XDMA_DESC_ADJACENT_MASK
last_blk_desc = (sw_desc->desc_num - 1) & XDMA_DESC_ADJACENT_MASK;
val = (desc->desc_num - 1) & XDMA_DESC_ADJACENT_MASK;
desc += desc_num & XDMA_DESC_ADJACENT_MASK;
if (!(++desc_num & XDMA_DESC_ADJACENT_MASK))