FreeBSD Device Drivers

Packet Transmission

To transmit a packet, the network stack calls a driver’s output routine. All output routines end by calling their interface’s transmit or start routine. Here is em(4)’s start routine:

static void
em_start(struct ifnet *ifp)
{
        struct adapter *adapter = ifp->if_softc;
        struct tx_ring *txr = adapter->tx_rings;

        if (ifp->if_drv_flags & IFF_DRV_RUNNING) {
              EM_TX_LOCK(txr);
              em_start_locked(ifp, txr);
                EM_TX_UNLOCK(txr);
        }
}

This start routine acquires a lock and then calls em_start_locked.

em_start_locked Function

The em_start_locked function ...