On Wed, 2021-08-04 at 07:13 +0000, Lin, Wayne wrote:

[Public]

quoted

-----Original Message-----
From: Lyude Paul <lyude@redhat.com>
Sent: Wednesday, August 4, 2021 8:09 AM
To: Lin, Wayne <redacted>; dri-devel@lists.freedesktop.org
Cc: Kazlauskas, Nicholas <redacted>; Wentland, Harry <
Harry.Wentland@amd.com>; Zuo, Jerry
[off-list ref]; Wu, Hersen [off-list ref]; Juston Li <
juston.li@intel.com>; Imre Deak [off-list ref];
Ville Syrjälä [off-list ref]; Wentland, Harry <
Harry.Wentland@amd.com>; Daniel Vetter [off-list ref];
Sean Paul [off-list ref]; Maarten Lankhorst <
maarten.lankhorst@linux.intel.com>; Maxime Ripard [off-list ref];
Thomas Zimmermann [off-list ref]; David Airlie [off-list ref];
Daniel Vetter [off-list ref]; Deucher,
Alexander [off-list ref]; Siqueira, Rodrigo <
Rodrigo.Siqueira@amd.com>; Pillai, Aurabindo
[off-list ref]; Eryk Brol [off-list ref]; Bas
Nieuwenhuizen [off-list ref]; Cornij, Nikola
[off-list ref]; Jani Nikula [off-list ref]; Manasi
Navare [off-list ref]; Ankit Nautiyal
[off-list ref]; José Roberto de Souza
[off-list ref]; Sean Paul [off-list ref]; Ben Skeggs
[off-list ref]; stable@vger.kernel.org
Subject: Re: [PATCH 2/4] drm/dp_mst: Only create connector for connected
end device

On Tue, 2021-08-03 at 19:58 -0400, Lyude Paul wrote:

quoted

On Wed, 2021-07-21 at 00:03 +0800, Wayne Lin wrote:

quoted

[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.

Hi Lyude,
Really appreciate for your time to elaborate in such detail. Thanks!

I come up with this commit because I observed something confusing when I was
analyzing
MST connectors' life cycle. Take the topology instance you mentioned below

Root MSTB -> Output_Port 1 -> MSTB 1.1 ->Output_Port 1(Connected w/ display)
                    |                                                    -

quoted

Output_Port 2 (Disconnected)

                    -> Output_Port 2 -> MSTB 2.1 ->Output_Port 1
(Disconnected)
                                                                          ->
Output_Port 2 (Disconnected)
Which is exactly the topology of Startech DP 1-to-4 hub. There are 3 1-to-2
branch chips
within this hub. With our MST implementation today, we'll create drm
connectors for all
output ports. Hence, we totally create 6 drm connectors here. However,
Output ports of
Root MSTB are not connected to a stream sink. They are connected with branch
devices.
Thus, creating drm connector for such port looks a bit strange to me and
increases
complexity to tracking drm connectors.  My thought is we only need to create
drm
connector for those connected end device. Once output port is connected then
we can
determine whether to add on a drm connector for this port based on the peer
device type.
Hence, this commit doesn't try to break the locking logic but add more
constraints when
We try to add drm connector. Please correct me if I misunderstand anything
here. Thanks!

Sorry-I will respond to this soon, some more stuff came up at work so it might
take me a day or two

quoted

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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).

Whoops. One thing I totally forgot to mention here: the reason this is a
problem is because we'd now have two drm_connectors
which both have the same drm_dp_mst_port pointer.

quoted

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

I think input ports' DisplayPort_Device_Plug_Status field is still set to 1?
But yes,
according to DP1.4 spec 2.11.9.3, when MST device whose DPRX detected the
connection status change shall broadcast CSN downstream only. Hence, we'll
never
receive a CSN for this case.

quoted

quoted

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.

Based on DP1.4 spec 2.5.1. Physical input ports are assigned smaller port
numbers than physical output ports. For concentrator product, if there are 2
input ports of it's branch device, then their port numbers are port 0 & port
1
which can refer to figure 2-122 of DP1.4.

quoted

quoted

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.

Thanks again Lyude! Much appreciated for your time and help! And please
correct me if I misunderstand anything here : )

quoted

quoted

quoted

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 <redacted>
Cc: Imre Deak <redacted>
Cc: Ville Syrjälä <redacted>
Cc: Harry Wentland <redacted>
Cc: Daniel Vetter <redacted>
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 <redacted>
Cc: Daniel Vetter <redacted>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Nicholas Kazlauskas <redacted>
Cc: Rodrigo Siqueira <redacted>
Cc: Aurabindo Pillai <redacted>
Cc: Eryk Brol <redacted>
Cc: Bas Nieuwenhuizen <redacted>
Cc: Nikola Cornij <redacted>
Cc: Wayne Lin <redacted>
Cc: "Ville Syrjälä" <redacted>
Cc: Jani Nikula <redacted>
Cc: Manasi Navare <redacted>
Cc: Ankit Nautiyal <redacted>
Cc: "José Roberto de Souza" <redacted>
Cc: Sean Paul <redacted>
Cc: Ben Skeggs <redacted>
Cc: dri-devel@lists.freedesktop.org
Cc: <redacted> # v5.5+
Signed-off-by: Wayne Lin <redacted>
---
 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)

--
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

Regards,
Wayne Lin

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat