RE: [PATCH] Revert "drm/dp_mst: Remove single tx msg restriction."

["Lin, Wayne" <Wayne.Lin@amd.com>]

[AMD Public Use]



> -----Original Message-----
> From: Wentland, Harry <Harry.Wentland@amd.com>
> Sent: Friday, April 24, 2020 1:53 AM
> To: Lyude Paul <lyude@redhat.com>; dri-devel@lists.freedesktop.org
> Cc: David Airlie <airlied@linux.ie>; linux-kernel@vger.kernel.org; Sean Paul
> <seanpaul@chromium.org>; Thomas Zimmermann <tzimmermann@suse.de>;
> Lin, Wayne <Wayne.Lin@amd.com>; Lin, Wayne <Wayne.Lin@amd.com>
> 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@amd.com>

> > 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@amd.com>
> 
> Harry
> 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > Fixes: 6bb0942e8f46 ("drm/dp_mst: Remove single tx msg restriction.")
> > Cc: Wayne Lin <Wayne.Lin@amd.com>
> > Cc: Sean Paul <seanpaul@chromium.org>
> > ---
> >  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