Understanding Linux Network Internals

Interaction Between Devices and Kernel

Nearly all devices (including NICs) interact with the kernel in one of two ways:

Polling

Driven on the kernel side. The kernel checks the device status at regular intervals to see if it has anything to say.

Interrupt

Driven on the device side. The device sends a hardware signal (by generating an interrupt) to the kernel when it needs the kernel's attention.

In Chapter 9, you can find a detailed discussion of NIC driver design alternatives as well as software interrupts. You will also see how Linux can use a combination of polling and interrupts to increase performance. In this chapter, we will look only at the interrupt-based case.

I won't go into detail on how interrupts are reported by the hardware, the difference between hardware exceptions and device interrupts, how the driver and bus kernel infrastructures are designed, etc.; you can refer to Linux Device Drivers and Understanding the Linux Kernel for those topics. But I'll give a brief overview on interrupts to help you understand how device drivers initialize and register the devices they are responsible for, with special attention to the networking aspect.

Hardware Interrupts

You do not need to know the low-level background about how hardware interrupts are handled. However, there are details worth mentioning because they can make it easier to understand how NIC device drivers are written, and therefore how they interact with the upper networking layers.

Every interrupt runs a function ...