alexforencich
commented
1 year ago There are two main checks that need to be done when issuing AXI operations. First is to check the size of the operation vs. the max burst size, and to cut the transfer at the max burst size if it is larger than the max burst size. The second is to check for 4K address boundary crossings. Bursts cannot cross 4k address boundaries, so if there is a crossing then the burst has to be split on that crossing, and you can potentially get a 4k address boundary crossing even when transferring 2 data bytes. The thing to note is that any burst 4k or larger must necessarily cross a 4k boundary, which renders the burst size check redundant as the 4k boundary check will also limit the burst size to 4k. You're never going to have a situation where the max burst size check will fail but the 4k check will pass - either the 4k check fails and the transfer is cut on a 4k boundary, or they both fail and the transfer is cut on a 4k boundary, in both cases the result is the same. Hence this simple optimization to explicitly skip the burst size check if the max burst size is 4k or larger, which saves some logic resources and improves timing performance without affecting functionality.
qiweiii-git
axi_dma_wr lines 465
if (op_word_count_reg <= AXI_MAX_BURST_SIZE - (addr_reg & OFFSET_MASK) || AXI_MAX_BURST_SIZE >= 4096) begin// packet smaller than max burst sizehttps://github.com/alexforencich/verilog-axi/blob/38915fb5330cb8270b454afc0140a94489dc56db/rtl/axi_dma_wr.v#LL465C26-L465C26If AXI_MAX_BURST_LEN set to 256 and AXI_DATA_WIDTH set to 128, the AXI_MAX_BURST_SIZE will be 4096, it will always go to the 'if' and assume the packet smaller than max burst size, if we send a data stream large than the max burst size, the axi_dma will hanging. Did I miss some limitation of the axi_dma?