ddi_dma_nextwin() shifts the current DMA window win within the object referred to by handle to the
next DMA window nwin. If the current window is NULL, the first window within the object is returned. A DMA window is a portion of a DMA object or might be the entire object. A DMA window has system resources allocated to it and is
prepared to accept data transfers. Examples of system resources are DVMA mapping resources and intermediate transfer buffer resources.
All DMA objects require a window. If the DMA window represents the whole DMA object it has system
resources allocated for the entire data transfer. However, if the system is unable to setup the entire DMA object due to system resource limitations, the driver writer may
allow the system to allocate system resources for less than the entire DMA object. This can be accomplished by specifying the DDI_DMA_PARTIAL flag as a parameter to ddi_dma_buf_setup(9F) or ddi_dma_addr_setup(9F) or as part of a ddi_dma_req(9S) structure in a call to ddi_dma_setup(9F).
Only the window that has resources allocated is valid per object at any one time. The currently valid window is the one that was most recently returned from ddi_dma_nextwin().
Furthermore, because a call to ddi_dma_nextwin() will reallocate system resources to the new window, the previous window will become invalid. It is a severe error
to call ddi_dma_nextwin() before any transfers into the current window are complete.
ddi_dma_nextwin() takes care of underlying memory synchronizations required to shift the window. However, if you want to access the data before or after moving the window, further
synchronizations using ddi_dma_sync(9F) are required.