When a buf structure pointer is passed into the device driver's strategy(9E) routine, the data buffer referred to by b_un.b_addr is not necessarily mapped in the kernel's address space. This means that the driver cannot directly access the data. Most block-oriented devices have DMA capability, and therefore do not need to access the data buffer directly. Instead, they use the DMA mapping routines to allow the device's DMA engine to do the data transfer. For details about using DMA, see Chapter 7, DMA.
If a driver needs to directly access the data buffer (as opposed to having the device access the data), it must first map the buffer into the kernel's address space using bp_mapin(9F). bp_mapout(9F) should be used when the driver no longer needs to access the data directly.
bp_mapout(9F) should only be called on buffers which have been allocated and are owned by the device driver. It must not be called on buffers passed to the driver through the strategy(9E) entry point (for example a filesystem). Because bp_mapin(9F) does not keep a reference count, bp_mapout(9F) will remove any kernel mapping that a layer above the device driver might rely on.