VIAFB_DMA_MAGIC
descr->next_desc_low = (nextdesc&0xfffffff0) | VIAFB_DMA_MAGIC;
descr[-1].next_desc_low = VIAFB_DMA_FINAL_SEGMENT|VIAFB_DMA_MAGIC;
viafb_mmio_write(VDMA_DPRL0, descr_handle | VIAFB_DMA_MAGIC);