What could be the reason for 6lo send queue heap up?

[lancersky]

Describe the bug A clear and concise description of what the bug is.

To Reproduce Information to reproduce the behavior, including:

1. Git commit id
   openthread/ot-br-posix@5be3594
   RCP(EFR32MG21F768) firmware base on GSDK4.0.2.
2. IEEE 802.15.4 hardware platform
3. Build steps
4. Network topology

Expected behavior A clear and concise description of what you expected to happen.

Console/log output If applicable, add console/log output to help explain your problem.

Additional context Add any other context about the problem here.

[jwhui]

The 6lo send queue not being serviced is typically an indicator that the radio driver is stuck. In particular, the radio was requested to send a packet, but the radio driver never provided indication that it completed its transmission operation.

See:

openthread/src/core/mac/mac.cpp

Line 921 in 424005f

void Mac::BeginTransmit(void)

openthread/src/core/mac/mac.cpp

Line 1259 in 424005f

void Mac::HandleTransmitDone(TxFrame &aFrame, RxFrame *aAckFrame, Error aError)