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SubjectRE: [RESEND RFC 15/18] drm/display/dp_mst: Skip releasing payloads if last connected port isn't connected
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> -----Original Message-----
> From: Lyude Paul <lyude@redhat.com>
> Sent: Wednesday, June 8, 2022 3:30 AM
> To: dri-devel@lists.freedesktop.org; nouveau@lists.freedesktop.org; amd-
> gfx@lists.freedesktop.org
> Cc: Lin, Wayne <Wayne.Lin@amd.com>; Ville Syrjälä
> <ville.syrjala@linux.intel.com>; Zuo, Jerry <Jerry.Zuo@amd.com>; Jani Nikula
> <jani.nikula@intel.com>; Imre Deak <imre.deak@intel.com>; Daniel Vetter
> <daniel.vetter@ffwll.ch>; Sean Paul <sean@poorly.run>; David Airlie
> <airlied@linux.ie>; Daniel Vetter <daniel@ffwll.ch>; Thomas Zimmermann
> <tzimmermann@suse.de>; Lakha, Bhawanpreet
> <Bhawanpreet.Lakha@amd.com>; open list <linux-kernel@vger.kernel.org>
> Subject: [RESEND RFC 15/18] drm/display/dp_mst: Skip releasing payloads if
> last connected port isn't connected
>
> In the past, we've ran into strange issues regarding errors in response to
> trying to destroy payloads after a port has been unplugged. We fixed this
> back in:
>
> This is intended to replace the workaround that was added here:
>
> commit 3769e4c0af5b ("drm/dp_mst: Avoid to mess up payload table by
> ports in stale topology")
>
> which was intended fix to some of the payload leaks that were observed
> before, where we would attempt to determine if the port was still
> connected to the topology before updating payloads using
> drm_dp_mst_port_downstream_of_branch. This wasn't a particularly good
> solution, since one of the points of still having port and mstb validation is to
> avoid sending messages to newly disconnected branches wherever possible
> - thus the required use of drm_dp_mst_port_downstream_of_branch
> would indicate something may be wrong with said validation.
>
> It seems like it may have just been races and luck that made
> drm_dp_mst_port_downstream_of_branch work however, as while I was
> trying to figure out the true cause of this issue when removing the legacy
> MST code - I discovered an important excerpt in section 2.14.2.3.3.6 of the DP
> 2.0
> specs:
>
> "BAD_PARAM - This reply is transmitted when a Message Transaction
> parameter is in error; for example, the next port number is invalid or /no
> device is connected/ to the port associated with the port number."
>
> Sure enough - removing the calls to
> drm_dp_mst_port_downstream_of_branch()
> and instead checking the ->ddps field of the parent port to see whether we
> should release a given payload or not seems to totally fix the issue. This does
> actually make sense to me, as it seems the implication is that given a
> topology where an MSTB is removed, the payload for the MST parent's port
> will be released automatically if that port is also marked as disconnected.
> However, if there's another parent in the chain after that which is connected
> - payloads must be released there with an ALLOCATE_PAYLOAD message.
>
> So, let's do that!
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: Wayne Lin <Wayne.Lin@amd.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Fangzhi Zuo <Jerry.Zuo@amd.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Imre Deak <imre.deak@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Sean Paul <sean@poorly.run>
> ---
> drivers/gpu/drm/display/drm_dp_mst_topology.c | 51 +++++++------------
> 1 file changed, 17 insertions(+), 34 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index dd314586bac3..70adb8db4335 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -3137,7 +3137,7 @@ static struct drm_dp_mst_port
> *drm_dp_get_last_connected_port_to_mstb(struct drm static struct
> drm_dp_mst_branch * drm_dp_get_last_connected_port_and_mstb(struct
> drm_dp_mst_topology_mgr *mgr,
> struct drm_dp_mst_branch *mstb,
> - int *port_num)
> + struct drm_dp_mst_port **last_port)
> {
> struct drm_dp_mst_branch *rmstb = NULL;
> struct drm_dp_mst_port *found_port;
> @@ -3153,7 +3153,8 @@
> drm_dp_get_last_connected_port_and_mstb(struct
> drm_dp_mst_topology_mgr *mgr,
>
> if (drm_dp_mst_topology_try_get_mstb(found_port-
> >parent)) {
> rmstb = found_port->parent;
> - *port_num = found_port->port_num;
> + *last_port = found_port;
> + drm_dp_mst_get_port_malloc(found_port);
> } else {
> /* Search again, starting from this parent */
> mstb = found_port->parent;
> @@ -3170,7 +3171,7 @@ static int drm_dp_payload_send_msg(struct
> drm_dp_mst_topology_mgr *mgr,
> int pbn)
> {
> struct drm_dp_sideband_msg_tx *txmsg;
> - struct drm_dp_mst_branch *mstb;
> + struct drm_dp_mst_branch *mstb = NULL;
> int ret, port_num;
> u8 sinks[DRM_DP_MAX_SDP_STREAMS];
> int i;
> @@ -3178,12 +3179,22 @@ static int drm_dp_payload_send_msg(struct
> drm_dp_mst_topology_mgr *mgr,
> port_num = port->port_num;
> mstb = drm_dp_mst_topology_get_mstb_validated(mgr, port-
> >parent);
> if (!mstb) {
> - mstb = drm_dp_get_last_connected_port_and_mstb(mgr,
> - port->parent,
> - &port_num);
> + struct drm_dp_mst_port *rport = NULL;
> + bool ddps;
>
> + mstb = drm_dp_get_last_connected_port_and_mstb(mgr,
> port->parent,
> +&rport);
> if (!mstb)
> return -EINVAL;
> +
> + ddps = rport->ddps;
> + port_num = rport->port_num;
> + drm_dp_mst_put_port_malloc(rport);
> +
> + /* If the port is currently marked as disconnected, don't send
> a payload message */
> + if (!ddps) {
Hi Lyude,

