Overview

The goal of this chapter is to cover some advanced IXP12xx programming topics done infrequently, such as programming the ready-bus sequencer, or done only in certain applications, such as using the CRC features of the IXP1240 and IXP1250, or represent performance optimizations to ingress and egress that work only with certain hardware configurations.

You can choose to read, or skip, the sections of this chapter as they relate to your needs. The details of this chapter are probably best read as a reference after you discover you need to take advantage of them. The details of the receive and transmit operations in this chapter assume a familiarity with the previous chapters in this book.

This chapter covers the following four programming topics:

• Receiving packets from MAC fast ports. Certain IX bus MAC ports support speculative receive-requests. Such MAC ports are called fast ports. Fast ports allow receive-requests to be issued without first checking the receive-ready bits. Such speculative receive-requests can provide a large, and often necessary, performance gain on high-speed ports. The tradeoff is yet more receive complexity.

• Transmitting packets on multiple ports with different speeds. Due to the design of the transmit state machine and TFIFOs, dealing with ports of different speeds can be tricky. Preventing slower ports from affecting the transmit speed of faster ports requires some tradeoffs in the transmit code.

• Programming the ready-bus sequencer. The FBI s ready-bus sequencer can be programmed, with a limited instruction set, to probe, and inform the microengines about, the...