FreeBSD Device Drivers

Creating DMA Tags

As mentioned earlier, DMA tags describe the characteristics and restrictions of DMA transactions and are created by using the bus_dma_tag_create function.

#include <machine/bus.h>

int
bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
    bus_size_t boundary, bus_addr_t lowaddr, bus_addr_t highaddr,
    bus_dma_filter_t *filtfunc, void *filtfuncarg, bus_size_t maxsize,
    int nsegments, bus_size_t maxsegsz, int flags,
    bus_dma_lock_t *lockfunc, void *lockfuncarg, bus_dma_tag_t *dmat);

Here, the parent argument identifies the parent DMA tag. To create a top-level DMA tag, pass bus_get_dma_tag(device_t dev) as parent.

The alignment argument denotes the physical alignment, in bytes, of each DMA segment. Recall that DMA maps represent ...