Design

Describes different modes of operations of the DMA Framework .

Some DMA controllers provide several modes of operation. For example, one ASSP provides both single-buffer mode and scatter-gather mode transfers.

There are two options:

• Select a single mode of operation.

• Select a multiple mode of operation, and split the physical DMA controller into several logical controllers, one for each mode to be supported. If this option is chosen, the PSL must include one concrete controller class per logical controller; a controller class is derived from TDmac.

  If the DMA controller supports more than one mode of operation, implement the simplest one first. Implementing single-buffer mode first allows most of the the PSL (platform-specific layer) to be debugged before you start to implement the more complex scatter-gather functionality.

One reference implementation adopts a mixed strategy; the mode of operation is selectable at build-time by defining the macro __DESFETCHMODE__ to exercise and demonstrate two different modes of operation.

Scatter-gather support is essentially a superset of single-buffer support because a scatter-gather list is an ordered set of variable sized buffers, and means that code shown here demonstrates the implementation of a scatter-gather DMA controller.

Almost all modern MCUs use scatter-gather capable DMA controllers. Note that we refer to descriptor fetch mode synonymously with scatter-gather mode, as scatter-gather is implemented by passing the DMA controller a linked list of descriptors describing the buffers to be transferred. Non–descriptor mode requires that the DMA controller registers are programmed by software to describe the buffer transfer required. Since each transfer must be handled by an interrupt service routine triggered when the DMA controller has completed the transfer, non-descriptor mode is not capable of performing transfers past buffer boundaries.