Thanks for driving this!
Shouldn't we still send ALLOCATE_PAYLOAD with PBN 0 to the last connected
Port even its peer device is disconnected? We rely on this "path msg" to update
all payload ID tables along the virtual payload channel.

commit 3769e4c0af5b ("drm/dp_mst: Avoid to mess up payload table by
ports in stale topology") was trying to skip updating payload for a target which is
no longer existing in the current topology rooted at mgr->mst_primary. I passed
"mgr->mst_primary" to drm_dp_mst_port_downstream_of_branch() previously.
Sorry, I might not fully understand the issue you've seen. Could you elaborate on
this more please?

Thanks!
> + ret = -EINVAL;
> + goto fail_put;
> + }
> }
>
> txmsg = kzalloc(sizeof(*txmsg), GFP_KERNEL); @@ -3384,7 +3395,6
> @@ int drm_dp_update_payload_part1(struct drm_dp_mst_topology_mgr
> *mgr, int start_s
> struct drm_dp_mst_port *port;
> int i, j;
> int cur_slots = start_slot;
> - bool skip;
>
> mutex_lock(&mgr->payload_lock);
> for (i = 0; i < mgr->max_payloads; i++) { @@ -3399,16 +3409,6 @@ int
> drm_dp_update_payload_part1(struct drm_dp_mst_topology_mgr *mgr,
> int start_s
> port = container_of(vcpi, struct drm_dp_mst_port,
> vcpi);
>
> - mutex_lock(&mgr->lock);
> - skip
> = !drm_dp_mst_port_downstream_of_branch(port, mgr->mst_primary);
> - mutex_unlock(&mgr->lock);
> -
> - if (skip) {
> - drm_dbg_kms(mgr->dev,
> - "Virtual channel %d is not in current
> topology\n",
> - i);
> - continue;
> - }
> /* Validated ports don't matter if we're releasing
> * VCPI
> */
> @@ -3509,7 +3509,6 @@ int drm_dp_update_payload_part2(struct
> drm_dp_mst_topology_mgr *mgr)
> struct drm_dp_mst_port *port;
> int i;
> int ret = 0;
> - bool skip;
>
> mutex_lock(&mgr->payload_lock);
> for (i = 0; i < mgr->max_payloads; i++) { @@ -3519,13 +3518,6 @@ int
> drm_dp_update_payload_part2(struct drm_dp_mst_topology_mgr *mgr)
>
> port = container_of(mgr->proposed_vcpis[i], struct
> drm_dp_mst_port, vcpi);
>
> - mutex_lock(&mgr->lock);
> - skip = !drm_dp_mst_port_downstream_of_branch(port,
> mgr->mst_primary);
> - mutex_unlock(&mgr->lock);
> -
> - if (skip)
> - continue;
> -
> drm_dbg_kms(mgr->dev, "payload %d %d\n", i, mgr-
> >payloads[i].payload_state);
> if (mgr->payloads[i].payload_state == DP_PAYLOAD_LOCAL)
> {
> ret = drm_dp_create_payload_step2(mgr, port, mgr-
> >proposed_vcpis[i]->vcpi, &mgr->payloads[i]); @@ -4780,18 +4772,9 @@
> EXPORT_SYMBOL(drm_dp_mst_reset_vcpi_slots);
> void drm_dp_mst_deallocate_vcpi(struct drm_dp_mst_topology_mgr *mgr,
> struct drm_dp_mst_port *port)
> {
> - bool skip;
> -
> if (!port->vcpi.vcpi)
> return;
>
> - mutex_lock(&mgr->lock);
> - skip = !drm_dp_mst_port_downstream_of_branch(port, mgr-
> >mst_primary);
> - mutex_unlock(&mgr->lock);
> -
> - if (skip)
> - return;
> -
> drm_dp_mst_put_payload_id(mgr, port->vcpi.vcpi);
> port->vcpi.num_slots = 0;
> port->vcpi.pbn = 0;
> --
> 2.35.3
--
Wayne Lin
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