An optimal AXI4 interface is one in which the design never stalls while waiting to access the bus, and after bus access is granted, the bus never stalls while waiting for the design to read/write. To create the optimal AXI4 interface, the following options are provided in the INTERFACE pragma or directive to specify the behavior of the bursts and optimize the efficiency of the AXI4 interface. Refer to AXI Burst Transfers for more information on burst transfers.
Some of these options use internal storage to buffer data and may have an impact on area and resources:
-
latency: Specifies the expected latency of the AXI4 interface, allowing the design to initiate a bus request a number of cycles (latency) before the read or write is expected. If this figure is too low, the design will be ready too soon and may stall waiting for the bus. If this figure is too high, bus access may be granted but the bus may stall waiting on the design to start the access. -
max_read_burst_length: Specifies the maximum number of data values read during a burst transfer. -
num_read_outstanding: Specifies how many read requests can be made to the AXI4 bus, without a response, before the design stalls. This implies internal storage in the design, a FIFO of size:num_read_outstanding*max_read_burst_length*word_size. -
max_write_burst_length: Specifies the maximum number of data values written during a burst transfer. -
num_write_outstanding: Specifies how many write requests can be made to the AXI4 bus, without a response, before the design stalls. This implies internal storage in the design, a FIFO of size:num_read_outstanding*max_read_burst_length*word_size
The following example can be used to help explain these options:
#pragma HLS interface mode=m_axi port=input offset=slave bundle=gmem0
depth=1024*1024*16/(512/8)
latency=100
num_read_outstanding=32
num_write_outstanding=32
max_read_burst_length=16
max_write_burst_length=16
The interface is specified as having a latency of 100. Vitis HLS seeks to schedule the request for burst access 100 clock
cycles before the design is ready to access the AXI4 bus. To further
improve bus efficiency, the options num_write_outstanding and num_read_outstanding ensure the design contains enough buffering to store
up to 32 read and write accesses. This allows the design to continue processing until
the bus requests are serviced. Finally, the options max_read_burst_length and max_write_burst_length ensure the maximum burst size is 16 and that the
AXI4 interface does not hold the bus for longer than this.
These options allow the behavior of the AXI4 interface to be optimized for the system in which it will operate. The efficiency of the operation does depend on these values being set accurately.