Chapter 6. IO Ordering

Chapter 6. I/O Ordering

The Previous Chapter

The previous chapter described the rules governing acceptance, forwarding, and rejection of packets seen by HyperTransport devices. Several factors come into play in routing, including the packet type, the direction it is moving, and the device type which sees it. A related topic also covered in this chapter is the fairness algorithm used by a tunnel device as it inserts its own packets into the traffic it forwards upstream on behalf of devices below it. The HyperTransport specification provides a fairness algorithm and a hardware method for tunnel management packet insertion.

This Chapter

This chapter describes the ordering rules which apply to packets associated with the three types of HyperTransport I/O traffic: PIO, DMA, and Peer-to-Peer. Depending on whether compatibility with the full producer-consumer ordering model used in PCI is required or relaxed ordering is permissible, attribute bits in request and response packets may be set or cleared. These bits are defined by the requester and are used by devices in the path to the target, and within the target, to enforce proper ordering. HyperTransport applies dedicated sets of ordering rules for upstream I/O traffic, downstream I/O traffic, and the special ordering required of host bridges and in double-hosted chains. Refer to Chapter 20, entitled "I/O Compatibility," on page 457 for a description of the additional ordering requirements when interfacing HyperTransport to other compatible protocols (e.g. PCI, PCI-X, and AGP).

The Next Chapter

In the next chapter, examples are presented which apply the packet principles described in the preceding chapter. The examples also entail more complex system transactions than discussed previously, including reads, posted and non-posted writes , and atomic read-modify-write operations.