Is there a trade-off for memory to memory DMA transfer when the data size is small?

Question

I am learning about the STM32 F4 microcontroller. I\'m trying to find out about limitations for using DMA.

Per my understanding and research, I know that if the data si

Answer 1

The information is right in that using a DMA requires some development work and some runtime to manage the DMA transfer itself (see also a related question here), which may not be worth the benefits of using DMA. That is, for small portions of data one doesn't gain as much performance (or latency) as during big transfers. On embedded systems, DMA controllers (and their channels) are limited resources so it is important to consider which part of the function benefits from such a resource most. Therefore, one would usually prefer using DMA for the data transfers to/from disks (if it is about "payload data" such as large files or video streams) over slow serial connections.

The information is wrong, however, in that DMA is not worth using on serial interfaces as those only transfer a single bit at a time. Please note that microcontrollers (as your STM32F4) have built-in peripheral components that convert the serial bit-by-bit stream into a byte-by-byte or word-by-word stream, which can easily be tranferred by DMA in a helpful way - especially if the size of the packets is known in advance and software doesn't have to analyse a non-formatted stream. Furthermore, not every serial connection is "slow" at all. If the project uses, e. g., an SPI flash chip, then the SPI serial connection is the one used for data transfer.