If using flow control with a high CTS the thread may be
blocked forever on the second transmit attempt due to waiting
on the txdma semaphore. The calling thread can then never
make progress and release any resources it has taken, thus
may cause a deadlock in other parts of the system.
The implementation differs in behavior from interrupt-driven
TX. It should not implicitly wait on a taken semaphore but
return immediately and let the upper layers decide on what to
do next.
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