[PATCH 1/2] platform/surface: aggregator: Do not check for repeated unsequenced packets

[Maximilian Luz <luzmaximilian@gmail.com>]

Currently, we check any received packet whether we have already seen it
previously, regardless of the packet type (sequenced / unsequenced). We
do this by checking the sequence number. This assumes that sequence
numbers are valid for both sequenced and unsequenced packets. However,
this assumption appears to be incorrect.

On some devices, the sequence number field of unsequenced packets (in
particular HID input events on the Surface Pro 9) is always zero. As a
result, the current retransmission check kicks in and discards all but
the first unsequenced packet, breaking (among other things) keyboard and
touchpad input.

Note that we have, so far, only seen packets being retransmitted in
sequenced communication. In particular, this happens when there is an
ACK timeout, causing the EC (or us) to re-send the packet waiting for an
ACK. Arguably, retransmission / duplication of unsequenced packets
should not be an issue as there is no logical condition (such as an ACK
timeout) to determine when a packet should be sent again.

Therefore, remove the retransmission check for unsequenced packets
entirely to resolve the issue.

Fixes: c167b9c7e3d6 ("platform/surface: Add Surface Aggregator subsystem")
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
---
 .../surface/aggregator/ssh_packet_layer.c     | 24 +++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/platform/surface/aggregator/ssh_packet_layer.c b/drivers/platform/surface/aggregator/ssh_packet_layer.c
index 6748fe4ac5d5..def8d7ac541f 100644
--- a/drivers/platform/surface/aggregator/ssh_packet_layer.c
+++ b/drivers/platform/surface/aggregator/ssh_packet_layer.c
@@ -1596,16 +1596,32 @@ static void ssh_ptl_timeout_reap(struct work_struct *work)
 		ssh_ptl_tx_wakeup_packet(ptl);
 }
 
-static bool ssh_ptl_rx_retransmit_check(struct ssh_ptl *ptl, u8 seq)
+static bool ssh_ptl_rx_retransmit_check(struct ssh_ptl *ptl, const struct ssh_frame *frame)
 {
 	int i;
 
+	/*
+	 * Ignore unsequenced packets. On some devices (notably Surface Pro 9),
+	 * unsequenced events will always be sent with SEQ=0x00. Attempting to
+	 * detect retransmission would thus just block all events.
+	 *
+	 * While sequence numbers would also allow detection of retransmitted
+	 * packets in unsequenced communication, they have only ever been used
+	 * to cover edge-cases in sequenced transmission. In particular, the
+	 * only instance of packets being retransmitted (that we are aware of)
+	 * is due to an ACK timeout. As this does not happen in unsequenced
+	 * communication, skip the retransmission check for those packets
+	 * entirely.
+	 */
+	if (frame->type == SSH_FRAME_TYPE_DATA_NSQ)
+		return false;
+
 	/*
 	 * Check if SEQ has been seen recently (i.e. packet was
 	 * re-transmitted and we should ignore it).
 	 */
 	for (i = 0; i < ARRAY_SIZE(ptl->rx.blocked.seqs); i++) {
-		if (likely(ptl->rx.blocked.seqs[i] != seq))
+		if (likely(ptl->rx.blocked.seqs[i] != frame->seq))
 			continue;
 
 		ptl_dbg(ptl, "ptl: ignoring repeated data packet\n");
@@ -1613,7 +1629,7 @@ static bool ssh_ptl_rx_retransmit_check(struct ssh_ptl *ptl, u8 seq)
 	}
 
 	/* Update list of blocked sequence IDs. */
-	ptl->rx.blocked.seqs[ptl->rx.blocked.offset] = seq;
+	ptl->rx.blocked.seqs[ptl->rx.blocked.offset] = frame->seq;
 	ptl->rx.blocked.offset = (ptl->rx.blocked.offset + 1)
 				  % ARRAY_SIZE(ptl->rx.blocked.seqs);
 
@@ -1624,7 +1640,7 @@ static void ssh_ptl_rx_dataframe(struct ssh_ptl *ptl,
 				 const struct ssh_frame *frame,
 				 const struct ssam_span *payload)
 {
-	if (ssh_ptl_rx_retransmit_check(ptl, frame->seq))
+	if (ssh_ptl_rx_retransmit_check(ptl, frame))
 		return;
 
 	ptl->ops.data_received(ptl, payload);
-- 
2.38.1