[PATCH 2/3] drm/dp_mst: Skip CSN if topology probing is not done yet

[Lin, Wayne]

[Public]

> -----Original Message-----
> From: Lyude Paul <lyude at redhat.com>
> Sent: Thursday, June 27, 2024 12:21 AM
> To: Lin, Wayne <Wayne.Lin at amd.com>; amd-gfx at lists.freedesktop.org; dri-
> devel at lists.freedesktop.org
> Cc: jani.nikula at intel.com; imre.deak at intel.com; daniel at ffwll.ch; Wentland,
> Harry <Harry.Wentland at amd.com>; Zuo, Jerry <Jerry.Zuo at amd.com>;
> Wentland, Harry <Harry.Wentland at amd.com>; stable at vger.kernel.org
> Subject: Re: [PATCH 2/3] drm/dp_mst: Skip CSN if topology probing is not
> done yet
>
> Some comments down below:
>
> On Wed, 2024-06-26 at 16:48 +0800, Wayne Lin wrote:
> > [Why]
> > During resume, observe that we receive CSN event before we start
> > topology probing. Handling CSN at this moment based on uncertain
> > topology is unnecessary.
> >
> > [How]
> > Add checking condition in drm_dp_mst_handle_up_req() to skip handling
> > CSN if the topology is yet to be probed.
> >
> > Cc: Lyude Paul <lyude at redhat.com>
> > Cc: Harry Wentland <hwentlan at amd.com>
> > Cc: Jani Nikula <jani.nikula at intel.com>
> > Cc: Imre Deak <imre.deak at intel.com>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > Cc: stable at vger.kernel.org
> > Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>
> > ---
> >  drivers/gpu/drm/display/drm_dp_mst_topology.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > index 68831f4e502a..fc2ceae61db2 100644
> > --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > @@ -4069,6 +4069,7 @@ static int drm_dp_mst_handle_up_req(struct
> > drm_dp_mst_topology_mgr *mgr)
> >     if (up_req->msg.req_type == DP_CONNECTION_STATUS_NOTIFY) {
> >             const struct drm_dp_connection_status_notify *conn_stat =
> >                     &up_req->msg.u.conn_stat;
> > +           bool handle_csn;
> >
> >             drm_dbg_kms(mgr->dev, "Got CSN: pn: %d ldps:%d ddps:
> > %d mcs: %d ip: %d pdt: %d\n",
> >                         conn_stat->port_number,
> > @@ -4077,6 +4078,16 @@ static int drm_dp_mst_handle_up_req(struct
> > drm_dp_mst_topology_mgr *mgr)
> >                         conn_stat->message_capability_status,
> >                         conn_stat->input_port,
> >                         conn_stat->peer_device_type);
> > +
> > +           mutex_lock(&mgr->probe_lock);
> > +           handle_csn = mgr->mst_primary->link_address_sent;
> > +           mutex_unlock(&mgr->probe_lock);
> > +
> > +           if (!handle_csn) {
> > +                   drm_dbg_kms(mgr->dev, "Got CSN before finish
> > topology probing. Skip it.");
> > +                   kfree(up_req);
> > +                   goto out;
> > +           }
>
> Hm. I think you're definitely on the right track here with not handling CSNs
> immediately after resume. My one question though is whether dropping the
> event entirely here is a good idea? In theory, we could receive a CSN at any
> time during the probe - including receiving a CSN for a connector that we've
> already probed in the initial post-resume process, which could result in us
> missing CSNs coming out of resume and still having an outdated topology
> layout.
>
> I'm not totally sure about the solution I'm going to suggest but it seems like it
> would certainly be worth trying: what if we added a flag to
> drm_dp_mst_topology_mgr called something like "csn_during_resume"
> and simply set it to true in response to getting a CSN before we've finished
> reprobing? Then we at the end of the reprobe, we can simply restart the
> reprobing process if csn_during_resume gets set - which should still ensure
> we're up to date with reality.
>
Thanks for the review!

I understand your concern. My patch will just check whether mst manager starts
the probing process or not by confirming whether we sent LINK_ADDRESS to
the 1st mst branch already. It will drop the CSN event only when the event comes
earlier than the probing. The CSN events occur during topology probing should
still have chance to be handled after probing process release the mgr->probe_lock
I think. Does this make sense to you please? Thanks!

> >     } else if (up_req->msg.req_type ==
> > DP_RESOURCE_STATUS_NOTIFY) {
> >             const struct drm_dp_resource_status_notify *res_stat =
> >                     &up_req->msg.u.resource_stat;
>
> --
> Cheers,
>  Lyude Paul (she/her)
>  Software Engineer at Red Hat

--
Regards,
Wayne Lin