In relation to the JESD204 IP, what does the rx_start_of_frame represent?
The rx_start_of_frame is a 4-bit frame boundary indication. It indicates the byte position of the first byte of a frame in the received data in the following clock cycle.
This signal is asserted one cycle before the AXI4-Stream data.
The alignment of the very first valid byte is always in byte 0 if the multiframe size is a multiple of 4, and rx_buffer_delay is a multiple of 4.
(PG066) v7.0 includes information about timing of the rx_start_of_frame and rx_end_of_frame relative to the AXI data (see Figure 2-3 and Figure 2-4):