Asynchronous Buffer Port Access - 2023.2 English

AI Engine Kernel and Graph Programming Guide (UG1079)

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2023.2 English

In some situations, if you are not consuming a buffer port worth of data on every invocation of a kernel, or if you are not producing a buffer port worth of data on every invocation, then you can control the buffer synchronization by declaring the kernel port using async to declare the async buffer port in kernel function prototype. The example below illustrates that the kernel simple uses:

Synchronous input buffer port.
Asynchronous input buffer port.
Asynchronous output buffer port.

The declaration below informs the compiler to omit synchronization of the buffer named wts upon entry to the kernel. You must use buffer port synchronization member function shown inside the kernel code before accessing the buffer port using read/write iterators/references, as shown below.

void simple(adf::input_buffer<uint8>& ifm, adf::input_async_buffer<uint8>& wts, adf::output_async_buffer<uint8>& ofm)
    wts.acquire(); // acquire lock unconditionally inside the kernel
    if (<somecondition>) {
        ofm.acquire(); // acquire output buffer conditionally
    ... // do some computation
    wts.release(); // release input buffer port inside the kernel
    if (<somecondition>) {
        ofm.release(); // release output buffer port conditionally

The acquire() member function of the buffer object wts performs the appropriate synchronization and initialization to ensure that the buffer port object is available for read or write. This function keeps track of the appropriate buffer pointers and locks to be acquired internally, even if the buffer port is shared across AI Engine processors and can be double buffered. This function can be called unconditionally or conditionally under dynamic control and is potentially a blocking operation. It is your responsibility to ensure that the corresponding release() member function is executed sometime later (possibly even in a subsequent kernel call) to release the lock associated with that buffer object. Incorrect synchronization can lead to a deadlock in your code.

In the following example, the kernel located in tile 1 requests a lock acquisition (write access) three times per each run. The kernel located in tile 2 requests a lock acquisition (read access) twice per each run.
Figure 1. Lock Mechanism for Asynchronous Ping-pong Buffer Access

The lock acquisition and release is a kernel-only process. The main function is not taking care of the buffer synchronization; buffer synchronization is the user responsibility. Kernel in tile 1 requests three times the access to the ping pong buffer and tile 2 only twice. In order to balance the number of accesses, tile 1 should be run twice, and tile 2 should be run three times per iteration.

As seen in the figure, the lock acquisition occurs alternatively on the ping then pong buffer. The buffer choice is automatic. No user decision is needed at this point.

The minimum latency for lock acquisition is seven clock cycles during which the kernel is stalled. If the buffer is not available for acquisition, the kernel is stalled for a longer time (as indicated in red in the figure) until the buffer is available. Depending on the application, there might be time intervals where the ping and/or the pong buffer might not be locked at all.