This reverts commit 6bb0942e8f.
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.
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.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 6bb0942e8f ("drm/dp_mst: Remove single tx msg restriction.")
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200423164225.680178-1-lyude@redhat.com
Reviewed-by: Sean Paul <sean@poorly.run>
973a5909e9