Pipelining in AXI is a mechanism that allows multiple operations to be in progress simultaneously on the bus, significantly improving throughput.
Understanding the AXI Handshake
Before delving into pipelining, it's crucial to understand the fundamental data transfer mechanism in AXI. As the reference states: "A single cycle data transfer over an AXI interface involves an upstream entity asserting valid and data on the AXI interface, and a downstream entity accepting the data while driving a ready signal back upstream to confirm that the data can be accepted."
This valid/ready handshake is the core synchronization method for moving address, data, and control information across the AXI channels. Data is transferred only when both the VALID signal from the source and the READY signal from the destination are asserted simultaneously.
What Pipelining Enables
Without pipelining, a master might have to wait for the full completion of one transaction (address phase, data phase, response) before it can issue the address for the next transaction. Pipelining removes this limitation.
Pipelining in AXI allows:
- Overlapping Operations: Different phases of multiple transactions can occur concurrently. For example, while the data for transaction A is being transferred, the address for transaction B can be sent.
- Increased Throughput: By not waiting for each transaction to fully complete before starting the next, the bus can transfer data more efficiently.
How Pipelining Works in AXI
AXI achieves pipelining primarily through its channel structure and the ability to issue multiple outstanding transactions.
AXI is based on separate channels:
- Read Address Channel (
AR): Master sends read addresses. - Read Data Channel (
R): Slave returns read data and response. - Write Address Channel (
AW): Master sends write addresses. - Write Data Channel (
W): Master sends write data. - Write Response Channel (
B): Slave returns write response.
Pipelining is inherent in how these channels can operate relatively independently.
Key Aspects of AXI Pipelining:
- Separation of Address and Data: The address for a transaction is sent on one channel (AR or AW), and the corresponding data is sent on a different channel (R or W). These can be separated in time.
- Outstanding Transactions: An AXI master can issue multiple address requests (read or write) before the slave responds with the first piece of data or response. Each outstanding transaction is typically identified by a unique ID tag (
ARID,AWID). - Out-of-Order Completion (for Reads): With multiple outstanding read transactions, the slave can return the data for these transactions in a different order than the addresses were received, as long as the transaction ID is used to match the data to the correct address.
- Write Data Interleaving: Write data from different transactions can be interleaved on the W channel, using the
WIDsignal to identify which transaction the data belongs to.
Benefits of AXI Pipelining
| Feature | Non-Pipelined AXI Transaction | Pipelined AXI Transactions |
|---|---|---|
| Operation Flow | Sequential phases | Overlapping phases, concurrent transactions |
| Throughput | Lower | Higher |
| Latency | Single transaction focus | Multiple transactions in flight, potentially hiding latency |
| Bus Utilization | Lower | Higher |
Pipelining allows the master and slave to work on different parts of multiple transactions simultaneously, which is critical for high-performance systems where memory latency or peripheral response times can be significant.
Practical Example
Imagine a processor needing to read 10 separate data words from a memory controller over AXI.
- Without Pipelining: The processor sends address 1, waits for data 1 to return. Then sends address 2, waits for data 2, and so on.
- With Pipelining: The processor sends address 1, then immediately sends address 2, address 3, etc., up to the maximum outstanding limit. The memory controller receives addresses, fetches data in parallel, and sends data 1, data 2, data 3, etc., back to the processor as they become ready, potentially with some delay between address issue and the first data return, but subsequent data returns can be continuous.
This ability to pipeline address and data phases and handle multiple outstanding transactions is a key feature of the AXI protocol that distinguishes it from simpler bus protocols.
