Hey

I done a demo driver implementation:

https://github.com/StefJar/zephyr_stm32_uart3_dma_driver

Some problems I was facing:

- DMA API: the current one doesn't support ring buffer setups

- UART API: the current API is not mapping the IDLE event.

- UART API: no callbacks possible for event driven rx

With the current design of the UART and DMA API it seems to be challenging to get a nice fusion.

So I was implementing a simple API for the demo, making block writes possible and calling a callback when data is received. I didn't used the UART and DMA API because it would became a dirty hack, while calling functions form that API & overwriting/patching the ISRs. So I decided for a lean straight forward implementation.

In my case I am interested into SPI + DMA and USB ACM + DMA. From an energy perspective it is much better to use DMA transfers for the peripheral IOs (We can put the MCU to sleep).

Maybe it is worth to put a thought on using DMA for peripherals by default and only the special case is doing polling or RX/TX handling via uart etc. ISRs.

Stefan