TCPMv2: Handle timing corner case in TCPC PRL discard

If a message is received by the PD_INT task between the PRL_RX run and
PRL_TX run, we may end up sending a message to the TCPC even though the
PRL_RX will be forcing a discard next PRL cycle.  This will get the PE
out of sync, as it will assume the message was discarded when it was in
fact sent.

Peek into our receive queue right before sending a message to the TCPC,
in order to reduce the liklihood of a mix-up like this happening.

BRANCH=None
BUG=None
TEST=on voxel, run tast typec.Mode*.manual and observe we can
successfully exit DP mode even when a VDM:Attention command comes in
mid-PRL cycle

Signed-off-by: Diana Z <dzigterman@chromium.org>
Change-Id: I6979bf53972e40794dfae83f631b865a00d73302
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3192236
Reviewed-by: Scott Collyer <scollyer@chromium.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/driver/tcpm/tcpci.c b/driver/tcpm/tcpci.c
index 8a6aea2dbb..359cb04d30 100644
--- a/driver/tcpm/tcpci.c
+++ b/driver/tcpm/tcpci.c
@@ -1020,6 +1020,18 @@ int tcpci_tcpm_transmit(int port, enum tcpci_msg_type type,
 	}
 
 	/*
+	 * The PRL_RX state machine should force a discard of PRL_TX any time a
+	 * new message comes in.  However, since most of the PRL_RX runs on
+	 * the TCPC, we may receive a RX interrupt between the EC PRL_RX and
+	 * PRL_TX state machines running.  In this case, mark the message
+	 * discarded and don't tell the TCPC to transmit.
+	 */
+	if (tcpm_has_pending_message(port)) {
+		pd_transmit_complete(port, TCPC_TX_COMPLETE_DISCARDED);
+		return EC_ERROR_BUSY;
+	}
+
+	/*
 	 * We always retry in TCPC hardware since the TCPM is too slow to
 	 * respond within tRetry (~195 usec).
 	 *