Software can enable the FCI before enabling a channel. The DMA channel uses ARLEN on both the SRC and DST sides.
Number of SRC transaction = Number of DST transaction
SRC AXI transaction size/length = DST AXI transaction size/length
If a DMA channel is reading data from the flow controlling slave, each credit given to the DMA channel reads ARLEN x bus width (in bytes) worth of data. ARLEN x bus width (in bytes) worth of data is written to the FCI slave if the DMA channel is writing data to a slave.
The DMA channel can accept up to 32 credits. The slave can use this to pipeline credits to the DMA channel. Because of the aligned address requirement, each credit is the transfer size of ARLEN x bus width (bytes). A slave uses this to keep track of the number of bytes transferred. This information is used by slave to issue credits.