PCIe Protocol – Part 7 - logicmadness.com

Focuses on advanced Data Link Layer features: DLLPs, flow control, ACK/NAK, DLCMSM, and data integrity.

Table of Contents

1. Advanced Data Link Layer (DLL) Overview

The Data Link Layer (DLL) ensures reliable delivery of TLPs by:

Managing flow control via credits
Sending DLLPs (Data Link Layer Packets)
Handling ACK/NAK responses
Supporting replay mechanisms
Maintaining data integrity across the link

ASCII Layer Diagram

+--------------------+
| Transaction Layer  | <- TLPs
+--------------------+
| Data Link Layer    | <- DLLPs (ACK/NAK, Flow Control)
+--------------------+
| Physical Layer     | <- Electrical lane signals
+--------------------+

2. DLLPs (Data Link Layer Packets)

2.1 Types of DLLPs

DLLP Type	Purpose
ACK	Confirms TLP received correctly
NAK	Indicates TLP error or lost packet
Flow Control Update (FC)	Updates credit counts for header/data buffers
Sequence Number Update	Synchronizes flit sequence numbers

ASCII Example: ACK Flow

[TX Buffer] -> TLP -> [RX Buffer]
                      |
                   DLL Layer
                      |
                    ACK DLLP -> TX removes from replay buffer

3. Flow Control Mechanism

PCIe uses credit-based flow control to ensure no buffer overflows:

Header Credits: Track available header buffer space
Data Credits: Track available data buffer space
Posted / Non-Posted Credits: Separate for write vs read operations

Flow Control Update DLLP (FC Update)

[RX Device] -> sends FC Update DLLP -> [TX Device]
TX Device updates credits:
  Header Credits
  Posted Data Credits
  Non-Posted Data Credits

Transmission only proceeds if sufficient credits are available.

4. DLCMSM – Data Link Control and Status Machine

DLCMSM manages:

DLLP transmission/reception
Flow control updates
Error recovery and replay buffer management
Sequence number tracking

ASCII DLCMSM Example

[TX DLCMSM] <---> [RX DLCMSM]
Tracks:
- Sent TLPs
- Sequence numbers
- ACK/NAK feedback
- Credit availability

5. ACK/NAK Protocol

Signal	Action
ACK	Remove TLP from replay buffer
NAK	Re-transmit TLP from replay buffer
Discard	Drop invalid TLP after multiple retries

Replay Flow (ASCII)

TX Buffer: [TLP1][TLP2][TLP3]
RX: Receives TLP1, TLP3 (TLP2 corrupted)
RX sends NAK for TLP2
TX resends TLP2 -> RX receives correctly -> sends ACK

Ensures lossless delivery at high speeds.

6. Flow Control Initialization

At link startup, devices exchange FC credits
DLCMSM initializes buffers, sequence numbers, and thresholds
Periodic UpdateFC DLLPs ensure continuous credit synchronization

ASCII Initialization Example

[TX DLCMSM] --send--> INIT FC DLLP --> [RX DLCMSM]
[RX DLCMSM] --reply--> FC ACK DLLP --> [TX DLCMSM]

7. Data Integrity Mechanisms

CRC in TLP and DLLP: Detects errors during transmission
Replay Buffers: Store TLPs until ACK received
Sequence Numbers: Ensure correct order of packets
FEC (Gen6): Forward Error Correction adds redundancy for error recovery

ASCII Data Integrity Example

[TX TLP] -> [Physical Layer] -> [RX DLL]
        CRC Check
        |       \
       OK       Error -> NAK + Replay

8. DLLP Timing and Frequency

UpdateFC Frequency: How often FC DLLPs are sent
Must match traffic rate to avoid stalling the link
High-speed Gen5/Gen6 links require tight timing control

9. Summary – Advanced Data Link Layer

Responsibilities:

Transmit/receive DLLPs (ACK/NAK/Flow Control)
Initialize and track credit buffers
Manage DLCMSM for error handling and sequence numbers
Maintain data integrity using CRC, replay buffers, FEC
Ensure continuous flow control updates and link reliability