Stream FIFO Depth Specification - 2022.2 English

Vitis Model Composer User Guide (UG1483)

Document ID
UG1483
Release Date
2023-01-13
Version
2022.2 English

The AI Engine architecture uses streaming data extensively for communicating between two AI Engines, and for communicating between the AI Engine and the programmable logic (PL). This raises the potential for a resource deadlock when the data flow graph has reconvergent stream paths. If the pipeline depth of one path is longer than the other, the producer kernel can stall and might not be able to push data into the shorter path because of back pressure. At the same time, the consumer kernel is waiting to receive data on the longer path due to the lack of data. If the order of data production and consumption between two stream paths is different, a deadlock can happen even between two kernels that are directly connected with two stream paths.

Vitis Model Composer supports adding a FIFO_DEPTH between two AI Engine kernels or between an AI Engine and the programmable logic (PL) using the AIE FIFO block and automatically generates the graph code with fifo_depth constraint on a connection.

Figure 1. AIE FIFO

The AIE FIFO block specifies the FIFO depth on a particular path. Specifying the FIFO depth value can help avoid deadlock or stalling by creating more buffering in the paths. This block does not affect functional simulation and will only impact the generated graph code. You can set location constraints for the FIFO block using the Constraint manager. FIFOs can be either stream FIFOs, DMA FIFOs, or a combination of the two.

Figure 2. Block Parameters: AIE FIFO
Parameters
Use this tab to specify FIFO depth.
Constraints
Use this tab to set location constraints for the FIFO block using the Constraint manager.

Consider the following example where the AIE FIFO blocks are connected in two stream paths between AIE Kernel blocks.

Figure 3. AIE FIFO Blocks in Two Stream Paths

The FIFO depth value is are specified as 0 by default and the corresponding information is reflected on the block symbol. For example, if values 2 and 4 are specified as parameters of the two FIFO blocks, the values will update as shown in the following figures.

Figure 4. AIE FIFO
Figure 5. AIE FIFO
Figure 6. Block Symbol Updates

The stream FIFO values specified using the AIE Signal Spec block are automatically updated in the generated graph code (graph.h) with fifo_depth constraints as shown in the following code.

Snippet of graph.h

// create nets to specify connections
      adf::connect< adf::stream > net0 (In1, AIE_Kernel.in[0]);
      adf::connect< adf::stream > net1 (AIE_Kernel.out[0], AIE_Kernel1.in[0]);
      adf::fifo_depth(net1) = 2;
      adf::connect< adf::stream > net2 (AIE_Kernel.out[1], AIE_Kernel1.in[1]);
      adf::fifo_depth(net2) = 4;
      adf::connect< adf::stream > net3 (AIE_Kernel1.out[0], Out1);

Notice the two fifo_depth constraints corresponding to the two stream paths in the design. Using this fifo_depth constraint on a connection is useful as it creates more buffering in paths with back pressure, and hence avoids any deadlock.

Important: The AIE FIFO block can only be used in an AI Engine subsystem.