Re: [PATCH v3 2/5] drm/i915: Only use one link bw config for MST topologies

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Mar 09, 2018 at 04:32:28PM -0500, Lyude Paul wrote:
> When a DP MST link needs retraining, sometimes the hub will detect that
> the current link bw config is impossible and will update it's RX caps in
> the DPCD to reflect the new maximum link rate. Currently, we make the
> assumption that the RX caps in the dpcd will never change like this.
> This means if the sink changes it's RX caps after we've already set up
> an MST link and we attempt to add or remove another sink from the
> topology, we could put ourselves into an invalid state where we've tried
> to configure different sinks on the same MST topology with different
> link rates. We could also run into this situation if a sink reports a
> higher link rate after suspend, usually from us having trained it with a
> fallback bw configuration before suspending.
> 
> So: "lock" the bw config by only using the max DP link rate/lane count
> on the first modeset for an MST topology. For every modeset following,
> we instead use the last configured link bw for this topology. We only
> unlock the bw config when we've detected a new MST sink.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: Manasi Navare <manasi.d.navare@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c     | 11 +++++++++--
>  drivers/gpu/drm/i915/intel_dp_mst.c | 22 +++++++++++++++-------
>  drivers/gpu/drm/i915/intel_drv.h    |  6 ++++++
>  3 files changed, 30 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 5abf0c95725a..5645a194de92 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3871,18 +3871,25 @@ intel_dp_can_mst(struct intel_dp *intel_dp)
>  static void
>  intel_dp_configure_mst(struct intel_dp *intel_dp)
>  {
> +	bool was_mst;
> +
>  	if (!i915_modparams.enable_dp_mst)
>  		return;
>  
>  	if (!intel_dp->can_mst)
>  		return;
>  
> +	was_mst = intel_dp->is_mst;
>  	intel_dp->is_mst = intel_dp_can_mst(intel_dp);
>  
> -	if (intel_dp->is_mst)
> +	if (intel_dp->is_mst) {
>  		DRM_DEBUG_KMS("Sink is MST capable\n");
> -	else
> +
> +		if (!was_mst)
> +			intel_dp->mst_bw_locked = false;
> +	} else {
>  		DRM_DEBUG_KMS("Sink is not MST capable\n");
> +	}
>  
>  	drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr,
>  					intel_dp->is_mst);
> diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
> index c3de0918ee13..c0553456b18e 100644
> --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> @@ -42,7 +42,7 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
>  		to_intel_connector(conn_state->connector);
>  	struct drm_atomic_state *state = pipe_config->base.state;
>  	int bpp;
> -	int lane_count, slots;
> +	int lane_count, link_rate, slots;
>  	const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
>  	int mst_pbn;
>  	bool reduce_m_n = drm_dp_has_quirk(&intel_dp->desc,
> @@ -56,16 +56,22 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
>  			      bpp);
>  	}
>  	/*
> -	 * for MST we always configure max link bw - the spec doesn't
> -	 * seem to suggest we should do otherwise.
> +	 * for MST we always configure max link bw if we don't know better -
> +	 * the spec doesn't seem to suggest we should do otherwise. But,
> +	 * ensure it always stays consistent with the rest of this hub's
> +	 * state.
>  	 */
> -	lane_count = intel_dp_max_lane_count(intel_dp);
> +	if (intel_dp->mst_bw_locked) {
> +		lane_count = intel_dp->lane_count;
> +		link_rate = intel_dp->link_rate;

This feels like something we should be tracking in the MST state.

> +	} else {
> +		lane_count = intel_dp_max_lane_count(intel_dp);
> +		link_rate = intel_dp_max_link_rate(intel_dp);
> +	}
>  
>  	pipe_config->lane_count = lane_count;
> -
>  	pipe_config->pipe_bpp = bpp;
> -
> -	pipe_config->port_clock = intel_dp_max_link_rate(intel_dp);
> +	pipe_config->port_clock = link_rate;
>  
>  	if (drm_dp_mst_port_has_audio(&intel_dp->mst_mgr, connector->port))
>  		pipe_config->has_audio = true;
> @@ -221,6 +227,8 @@ static void intel_mst_pre_enable_dp(struct intel_encoder *encoder,
>  	connector->encoder = encoder;
>  	intel_mst->connector = connector;
>  
> +	intel_dp->mst_bw_locked = true;
> +
>  	DRM_DEBUG_KMS("active links %d\n", intel_dp->active_mst_links);
>  
>  	drm_dp_send_power_updown_phy(&intel_dp->mst_mgr, connector->port, true);
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index 3f19dc80997f..fc338529e918 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -1107,6 +1107,12 @@ struct intel_dp {
>  	bool can_mst; /* this port supports mst */
>  	bool is_mst;
>  	int active_mst_links;
> +	/* Set when we've already decided on a link bw for mst, to prevent us
> +	 * from setting different link bandwiths if the hub tries to confuse
> +	 * us by changing it later
> +	 */
> +	bool mst_bw_locked;
> +
>  	/* connector directly attached - won't be use for modeset in mst world */
>  	struct intel_connector *attached_connector;
>  
> -- 
> 2.14.3

-- 
Ville Syrjälä
Intel OTC