- Rx/Tx handler in usart2_dma.*
- CubeMX: USART and DMA init
- DMA CH2: Tx
- DMA CH3: Rx
- DMA Port0 for periph transfer
- DMA Port1 for mem transfer
- Continuous Rx handling via DMA pointer (no INT, no TC flag)
- Tx: collect data into tx buf or send immediately
