Re: [PATCH 2/4] drm/dp_mst: Only create connector for connected end device

[Lyude Paul <lyude@redhat.com>]

On Wed, 2021-08-11 at 09:49 +0000, Lin, Wayne wrote:
> [Public]
> 
> > -----Original Message-----
> > From: Lyude Paul <lyude@redhat.com>
> > Sent: Wednesday, August 11, 2021 4:45 AM
> > To: Lin, Wayne <Wayne.Lin@amd.com>; dri-devel@lists.freedesktop.org
> > Cc: Kazlauskas, Nicholas <Nicholas.Kazlauskas@amd.com>; Wentland, Harry <
> > Harry.Wentland@amd.com>; Zuo, Jerry
> > <Jerry.Zuo@amd.com>; Wu, Hersen <hersenxs.wu@amd.com>; Juston Li <
> > juston.li@intel.com>; Imre Deak <imre.deak@intel.com>;
> > Ville Syrjälä <ville.syrjala@linux.intel.com>; Daniel Vetter <
> > daniel.vetter@ffwll.ch>; Sean Paul <sean@poorly.run>; Maarten Lankhorst
> > <maarten.lankhorst@linux.intel.com>; Maxime Ripard <mripard@kernel.org>;
> > Thomas Zimmermann <tzimmermann@suse.de>;
> > David Airlie <airlied@linux.ie>; Daniel Vetter <daniel@ffwll.ch>; Deucher,
> > Alexander <Alexander.Deucher@amd.com>; Siqueira,
> > Rodrigo <Rodrigo.Siqueira@amd.com>; Pillai, Aurabindo <
> > Aurabindo.Pillai@amd.com>; Eryk Brol <eryk.brol@amd.com>; Bas
> > Nieuwenhuizen <bas@basnieuwenhuizen.nl>; Cornij, Nikola <
> > Nikola.Cornij@amd.com>; Jani Nikula <jani.nikula@intel.com>; Manasi
> > Navare <manasi.d.navare@intel.com>; Ankit Nautiyal <
> > ankit.k.nautiyal@intel.com>; José Roberto de Souza <jose.souza@intel.com>;
> > Sean Paul <seanpaul@chromium.org>; Ben Skeggs <bskeggs@redhat.com>; 
> > stable@vger.kernel.org
> > Subject: Re: [PATCH 2/4] drm/dp_mst: Only create connector for connected
> > end device
> > 
> > On Wed, 2021-08-04 at 07:13 +0000, Lin, Wayne wrote:
> > > [Public]
> > > 
> > > > -----Original Message-----
> > > > From: Lyude Paul <lyude@redhat.com>
> > > > Sent: Wednesday, August 4, 2021 8:09 AM
> > > > To: Lin, Wayne <Wayne.Lin@amd.com>; dri-devel@lists.freedesktop.org
> > > > Cc: Kazlauskas, Nicholas <Nicholas.Kazlauskas@amd.com>; Wentland,
> > > > Harry < Harry.Wentland@amd.com>; Zuo, Jerry <Jerry.Zuo@amd.com>; Wu,
> > > > Hersen <hersenxs.wu@amd.com>; Juston Li < juston.li@intel.com>; Imre
> > > > Deak <imre.deak@intel.com>; Ville Syrjälä
> > > > <ville.syrjala@linux.intel.com>; Wentland, Harry <
> > > > Harry.Wentland@amd.com>; Daniel Vetter <daniel.vetter@ffwll.ch>;
> > > > Sean Paul <sean@poorly.run>; Maarten Lankhorst <
> > > > maarten.lankhorst@linux.intel.com>; Maxime Ripard
> > > > <mripard@kernel.org>; Thomas Zimmermann <tzimmermann@suse.de>; David
> > > > Airlie <airlied@linux.ie>; Daniel Vetter <daniel@ffwll.ch>; Deucher,
> > > > Alexander <Alexander.Deucher@amd.com>; Siqueira, Rodrigo <
> > > > Rodrigo.Siqueira@amd.com>; Pillai, Aurabindo
> > > > <Aurabindo.Pillai@amd.com>; Eryk Brol <eryk.brol@amd.com>; Bas
> > > > Nieuwenhuizen <bas@basnieuwenhuizen.nl>; Cornij, Nikola
> > > > <Nikola.Cornij@amd.com>; Jani Nikula <jani.nikula@intel.com>; Manasi
> > > > Navare <manasi.d.navare@intel.com>; Ankit Nautiyal
> > > > <ankit.k.nautiyal@intel.com>; José Roberto de Souza
> > > > <jose.souza@intel.com>; Sean Paul <seanpaul@chromium.org>; Ben
> > > > Skeggs <bskeggs@redhat.com>; 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:
> > > > > 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.
> > > 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)
> > >                     |
> > > -
> > > > 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
> No worries. Much appreciated for your time!
> > 

Alright - finally got some time to respond to this. So this change still
doesn't really seem correct to me (if anyone watching this thread wants to
chime in to correct me btw feel free).

JFYI - I don't think the commit is trying to break anything intentionally,
it's just that there's a lot of moving pieces with the locking here that are
easy to trip over. That being said though, besides the locking issues after
thinking about this I'm still a bit skeptical on how much this would work or
even if we would want it.

To start off - my main issue with this is that it sounds like we're basically
entirely getting rid of the disconnected state for MST connectors, and then
only exposing the connector when something is connected. Unless I'm missing
something here, the PDT can pretty much change whenever something is
connected/disconnected or across suspend/resume reprobes. To do this with the
connector API would be very different from connector probing behavior for
other connector types, which already seems like an issue to me. This would
also break the ability to force a connector to be connected/disconnected, as
there would no longer be a way to force a disconnected MST connector on.

The other thing is I'm not entirely clear still on what's trying to be
accomplished here. If you're trying to identify DRM connectors, there's
already no guaranteed consistency with connector names which means that having
less connectors doesn't really make things any easier to identify. For
actually trying to figure out more details on connectors, if this is somethig
userspace needs, this seems like something we should just be adding in the
form of connector props.

With all of this being said, this ends up just seeming like we're adding
potentially a lot of complexity to how we create connectors and the
suspend/resume reprobing code. I think it'd be good to know what the precise
usecase for this actually is, if this is something you still think is needed.

> > > > > 
> > > > > 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.
> > > > 
> > > > > 
> > > > > 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.
> > > > > 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.
> > > > > 
> > > > > 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 : )
> > > > > 
> > > > > > 
> > > > > > 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)
> > > > > 
> > > > 
> > > > --
> > > > Cheers,
> > > >  Lyude Paul (she/her)
> > > >  Software Engineer at Red Hat
> > > Regards,
> > > Wayne Lin
> > > 
> > 
> > --
> > Cheers,
> >  Lyude Paul (she/her)
> >  Software Engineer at Red Hat
> --
> Regards,
> Wayne Lin
> 

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