Setting up DMA mappings
For any type of DMA transfer, one needs to provide source and destination addresses, as well as the number of words to transfer. In the case of a peripheral DMA, the peripheral's FIFO serves as either the source or the destination. When the peripheral serves as the source, a memory location (internal or external) serves as the destination address. When the peripheral serves as the destination, a memory location (internal or external) serves as the source address.
With a peripheral DMA, we specify either the source or the destination, depending on the direction of the transfer. In others words, a DMA transfer requires suitable memory mappings. This is what we will discuss in the following sections.
Cache coherency and DMA
As discussed in Chapter 11, Kernel Memory Management, copies of recently accessed memory areas are stored in the cache. This applies to DMA memory too. The reality is that memory shared between two independent devices is generally the source of cache...
