Initial Configuration Accesses

What's Going On During Initialization Time?

During initialization time, the startup configuration software is accessing the configuration registers within each function to determine the presence of a function as well as its resource requirements. Immediately after RST# is removed from a PCI or a PCI-X function, it may not be prepared to service configuration accesses on a timely basis. As an example, a function's configuration registers might not contain valid default values immediately after RST# is removed. Perhaps the function must start backloading this information into its configuration registers from a serial EEPROM. In this case, it could be a substantial amount of time after RST# removal before the function can provide read data from or accept write data into its configuration registers. For this reason, functions do not have obey the 16 clock first Data Phase completion rule during initialization time.

Definition of Initialization Period In PCI

As defined in the PCI 2.3 spec, Initialization Time (Trhfa) begins when RST# is deasserted and completes 2²⁵ PCI clocks later (32 mega-cycles). This parameter is referred to in the spec as Trhfa (Time from Reset High-to-First-Access). At a bus speed of 33MHz, this equates to 1.0066 seconds, while it equates to 0.5033 seconds at a bus speed of 66MHz. Run-time follows initialization-time. If a target is accessed during initialization-time, it is allowed to do any of the following:

• Ignore the request (except if it is a boot device). A boot device is one that must respond as a target in order to allow the processor to access the boot ROM. In a typical PC design, this would be ICH (IO Control Hub). Devices in the processor's path to the boot ROM should be prepared to be the target of a transaction immediately after Trhff expires (five clock cycles after RST# is deasserted).

• Claim the access and hold in Wait States until it can complete the request, not to exceed the end of Initialization Time.

• Claim the access and terminate with Retry.

Definition of Initialization Period In PCI-X

In PCI-X, Trhfa is 2²⁶ clocks (64 mega-cycles) in duration rather than 2²⁵ as it is in PCI. This is because the PCI-X clock speed can be substantially faster than (up to 133MHz) the PCI clock speed and if this parameter remained the same as the PCI Trhfa spec, Initialization Time would be reduced to 0.25 seconds (at a clock speed of 133MHz).

During Initialization Time, a PCI-X target has the same options available as a PCI target does (see previous section).

PCI Express and Initialization Time

Just as in PCI or PCI-X, some devices in a PCI Express environment may go through a rather long self-initialization sequence to before they are able to service configuration access requests.

When a PCI Express device receives a configuration request it may respond with a Configuration Request Retry Completion Status (CRS). Requester receipt of a Completion with Configuration Request Retry Status terminates the configuration access request on PCI Express.

Initial Configuration Access Failure Timeout

After a PCI Express device is reset, the Root Complex and/or system software must allow 1.0s (+50%/-0%) for the device to return a Successful Completion status before deciding that the device has malfunctioned. This is analogous to the PCI/PCI-X Trhfa parameter.

When attempting a configuration access to a device on a PCI or PCI-X bus downstream of a PCI Express-to-PCI or -PCI-X bridge, Trhfa must be taken into account.

Delay Prior To Initial Configuration Access to Device

After system hardware or software causes one or more devices to be reset, software must wait at least 100ms from the end of reset before issuing any configuration requests to those devices. This time period is allocated to allow the device(s) to complete internal initialization.

The system design must guarantee (in a design-specific manner) that all components that must be software visible at boot time are ready to receive configuration requests within 100ms of the deassertion of Fundamental Reset at the Root Complex.

A Device With a Lengthy Self-Initialization Period

If a PCI Express device requires additional time to finish its self initialization, the system design must provide a design-specific mechanism for re-issuing configuration requests terminated with CRS status after the initial 1s timeout has elapsed.

To ensure proper enumeration of the system in a system running legacy PCI/PCI-X based software, the Root Complex hardware must re-issue the configuration request.

RC Response To CRS Receipt During Run-Time

After initialization time has elapsed, the action(s) taken by the Root Complex upon receipt of a Configuration Request Retry Completion Status is implementation-specific. It may re-issue the configuration request as a new request or may indicate failed completion to the processor.

If the Root Complex is designed to automatically retry the request, the number of retries attempted before indicating a failure to the processor is design-specific.

During Run-Time, support for a Completion Timeout (and the duration of the timeout) for configuration requests are implementation-specific.

The default setting in a PCI Express-to-PCI or -PCI-X bridge prevents it from returning a Configuration Request Retry Status (CRS) for a configuration request that targets a PCI or PCI-X device downstream of the bridge. This can result in a lengthy completion delay that must be taken into account by the Completion Timeout value used by the Root Complex. Configuration software can enable such a bridge to return Configuration Request Retry Status by setting the Bridge Configuration Retry Enable bit in the bridge's Device Control register.