %d)\n",
> +		DRM_DEBUG_KMS("invalid link bandwidth: %x (link count: %d)\n",
>  			      dp_link_bw, dp_link_count);
>  		return false;
>  
> @@ -2096,6 +2096,77 @@ static bool drm_dp_get_vc_payload_bw(int dp_link_bw,
>  	return true;
>  }
>  
> +static void drm_dp_set_mstb_link_status(struct drm_dp_mst_branch *mstb,
> +					enum drm_link_status status)
> +{
> +	struct drm_dp_mst_branch *rmstb;
> +	struct drm_dp_mst_port *port;
> +
> +	list_for_each_entry(port, &mstb->ports, next) {
> +		rmstb = drm_dp_get_validated_mstb_ref(mstb->mgr, port->mstb);
> +		if (rmstb) {
> +			drm_dp_set_mstb_link_status(rmstb, status);
> +			drm_dp_put_mst_branch_device(rmstb);
> +		}
> +
> +		if (port->connector)
> +			port->connector->state->link_status = status;
> +	}
> +}
> +
> +/**
> + * drm_dp_mst_topology_mgr_lower_link_rate() - Override the DP link bw/count
> + * for all connectors in a given MST topology
> + * @mgr: manager to set state for
> + * @dp_link_bw: The new DP link bandwidth

I would rather call this argument as dp_link_rate since thats what we are passing
and not dp_link_bw.

> + * @dp_link_count: The new DP link count
> + *
> + * This is called by the driver when it detects that the current DP link for
> + * the given topology manager is unstable, and needs to be retrained at a
> + * lower link rate.
> + *
> + * This takes care of updating the link status on all downstream connectors
> + * along with recalculating the VC payloads. The driver should send a hotplug
> + * event after calling this function to notify userspace of the link status
> + * change.
> + *
> + * RETURNS:
> + *
> + * True for success, or negative error code on failure.
> + */
> +int drm_dp_mst_topology_mgr_lower_link_rate(struct drm_dp_mst_topology_mgr *mgr,
> +					    int dp_link_bw, int dp_link_count)
> +{
> +	struct drm_device *dev = mgr->dev;
> +	struct drm_dp_mst_branch *mst_primary;
> +	int new_pbn_div;
> +	int ret = 0;
> +
> +	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> +
> +	if (!drm_dp_get_vc_payload_bw(drm_dp_link_rate_to_bw_code(dp_link_bw),

This explains my comment above, we convert link rate to link_bw here so better
call that link_rate to begin with

> +				      dp_link_count, &new_pbn_div)) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	mst_primary = drm_dp_get_validated_mstb_ref(mgr, mgr->mst_primary);
> +	if (!mst_primary)
> +		goto out;
> +
> +	DRM_DEBUG_KMS("MST link failed to retrain, lowering pbn_div to %d\n",
> +		      new_pbn_div);

This debug message seems a little misleading since we are saying that it failed
to train link or retrain at the same params and now we are retraining with fallback
values. Clarify this a bit more in this message like "MST link training failed
or retrain at same params failed"..?

Manasi

> +	mgr->pbn_div = new_pbn_div;
> +
> +	drm_dp_set_mstb_link_status(mst_primary, DRM_MODE_LINK_STATUS_BAD);
> +
> +	drm_dp_put_mst_branch_device(mst_primary);
> +out:
> +	drm_modeset_unlock(&dev->mode_config.connection_mutex);
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_dp_mst_topology_mgr_lower_link_rate);
> +
>  /**
>   * drm_dp_mst_topology_mgr_set_mst() - Set the MST state for a topology manager
>   * @mgr: manager to set state for
> diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> index 7f78d26a0766..6261ec43a2c0 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -634,4 +634,7 @@ int drm_dp_atomic_release_vcpi_slots(struct drm_atomic_state *state,
>  int drm_dp_send_power_updown_phy(struct drm_dp_mst_topology_mgr *mgr,
>  				 struct drm_dp_mst_port *port, bool power_up);
>  
> +int drm_dp_mst_topology_mgr_lower_link_rate(struct drm_dp_mst_topology_mgr *mgr,
> +					    int dp_link_bw, int dp_link_count);
> +
>  #endif
> -- 
> 2.14.3
>