[AMBA][CHI] Channel-based architecture

 

Introduction

CHI defines ==five primary channels== for communication between a node and the interconnect.

The five Channels of AMBA CHI

1. Request Channel (TXREQ)

• Purpose
  • RN이 reading from memory, write to memory, or other cache-to-cache operation 같은 기본 기능을 요청할때 사용
• Direction
  • ==RN -> Interconnect==

Verilog Port Name	Description
TXREQFLITPEND	Flit[^1] 보류(Pending). Valid signal이 assert 되지 않았더라도 request가 보류되었음을 표시. QoS[^2] 스케줄링을 위해 사용.
TXREQVALID	Valid. High 일 때, TXREQFLIT의 데이터가 valid 상태임을 나타냄. interconnect에서 sampled 될 준비가 되었음을 표시.
TXREQFLIT	Flit Data. request packet 자체를 전달하는 payload bus. 아래의 주요 정보들을 포함한 여러 필드가 포함:  - QoS: Quality of Service. A ==priority level for the transaction==.  - TgtID: ==Target ID==. The destination node for the request.  - SrcID: ==Source ID==. The ID of the node sending the request.  - TxnID: ==Transaction ID==. A unique tag to identify this specific transaction.  - Opcode: The most important field. ==It defines the command== (e.g., ReadShared, WriteUnique, MakeInvalid).
TXREQCREDIT	Credit. ==CHI uses a credit-based flow control system==. INC는 RN에 credit을 부여하고, RN은 각각의 flit에 대해서 credit을 소비. 이 신호는 RN이 credit을 반환하고 있음을 표시.
### 2. Response Channel (TXRSP)
- Purpose
- RN이 수신한 snoop requests에 대한 응답
- Direction
- ==RN -> Interconnect==

Verilog Port Name	Description
TXRSPFLITPEND	Flit Pending. Same as the request channel, used for QoS.
TXRSPVALID	Valid. Indicates the response flit on TXRSPFLIT is valid.
TXRSPFLIT	Flit Data. The response packet payload. Key sub-fields include:  - TxnID: The ID of the snoop transaction this is a response to.  - Opcode: The response command (e.g., SnpRespData, SnpRespClean).  - Resp: The ==state of the cache line== (e.g., indicating if the data is clean or dirty).
TXRSPCREDIT	Credit. Returns a credit to the interconnect for the response channel.

3. Data Channel (TXDAT)

• Purpose
  • RN이 ==write data==를 interconnect로 보내기 위한 채널, ==snoop 요청에 대한 데이터==의 전송에도 사용
• Direction
  • ==RN -> Interconnect==

Verilog Port Name	Description
TXDATFLITPEND	Flit Pending. Used for QoS.
TXDATVALID	Valid. Indicates the data flit on TXDATFLIT is valid.
TXDATFLIT	Flit Data. The data packet payload. Key sub-fields include:  - TxnID: The ID of the transaction this data belongs to.  - Data: The actual data being written (e.g., 128 or 256 bits).  - BE: ==Byte Enables==. Specifies which bytes within the Data bus are valid. Essential for partial writes.
TXDATCREDIT	Credit. Returns a credit to the interconnect for the data channel.

4. Snoop Channel (RXSNP)

• Purpose
  • Interconnect가 snoop[^3] request를 RN에 보내는 채널
• Direction
  • ==Interconnect -> RN==

Verilog Port Name	Description
RXSNPFLITPEND	Flit Pending. Indicates a snoop is pending for QoS.
RXSNPVALID	Valid. Indicates the snoop flit on RXSNPFLIT is valid.
RXSNPFLIT	Flit Data. The snoop packet payload. Key sub-fields include:  - SrcID: The ID of the node that initiated the original request causing this snoop.  - TxnID: The unique ID for this snoop transaction.  - Addr: The memory address being snooped.  - Opcode: The snoop command (e.g., SnpShared, SnpUnique).
RXSNPCREDIT	Credit. The RN returns a credit to the interconnect for the snoop channel.

5. Receive Data & Completion Channel (RXDAT)

• Purpose
  • 중요한 2가지 목적의 복합채널
    • Read transaction에 의해 요청된 Data
    • Write transaction에 대한 최종 acknowledgement
• Direction
  • ==Interconnect -> RN==

Verilog Port Name	Description
RXDATFLITPEND	Flit Pending. Used for QoS.
RXDATVALID	Valid. Indicates the data/completion flit on RXDATFLIT is valid.
RXDATFLIT	Flit Data. The payload. Key sub-fields include:  - TxnID: The ID of the original transaction this is the response for.  - Opcode: The response command (e.g., CompData for read data, Comp for write completion).  - Resp: The final state of the transaction (e.g., OK, Error).  - Data: The actual read data, if applicable.
RXDATCREDIT	Credit. The RN returns a credit to the interconnect for this channel.
## Summary Table
Request Node(CPU Core) 관점에서의 summay

Channel Name	Direction	Purpose	Key Payload Fields
TXREQ	RN -> ICN	Send Read/Write/Cache-Mgmt requests	Opcode, Addr, TxnID
TXRSP	RN -> ICN	Respond to snoops	Opcode, Resp, TxnID
TXDAT	RN -> ICN	Send write data	Data, BE, TxnID
RXSNP	ICN -> RN	Receive snoop requests	Opcode, Addr, TxnID
RXDAT	ICN -> RN	Receive read data & write completions	Opcode, Data, Resp, TxnID

---

[^1]: "Flow control digit", transfer의 가장 기본단위
[^2]: Quality of Service
[^3]: snoop은 특정 메모리 address가 있는지, 그리고 그 상태가 어떤지 확인하기 위해 cache에 보내는 query. 이는 cache coherency를 위한 핵심 매커니즘.