Chapter 6.3 – Types of Credits in PCI Express

Posted, Non-Posted, and Completion Credits (PH/PD/NPH/NPD/CPLH/CPLD) Explained

1 . Introduction

Every TLP transmitted in PCIe falls into one of three transaction categories:

1. Posted (P)
   
2. Non-Posted (NP)
   
3. Completion (CPL)
   

To manage buffer availability for these transactions, PCIe defines six credit counters per Virtual Channel:

• PH, PD – Posted Header/Data
  
• NPH, NPD – Non-Posted Header/Data
  
• CPLH, CPLD – Completion Header/Data
  

Each represents how many packets (headers) or data doublewords (payloads) can be accepted by the receiver.

2 . Why Multiple Credit Types?

Different packet classes have different communication patterns:

If a single shared buffer were used, large posted writes could block important completions — causing deadlock.
Hence, PCIe defines separate logical buffers and credit pools.

3 . Posted Credits (PH / PD)

Purpose:

Used for packets that do not expect a response, like:

• Memory Write
  
• Message
  
• Message with Data
  

Credit Types:

• PH – Posted Header credit
  
• PD – Posted Data credit (measured in DWs)
  

Behavior:

• Consumed immediately upon TLP transmission.
  
• Not restored (no completion expected).
  
• Only replenished when receiver frees buffer and sends a Flow Control Update DLLP.
  

Design Note:

Posted transactions can be transmitted aggressively to maximize bandwidth since they are non-blocking.

4 . Non-Posted Credits (NPH / NPD)

Purpose:

Used for TLPs that require a completion, such as:

• Memory Read
  
• I/O Read / Write
  
• Configuration Read / Write
  

Credit Types:

• NPH – Non-Posted Header credit
  
• NPD – Non-Posted Data credit (for writes that expect a response)
  

Behavior:

• Consumed when a Non-Posted request is transmitted.
  
• The requester must track all outstanding Non-Posted requests using Tag + Requester ID.
  
• Credits are only freed when:
  
  • The completer sends a Completion (Cpl / CplD), and
    
  • The requester processes and retires that completion.
    

💡 Key point: Non-Posted credits ensure the receiver won’t be flooded with reads or I/O operations that it cannot service.

5 . Completion Credits (CPLH / CPLD)

Purpose:

Used by a Completer (target device) to send completion packets for previous requests.

Credit Types:

• CPLH – Completion Header credit
  
• CPLD – Completion Data credit
  

Behavior:

• Required before sending Completion or Completion with Data (CplD) TLPs.
  
• Ensures the Requester (upstream device) has buffer space ready for incoming completions.
  
• Prevents deadlock if the Requester cannot accept more completion data.
  

Example:

An Endpoint sending a read response consumes:

• 1 CPLH (header)
  
• N CPLD (based on payload length)
  

6 . Why Separate Credits per TLP Type?

Let’s understand with an example of potential deadlock:

Scenario:

• Endpoint’s Completion queue is full.
  
• Root Complex keeps sending more Non-Posted Reads.
  
• If there were a shared buffer, these reads would block the completion path.
  

Result: Both sides wait forever — deadlock.

Solution in PCIe:

• Separate credits (and thus buffers) for each class ensure forward progress for Completions even if Posted/Non-Posted queues are full.
  

7 . Credit Consumption Examples

Example 1 – Memory Write (Posted)

• Header → consumes 1 PH
  
• Payload (8 DW) → consumes 8 PD
  

Example 2 – Memory Read (Non-Posted)

• Header → 1 NPH
  
• No data payload → no NPD
  

Example 3 – Completion with Data (CplD)

• Header → 1 CPLH
  
• Payload (16 DW) → 16 CPLD
  

8 . Interaction Between Credit Types

Each transaction’s flow control type depends on its TLP class:

⚙️ Hardware must maintain independent counters for each pair (H, D) and block transmission if any required counter reaches zero.

9 . Flow Control at System Level

Each direction of the link (Tx/Rx) maintains its own six credit counters per Virtual Channel.
For most designs using a single VC (VC0), this means 6 counters each for Tx and Rx.

If multiple VCs exist (VC0, VC1, etc.), then each VC has an independent set — preventing one class of traffic from consuming another’s credit pool.

10 . Debug and Verification Tip

In simulation or protocol analysis:

• Credits for each class can be tracked via Flow Control DLLPs.
  
• If PH or NPH counters reach zero while TLPs are still issued → design bug.
  
• If CPLD credits never increment → completion flow blocked, potential deadlock.
  

Example UVM check:

 assert(!(send_tlp && credits[type]==0))
 else $fatal("Sent TLP without credits!");

NOTES

• PCIe defines three transaction classes and six credit counters for precise buffer control.
  
• Posted Credits → for unidirectional requests (no completion).
  
• Non-Posted Credits → for requests requiring completions.
  
• Completion Credits → for sending completions back to requesters.
  
• Separation of credit pools ensures deadlock-free operation and high link utilization.
  
• Credit starvation or mismanagement is a top cause of flow stalls — always verify credit accounting logic.