BSP and Device Driver Development Guide
This function is called each time the network stack wants to start the
transmitter. This occures whenever the network stack adds a packet
to a device's send queue and the IFF_OACTIVE bit in the
device's if_flags is not set.
For many devices this function need only set the IFF_OACTIVE bit in the
if_flags and send an event to the transmit task
indicating that a packet is in the driver transmit queue.
BSP and Device Driver Development Guide
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