HyperTransport Flow Control
The Previous Chapter
The previous chapter described the use of HyperTransport control and data packets to construct HyperTransport link transactions. Control packet types include Information, Request, and Response variants; data packets contain a payload of 0-64 valid bytes. The transmission, structure, and use of each packet type is presented.
This chapter describes HyperTransport flow control, used to throttle the movement of packets across each link interface. On a high-performance connection such as HyperTransport, efficient management of transaction flow is nearly as important as the raw bandwidth made possible by clock speed and data bus width. Topics covered here include background information on bus flow control and the initialization and use of the HyperTransport virtual channel flow control buffer mechanism defined for each transmitter-receiver pair.
The Next Chapter
The next chapter describes 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.
On any bus where an agent initiates the exchange of information (commands, data, status, etc.) with a target, a number of things can cause a delay (or even end) the normal completion of the intended transfer. The throttling of information delivery on a bus is referred to as flow control. PCI is a good example of a bus protocol which has reasonably high burst bandwidth, but is subject to performance hits caused by an unsophisticated flow control mechanism. Before looking at the HyperTransport approach to flow control, some of the general problems in bus flow control are described in the following section in terms of the PCI protocol. Refer to Figure 5-1 on page 100.
Figure 5-1: PCI Interface Handshake Signals
How PCI Handles Flow Control
While the PCI specification permits 64-bit data bus and 66MHz clock options, a generic PCI bus carries only 32 bits (4 bytes) of data and runs at a 33MHz clock speed. This means that the burst bandwidth for this bus is 132MB/s (4 bytes x 33MHz = 132MB/s). In many systems the PCI bus is populated by all sorts of high- and low-performance peripherals such as hard drives, graphics adapters, and serial port adapters. All PCI bus master devices must take turns accessing the shared bus and performing their transfers. The priority of a bus master in accessing the bus and the amount of time it is allowed to retain control of the bus is a function of PCI arbitration. In a typical computer system, the PCI arbiter logic resides in the system chipset.
Once a PCI bus master has won arbitration and verifies the bus is idle, it commences its transaction. After decoding the address and command sent by the master, one target claims the cycle by asserting a signal called DEVSEL#. At this point, if both devices are prepared, either write data will be sent by the initiator or read data will be returned by the target. For cases where either the master or target are not prepared for full-speed transfer of some or all of the data, flow control comes into play. In PCI there are a number of cases that must be dealt with.
PCI Target Flow Control Problems
PCI Target Not Ready To Start
In some cases, a PCI device being targeted for transmission is not prepared to transfer any data at all. This could happen if the target is off-line, does not have buffer space for write data being sent to it, or does not have requested read data available. It may also occur if the transaction must cross a bridge device to a different bus. Many bus protocols, including PCI, place a limit on how long the bus may be stalled before completing a transaction; in cases where a target can't meet the requirement for even the first data, a mechanism is required to indicate the transaction should be abandoned and re-attempted later. PCI calls the target cancellation of a transaction (without transferring any data) a Retry; a Retry is indicated when a target asserts the STOP# signal (instead if TRDY#) in the first data phase.
PCI Target Starts Data Transfer, But Can't Continue
Another possibility is that a transaction started properly, some data has transferred, but at some point before completion the target "realizes" it can't continue the transfer within the time allowed by the protocol. The target must indicate to the master that the transaction must be suspended (and resumed later at the point where it left off). PCI calls this target suspension of a transaction (with a partial transfer of data) a Disconnect. A Disconnect is signalled when the target asserts the STOP# signal in a data phase after the first one.
PCI Target Starts, Can Continue, But Needs More Time
Sometimes a transaction is underway and the target requires additional time to complete transmission of a particular data item; in this case, it does not need to suspend the transaction altogether, but simply stretch one or more data phases. The generic name for this is wait-state insertion. Wait states are a reasonable alternative to Retry and Disconnect if there are not too many of them; when there are excessive wait states, bus performance would be better served by the devices giving up the bus and allowing it to be used by other devices while they prepare for the resumption of the suspended transaction. PCI targets de-assert the TRDY# signal during any data phase to indicate wait states. A target must be prepared to complete each data phase within 8 PCI clocks (maximum of seven wait states), except for the first data phase which it must complete within 16 clocks. If a target cannot meet the "16 and 8 tick" rules for completing a data phase, it must signal Retry or Disconnect instead.
PCI Initiator Flow Control Problems
While many flow control problems are associated with the target of a transaction, there are a couple which may occur on the initiator side. Again, the cases are described in terms of PCI protocol.
PCI Initiator Starts, But Can't Continue
Some bus protocols also allow an initiator to break off a transaction early in the event it can't accept the next read data or source the next write data within the time allowed by the protocol even with wait states. PCI initiators suspend transactions simply by de-asserting the FRAME# signal early. As a rule, the master will re-arbitrate later for the PCI bus and perform a new transaction which picks up from where it left off previously.
PCI Initiator Starts, Can Continue, But Needs Wait-States
Some bus protocols allow an initiator to insert wait states in a transfer, just as the target may. Other bus protocols (e.g. PCI-X) only allow targets to insert wait states based on the assumption that a device which starts a transaction should be ready to complete it before requesting the bus. In any case, PCI initiators de-assert the IRDY# signal to indicate wait states. An initiator must be prepared to complete each data phase within 8 clocks (maximum of seven wait states); if it can't meet this rule for any data phase, it must instead suspend the transaction by de-asserting FRAME#.
All PCI Flow Control Problems Hurt Performance
Each of the initiator and target flow control problems just described impact PCI bus performance for both the devices involved in the transfer, and for devices waiting to access the bus. While not every transaction is afflicted with target retries and disconnects, or early de-assertion of FRAME# by initiators, they happen enough to make effective bandwidth considerably less than 132MB/s on the PCI bus. In addition, arbitration and flow control uncertainties make system performance difficult to estimate.