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

[Lyude Paul]

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. 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