Protocol Layers
The layers of the protocol are the AXI4-Stream layer, the transaction layer, the data link layer and
the physical layer, and they are described in subsequent sections.
AXI4-Stream Layer
The AXI4-Stream layer implements Xilinx-specific requirements. In the transmit or outbound
direction, the AXI4 layer interfaces the transaction layer with two AXI4-Stream interfaces. In the
receive or inbound direction, the transaction layer output is forwarded to two AXI4-Stream
interfaces. Application designs can attach to the AXI4-Stream interfaces, exchange information
with the Versal
®
ACAP CPM Mode for PCI Express encoded as a Xilinx-specific streaming
protocol implementation, and run on top of the industry standard AXI4-Stream interface. The
CPM4 PCIe controllers support management of up to 256 (extended tag) or 768 (10 bit Tag)
outstanding customer initiated read requests, as part of the streaming protocol. The AXI4-Stream
layer supports:
• Reception and transmission of address translation services (ATS) invalid requests, ATS invalid
completions, ATS page requests and ATS PRG response message TLPs, which enable ATS to
be implemented in the fabric logic.
• AXI4-Stream interface widths of 64 bits, 128 bits, 256 bits, 512 bits, and 1024 bits.
Transaction Layer
The transaction layer is the upper layer of the PCI Express architecture, and its primary function
is to accept, buffer, and forward transaction layer packets (TLPs). TLPs communicate information
with the use of memory, I/O, configuration, and message transactions. To maximize the efficiency
of communication between devices, the transaction layer enforces PCI-compliant transaction
ordering rules and supports relaxed ordering (RO) of received transactions. The transaction layer
also manages TLP buffer space through credit-based flow control. The transaction layer
implements built-in tag management for transmitted non-posted transactions. It also implements
cut-through forwarding of transactions in the transmit (or outbound) direction.
CCIX Transaction Layer
The Cache Coherent Interconnect for Accelerators (CCIX) transaction layer requirements are
implemented by the optional virtual channel 1 (VC1) in the design. Note that VC1 storage is in
addition to the PCI Express-compliant virtual channel 0 (VC0) storage. The CCIX transaction layer
interfaces with the CCIX protocol layer is implemented externally to the PCIe ports over the
CCIX transaction layer (ARM CXS) hard interface. For more information, see the Versal ACAP CPM
CCIX Architecture Manual (
).
Chapter 1: Overview
PG346 (v3.3) November 16, 2022
CPM Mode for PCI Express
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