udma_alloc_tr_desc
d = udma_alloc_tr_desc(uc, tr_size, num_tr, dir);
d = udma_alloc_tr_desc(uc, tr_size, num_tr, dir);
d = udma_alloc_tr_desc(uc, tr_size, periods * num_tr, dir);
d = udma_alloc_tr_desc(uc, tr_size, num_tr, DMA_MEM_TO_MEM);