Linux Device Drivers, Second Edition

The Interrupt Handler

Most hardware interfaces are controlled by means of an interrupt handler. The interface interrupts the processor to signal one of two possible events: a new packet has arrived or transmission of an outgoing packet is complete. This generalization doesn’t always apply, but it does account for all the problems related to asynchronous packet transmission. Parallel Line Internet Protocol (PLIP) and Point-to-Point Protocol (PPP) are examples of interfaces that don’t fit this generalization. They deal with the same events, but the low-level interrupt handling is slightly different.

The usual interrupt routine can tell the difference between a new-packet-arrived interrupt and a done-transmitting notification by checking a status register found on the physical device. The snull interface works similarly, but its status word is implemented in software and lives in dev->priv. The interrupt handler for a network interface looks like this:

void snull_interrupt(int irq, void *dev_id, struct pt_regs *regs) { int statusword; struct snull_priv *priv; /* * As usual, check the "device" pointer for shared handlers. * Then assign "struct device *dev" */ struct net_device *dev = (struct net_device *)dev_id; /* ... and check with hw if it's really ours */ if (!dev /*paranoid*/ ) return; /* Lock the device */ priv = (struct snull_priv *) dev->priv; spin_lock(&priv->lock); /* retrieve statusword: real netdevices use I/O instructions */ statusword = priv->status; if (statusword ...