Understanding Linux Network Internals

Notifying Drivers When Frames Are Received

In Chapter 5, I mentioned that devices and the kernel can use two main techniques for exchanging data: polling and interrupts. I also said that a combination of the two is also a valid option. This section offers a brief overview of the most common ways for a driver to notify the kernel about the reception of a frame, along with the main pros and cons for each one. Some approaches depend on the availability of specific features on the devices (such as ad hoc timers), and some need changes to the driver, the operating system, or both.

Figure 9-2. Ingress path (frame reception)

This discussion could theoretically apply to any device type, but it best describes those devices like network cards that can generate a high number of interactions (that is, the reception of frames).

Polling

With this technique, the kernel constantly keeps checking whether the device has anything to say. It can do that by continually reading a memory register on the device, for instance, or returning to check it when a timer expires. As you can imagine, this approach can easily waste quite a lot of system resources, and is rarely employed if the operating system and device can use other techniques such as interrupts. Still, there are cases where polling is the best approach. We will come back to this point later.

Interrupts

Here the device driver, on behalf of the kernel, ...