When the driver is unloaded, the DMA handle must be freed. ddi_dma_free_handle(9F) destroys the DMA handle and any residual resources the system may be caching on the handle. Any further references of the DMA handle will have undefined results. In ddi_dma_free_handle(9F), handlep is a pointer to the DMA handle.
void ddi_dma_free_handle(ddi_dma_handle_t *handlep);