Match baud rate, data bits, parity, stop bits, and flow control between driver and device. Device tree properties must match hardware strap pins on the peripheral.
Interrupt vs DMA: DMA for bulk, interrupt for console.
FIFO watermark: leave measured headroom for worst-case service latency and bytes already in flight; no one percentage fits every baud and FIFO.
Loopback test isolates TX path from external device issues.