[AMD Public Use]
> -----Original Message-----
> From: Wentland, Harry <Harry.Wentland@xxxxxxx>
> Sent: Friday, April 24, 2020 1:53 AM
> To: Lyude Paul <lyude@xxxxxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: David Airlie <airlied@xxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Sean Paul
> <seanpaul@xxxxxxxxxxxx>; Thomas Zimmermann <tzimmermann@xxxxxxx>;
> Lin, Wayne <Wayne.Lin@xxxxxxx>; Lin, Wayne <Wayne.Lin@xxxxxxx>
> Subject: Re: [PATCH] Revert "drm/dp_mst: Remove single tx msg restriction."
>
> On 2020-04-23 12:42 p.m., Lyude Paul wrote:
> > This reverts commit 6bb0942e8f46863a745489cce27efe5be2a3885e.
> >
> > Unfortunately it would appear that the rumors we've heard of sideband
> > message interleaving not being very well supported are true. On the
> > Lenovo ThinkPad Thunderbolt 3 dock that I have, interleaved messages
> > appear to just get dropped:
> >
> > [drm:drm_dp_mst_wait_tx_reply [drm_kms_helper]] timedout msg send
> > 00000000571ddfd0 2 1
> > [dp_mst] txmsg cur_offset=2 cur_len=2 seqno=1 state=SENT path_msg=1
> dst=00
> > [dp_mst] type=ENUM_PATH_RESOURCES contents:
> > [dp_mst] port=2
> >
> > DP descriptor for this hub:
> > OUI 90-cc-24 dev-ID SYNA3 HW-rev 1.0 SW-rev 3.12 quirks 0x0008
> >
> > It would seem like as well that this is a somewhat well known issue in
> > the field. From section 5.4.2 of the DisplayPort 2.0 specification:
> >
> > There are MST Sink/Branch devices in the field that do not handle
> > interleaved message transactions.
> >
> > To facilitate message transaction handling by downstream devices, an
> > MST Source device shall generate message transactions in an atomic
> > manner (i.e., the MST Source device shall not concurrently interleave
> > multiple message transactions). Therefore, an MST Source device shall
> > clear the Message_Sequence_No value in the Sideband_MSG_Header to
> 0.
> >
> > MST Source devices that support field policy updates by way of
> > software should update the policy to forego the generation of
> > interleaved message transactions.
> >
Hi Paul,
Appreciate for your time!
Didn't notice it on DP 2.0 spec before :)
Acked-by: Wayne Lin <wayne.lin@xxxxxxx>
> > This is a bit disappointing, as features like HDCP require that we
> > send a sideband request every ~2 seconds for each active stream.
> > However, there isn't really anything in the specification that allows
> > us to accurately probe for interleaved messages.
> >
> > If it ends up being that we -really- need this in the future, we might
> > be able to whitelist hubs where interleaving is known to work-or maybe
> > try some sort of heuristics. But for now, let's just play it safe and
> > not use it.
> >
>
> Sounds like the DP spec would need an addition bit to indicate actual support
> of interleaved messages by the RX.
>
> Acked-by: Harry Wentland <harry.wentland@xxxxxxx>
>
> Harry
>
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > Fixes: 6bb0942e8f46 ("drm/dp_mst: Remove single tx msg restriction.")
