Understanding the Linux Kernel, Second Edition

Sending Packets to the Network Card

A network card device driver is usually started either when the kernel inserts a packet in its transmit queue (as described in the previous section), or when a packet is received from the communication channel. Let’s focus here on packet transmission.

As we have seen, the qdisc_run( ) function is invoked whenever the kernel wishes to activate a network card device driver; it is also executed by the NET_TX_SOFTIRQ softirq, which is implemented by the net_tx_action( ) function (see Section 4.7).

Essentially, the qdisc_run( ) function checks whether the network card device is idle and can thus transmit the packets in the queue. If the device cannot do this — for instance, because the card is already busy in transmitting or receiving a packet, the queue has been stopped to avoid flooding the communication channel, or for whatever other reason — the NET_TX_SOFTIRQ softirq is activated and the current execution of qdisc_run( ) is terminated. At a later time, when the scheduler selects a ksoftirqd_CPUn kernel thread, the net_tx_action( ) function invokes qdisc_run( ) again to retry the packet transmission.

In particular, qdisc_run( ) performs the following actions:

1. Checks whether the packet queue is “stopped” — that is, whether a suitable bit in the state field of the net_device network card object is set. If it is stopped, the function returns immediately.

2. Invokes the qdisc_restart( ) function, which in turn performs the following actions:

   1. Invokes ...