[PATCH 2/3] drm/dp_mst: Don't show connectors as connected before probing available PBN

[Lyude Paul]

final correction: I probably could actually get rid of this patch and do this
a bit more nicely by just making sure that we reprobe path resources for
connectors while under drm_dp_mst_topology_mgr->base.lock on CSNs, instead of
punting it off to the link address work like we seem to be doing. So, going to
try doing that instead.

On Fri, 2020-03-06 at 15:03 -0500, Lyude Paul wrote:
> On Thu, 2020-03-05 at 20:29 +0200, Ville Syrjälä wrote:
> > On Thu, Mar 05, 2020 at 01:13:36PM -0500, Lyude Paul wrote:
> > > On Thu, 2020-03-05 at 15:11 +0200, Ville Syrjälä wrote:
> > > > On Wed, Mar 04, 2020 at 05:36:12PM -0500, Lyude Paul wrote:
> > > > > It's next to impossible for us to do connector probing on topologies
> > > > > without occasionally racing with userspace, since creating a
> > > > > connector
> > > > > itself causes a hotplug event which we have to send before probing
> > > > > the
> > > > > available PBN of a connector. Even if we didn't have this hotplug
> > > > > event
> > > > > sent, there's still always a chance that userspace started probing
> > > > > connectors before we finished probing the topology.
> > > > > 
> > > > > This can be a problem when validating a new MST state since the
> > > > > connector will be shown as connected briefly, but without any
> > > > > available
> > > > > PBN - causing any atomic state which would enable said connector to
> > > > > fail
> > > > > with -ENOSPC. So, let's simply workaround this by telling userspace
> > > > > new
> > > > > MST connectors are disconnected until we've finished probing their
> > > > > PBN.
> > > > > Since we always send a hotplug event at the end of the link address
> > > > > probing process, userspace will still know to reprobe the connector
> > > > > when
> > > > > we're ready.
> > > > > 
> > > > > Signed-off-by: Lyude Paul <lyude at redhat.com>
> > > > > Fixes: cd82d82cbc04 ("drm/dp_mst: Add branch bandwidth validation to
> > > > > MST
> > > > > atomic check")
> > > > > Cc: Mikita Lipski <mikita.lipski at amd.com>
> > > > > Cc: Alex Deucher <alexander.deucher at amd.com>
> > > > > Cc: Sean Paul <seanpaul at google.com>
> > > > > Cc: Hans de Goede <hdegoede at redhat.com>
> > > > > ---
> > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 13 +++++++++++++
> > > > >  1 file changed, 13 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > index 207eef08d12c..7b0ff0cff954 100644
> > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > @@ -4033,6 +4033,19 @@ drm_dp_mst_detect_port(struct drm_connector
> > > > > *connector,
> > > > >  			ret = connector_status_connected;
> > > > >  		break;
> > > > >  	}
> > > > > +
> > > > > +	/* We don't want to tell userspace the port is actually
> > > > > plugged into
> > > > > +	 * anything until we've finished probing it's available_pbn,
> > > > > otherwise
> > > > 
> > > > "its"
> > > > 
> > > > Why is the connector even registered before we've finished the probe?
> > > > 
> > > Oops, I'm not sure how I did this by accident but the explanation I gave
> > > in
> > > the commit message was uh, completely wrong. I must have forgotten that
> > > I
> > > made
> > > sure we didn't expose connectors before probing their PBN back when I
> > > started
> > > my MST cleanup....
> > > 
> > > So: despite what I said before it's not actually when new connectors are
> > > created, it's when downstream hotplugs happen which means that the
> > > conenctor's
> > > always going to be there before we probe the available_pbn.
> > 
> > Not sure I understand. You're saying this is going to change for already
> > existing connectors when something else gets plugged in, and either we
> > zero it out during the probe or it always was zero to begin with for
> > whatever reason?
> 
> ok-me and Sean Paul did some playing around with available_pbn and full_pbn
> (I'll get into this one in a moment), and I also played around with a couple
> of different hubs and have a much better understanding of how this should
> work
> now.
> 
> So: first off tl;dr available_pbn is absolutely not what we want in
> basically
> any scenario, we actually want to use the full_pbn field that we get when
> sending ENUM_PATH_RESOURCES. Second, full_pbn represents the _smallest_
> bandwidth limitation encountered in the path between the root MSTB and each
> connected sink. Remember that there's technically a DisplayPort link trained
> between each branch device going down the topology, so that bandwidth
> limitation basically equates to "what is the lowest trained link rate that
> exists down the path to this port?". This also means that full_pbn will
> potentially change every time a new connector is plugged in, as some hubs
> will be clever and optimize the link rate they decide to use. Likewise,
> since there's not going to be anything trained on a disconnected port (e.g.
> ddps=0) there's no point in keeping full_pbn around for disconnected ports,
> since otherwise we might let userspace see a connected port with a stale
> full_pbn value.
> 
> So-IMHO the behavior of not letting connectors show as connected until we
> also
> have their full_pbn probed should definitely be the right solution here.
> Especially if we want to eventually start pruning modes based on full_pbn at
> some point in the future.
> 
> > > I did just notice
> > > though that we send a hotplug on connection status notifications even
> > > before
> > > we've finished the PBN probe, so I might be able to improve on that as
> > > well.
> > > We still definitely want to report the connector as disconnected before
> > > we
> > > have the available PBN though, in case another probe was already going
> > > before
> > > we got the connection status notification.
> > > 
> > > I'll make sure to fixup the explanation in the commit message on the
> > > next
> > > respin
> > > 
> > > > > +	 * userspace will see racy atomic check failures
> > > > > +	 *
> > > > > +	 * Since we always send a hotplug at the end of probing
> > > > > topology
> > > > > +	 * state, we can just let userspace reprobe this connector
> > > > > later.
> > > > > +	 */
> > > > > +	if (ret == connector_status_connected && !port-
> > > > > >available_pbn) 
> > > > > {
> > > > > +		DRM_DEBUG_KMS("[CONNECTOR:%d:%s] not ready yet (PBN
> > > > > not
> > > > > probed)\n",
> > > > > +			      connector->base.id, connector->name);
> > > > > +		ret = connector_status_disconnected;
> > > > > +	}
> > > > >  out:
> > > > >  	drm_dp_mst_topology_put_port(port);
> > > > >  	return ret;
> > > > > -- 
> > > > > 2.24.1
> > > > > 
> > > > > _______________________________________________
> > > > > dri-devel mailing list
> > > > > dri-devel at lists.freedesktop.org
> > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > -- 
> > > Cheers,
> > > 	Lyude Paul (she/her)
> > > 	Associate Software Engineer at Red Hat
-- 
Cheers,
	Lyude Paul (she/her)
	Associate Software Engineer at Red Hat