> > Cc: Wayne Lin <Wayne.Lin@xxxxxxx>
> > Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> > ---
> > drivers/gpu/drm/drm_dp_mst_topology.c | 14 ++++++++++++--
> > include/drm/drm_dp_mst_helper.h | 5 +++++
> > 2 files changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > index 21f10ceb3d6c..03a1496f6120 100644
> > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > @@ -1205,6 +1205,8 @@ static int drm_dp_mst_wait_tx_reply(struct
> drm_dp_mst_branch *mstb,
> > txmsg->state == DRM_DP_SIDEBAND_TX_SENT) {
> > mstb->tx_slots[txmsg->seqno] = NULL;
> > }
> > + mgr->is_waiting_for_dwn_reply = false;
> > +
> > }
> > out:
> > if (unlikely(ret == -EIO) && drm_debug_enabled(DRM_UT_DP)) { @@
> > -1214,6 +1216,7 @@ static int drm_dp_mst_wait_tx_reply(struct
> drm_dp_mst_branch *mstb,
> > }
> > mutex_unlock(&mgr->qlock);
> >
> > + drm_dp_mst_kick_tx(mgr);
> > return ret;
> > }
> >
> > @@ -2789,9 +2792,11 @@ static void process_single_down_tx_qlock(struct
> drm_dp_mst_topology_mgr *mgr)
> > ret = process_single_tx_qlock(mgr, txmsg, false);
> > if (ret == 1) {
> > /* txmsg is sent it should be in the slots now */
> > + mgr->is_waiting_for_dwn_reply = true;
> > list_del(&txmsg->next);
> > } else if (ret) {
> > DRM_DEBUG_KMS("failed to send msg in q %d\n", ret);
> > + mgr->is_waiting_for_dwn_reply = false;
> > list_del(&txmsg->next);
> > if (txmsg->seqno != -1)
> > txmsg->dst->tx_slots[txmsg->seqno] = NULL; @@ -2831,7
> +2836,8 @@
> > static void drm_dp_queue_down_tx(struct drm_dp_mst_topology_mgr *mgr,
> > drm_dp_mst_dump_sideband_msg_tx(&p, txmsg);
> > }
> >
> > - if (list_is_singular(&mgr->tx_msg_downq))
> > + if (list_is_singular(&mgr->tx_msg_downq) &&
> > + !mgr->is_waiting_for_dwn_reply)
> > process_single_down_tx_qlock(mgr);
> > mutex_unlock(&mgr->qlock);
> > }
> > @@ -3823,6 +3829,7 @@ static int drm_dp_mst_handle_down_rep(struct
> drm_dp_mst_topology_mgr *mgr)
> > mutex_lock(&mgr->qlock);
> > txmsg->state = DRM_DP_SIDEBAND_TX_RX;
> > mstb->tx_slots[seqno] = NULL;
> > + mgr->is_waiting_for_dwn_reply = false;
> > mutex_unlock(&mgr->qlock);
> >
> > wake_up_all(&mgr->tx_waitq);
> > @@ -3830,6 +3837,9 @@ static int drm_dp_mst_handle_down_rep(struct
> drm_dp_mst_topology_mgr *mgr)
> > return 0;
> >
> > out_clear_reply:
> > + mutex_lock(&mgr->qlock);
> > + mgr->is_waiting_for_dwn_reply = false;
> > + mutex_unlock(&mgr->qlock);
> > if (msg)
> > memset(msg, 0, sizeof(struct drm_dp_sideband_msg_rx));
> > out:
> > @@ -4683,7 +4693,7 @@ static void drm_dp_tx_work(struct work_struct
> *work)
> > struct drm_dp_mst_topology_mgr *mgr = container_of(work, struct
> > drm_dp_mst_topology_mgr, tx_work);
> >
> > mutex_lock(&mgr->qlock);
> > - if (!list_empty(&mgr->tx_msg_downq))
> > + if (!list_empty(&mgr->tx_msg_downq) &&
> > +!mgr->is_waiting_for_dwn_reply)
> > process_single_down_tx_qlock(mgr);
> > mutex_unlock(&mgr->qlock);
> > }
> > diff --git a/include/drm/drm_dp_mst_helper.h
> > b/include/drm/drm_dp_mst_helper.h index 2d7c26592c05..96bcf33c03d3
> > 100644
> > --- a/include/drm/drm_dp_mst_helper.h
> > +++ b/include/drm/drm_dp_mst_helper.h
> > @@ -592,6 +592,11 @@ struct drm_dp_mst_topology_mgr {
> > */
> > bool payload_id_table_cleared : 1;
> >
> > + /**
> > + * @is_waiting_for_dwn_reply: whether we're waiting for a down reply.
> > + */
> > + bool is_waiting_for_dwn_reply : 1;
> > +
> > /**
> > * @mst_primary: Pointer to the primary/first branch device.
> > */
> >
--
Best regards,
Wayne Lin
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