PCI Express Base Specification v1.0 - 2002.pdf

PCI EXPRESS BASE SPECIFICATION, REV. 1.0Isochronous traffic regulation from any ingress port is implemented as part of the PortArbitration of the target Egress Port. Specifically, a time-based WRR Port Arbitration isused to schedule isochronous read and/or write request transactions. The N max virtualtimeslots (t) within the isochronous time period (T) are represented by the time-based WRRTable in the PCI Express Virtual Channel Capability Structure detailed in Section 5.11. Thetable consists of N max entries. A table entry represents one virtual timeslot. An ingress Portis assigned with N link virtual timeslots when N link entries in the target Egress Port's time-basedWRR Port Table are set to the ingress port's Port Number.The above isochronous traffic regulation mechanism only applies to request transactions butnot to completion transactions. As read completion transactions only come from upstreamport and go to downstream ports, no Port Arbitration is needed. When Endpoint-to-Root-Complex and Peer-to-Peer communications co-exist in a Switch, a downstream (egress) portmay mix isochronous write requests and read completions in the same direction. In the caseof contention, the Egress Port must allow write requests to pass read completions to ensurethe Switch meet latency requirement for isochronous requests.A.10.4.Root ComplexA Root Complex may have multiple Root Ports capable of supporting isochronoustransactions. Before a Root Complex starts serving isochronous transactions for a Root Port,the port must be configured by the operating system-level PCI Express configurationsoftware to enable an Isochronous VC using the following configuration steps:• Configuration of an Isochronous Virtual Channel that Isochronous Traffic Class ismapped to.• Configuration of the Root Port as an Ingress Port:o Configuration (or reconfiguration if the Isochronous VC in RCRB is alreadyenabled) of the time-based WRR Port Arbitration Table of the targeting RCRBto include N link entries set to the ingress port's Port Number. Here N link is theisochronous allocation for the ingress port.o Enabling the targeting RCRB to load newly programmed Port ArbitrationTable.• Configuration of the Root Port as an Egress Port:o If supported, configuration of the Root Port's VC Arbitration Table with largeweights assigned to the Isochronous VC.• Enabling of the Isochronous VC for the Root Port.A Root Complex must observe the maximum isochronous transaction latency (L Completer ormore precisely L Root_Complex ) that applies to all the Root Ports in the Root Complex. How aRoot Complex schedules memory cycles for PCI Express isochronous transactions andother memory transactions is outside of the scope of this specification as long as L Root_Complex ismet for PCI Express isochronous transactions.When an Isochronous VC is enabled for a Root Port, the Root Complex must enforceproper traffic regulation to ensure that isochronous traffic from the Root Port confirms to397