On Wed, 2021-07-21 at 00:03 +0800, Wayne Lin wrote:
[Why] Currently, we will create connectors for all output ports no matter it's connected or not. However, in MST, we can only determine whether an output port really stands for a "connector" till it is connected and check its peer device type as an end device.
What is this commit trying to solve exactly? e.g. is AMD currently running into issues with there being too many DRM connectors or something like that? Ideally this is behavior I'd very much like us to keep as-is unless there's good reason to change it.
Some context here btw - there's a lot of subtleties with MST locking that isn't immediately obvious. It's been a while since I wrote this code, but if I recall correctly one of those subtleties is that trying to create/destroy connectors on the fly when ports change types introduces a lot of potential issues with locking and some very complicated state transitions. Note that because we maintain the topology as much as possible across suspend/resumes this means there's a lot of potential state transitions with drm_dp_mst_port and drm_dp_mst_branch we need to handle that would typically be impossible to run into otherwise.
An example of this, if we were to try to prune connectors based on PDT on the fly: assume we have a simple topology like this
Root MSTB -> Port 1 -> MSTB 1.1 (Connected w/ display) -> Port 2 -> MSTB 2.1
We suspend the system, unplug MSTB 1.1, and then resume. Once the system starts reprobing, it will notice that MSTB 1.1 has been disconnected. Since we no longer have a PDT, we decide to unregister our connector. But there's a catch! We had a display connected to MSTB 1.1, so even after unregistering the connector it's going to stay around until userspace has committed a new mode with the connector disabled.
Now - assuming we're still in the same spot in the resume processs, let's assume somehow MSTB 1.1 is suddenly plugged back in. Once we've finished responding to the hotplug event, we will have created a connector for it. Now we've hit a bug - userspace hasn't removed the previous zombie connector which means we have references to the drm_dp_mst_port in our atomic state and potentially also our payload tables (?? unsure about this one).
So then how do we manage to add/remove connectors for input connectors on the fly? Well, that's one of the fun normally-impossible state transitions I mentioned before. According to the spec input ports are always disconnected, so we'll never receive a CSN for them. This means in theory the only possible way we could have a connector go from being an input connector to an output connector connector would be if the entire topology was swapped out during suspend/resume, and the input/output ports in the two topologies topology happen to be in different places. Since we only have to reprobe once during resume before we get hotplugging enabled, we're guaranteed this state transition will only happen once in this state - which means the second replug I described in the previous paragraph can never happen.
Note that while I don't actually know if there's topologies with input ports at indexes other than 0, since the specification isn't super clear on this bit we play it safe and assume it is possible.
Anyway-this is -all- based off my memory, so please point out anything here that I've explained that doesn't make sense or doesn't seem correct :). It's totally possible I might have misremembered something.
In current code, we have chance to create connectors for output ports connected with branch device and these are redundant connectors. e.g. StarTech 1-to-4 DP hub is constructed by internal 2 layer 1-to-2 branch devices. Creating connectors for such internal output ports are redundant.
[How] Put constraint on creating connector for connected end device only.
Fixes: 6f85f73821f6 ("drm/dp_mst: Add basic topology reprobing when resuming") Cc: Juston Li juston.li@intel.com Cc: Imre Deak imre.deak@intel.com Cc: Ville Syrjälä ville.syrjala@linux.intel.com Cc: Harry Wentland hwentlan@amd.com Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: Sean Paul sean@poorly.run Cc: Lyude Paul lyude@redhat.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Maxime Ripard mripard@kernel.org Cc: Thomas Zimmermann tzimmermann@suse.de Cc: David Airlie airlied@linux.ie Cc: Daniel Vetter daniel@ffwll.ch Cc: Alex Deucher alexander.deucher@amd.com Cc: Nicholas Kazlauskas nicholas.kazlauskas@amd.com Cc: Rodrigo Siqueira Rodrigo.Siqueira@amd.com Cc: Aurabindo Pillai aurabindo.pillai@amd.com Cc: Eryk Brol eryk.brol@amd.com Cc: Bas Nieuwenhuizen bas@basnieuwenhuizen.nl Cc: Nikola Cornij nikola.cornij@amd.com Cc: Wayne Lin Wayne.Lin@amd.com Cc: "Ville Syrjälä" ville.syrjala@linux.intel.com Cc: Jani Nikula jani.nikula@intel.com Cc: Manasi Navare manasi.d.navare@intel.com Cc: Ankit Nautiyal ankit.k.nautiyal@intel.com Cc: "José Roberto de Souza" jose.souza@intel.com Cc: Sean Paul seanpaul@chromium.org Cc: Ben Skeggs bskeggs@redhat.com Cc: dri-devel@lists.freedesktop.org Cc: stable@vger.kernel.org # v5.5+ Signed-off-by: Wayne Lin Wayne.Lin@amd.com
drivers/gpu/drm/drm_dp_mst_topology.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c index 51cd7f74f026..f13c7187b07f 100644 --- a/drivers/gpu/drm/drm_dp_mst_topology.c +++ b/drivers/gpu/drm/drm_dp_mst_topology.c @@ -2474,7 +2474,8 @@ drm_dp_mst_handle_link_address_port(struct drm_dp_mst_branch *mstb, if (port->connector) drm_modeset_unlock(&mgr->base.lock); - else if (!port->input) + else if (!port->input && port->pdt != DP_PEER_DEVICE_NONE && + drm_dp_mst_is_end_device(port->pdt, port->mcs)) drm_dp_mst_port_add_connector(mstb, port); if (send_link_addr && port->mstb) { @@ -2557,6 +2558,10 @@ drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch *mstb, dowork = false; } + if (!port->input && !port->connector && new_pdt != DP_PEER_DEVICE_NONE && + drm_dp_mst_is_end_device(new_pdt, new_mcs)) + create_connector = true;
if (port->connector) drm_modeset_unlock(&mgr->base.lock); else if (create_connector)