摘自STM32F2xx_StdPeriph_Lib_V1.1.0 DMA例程readme.txt

There are different options to check on the DMA end of transfer:

1. Use DMA Transfer Complete interrupt. // 传输完成中断
2. Use DMA enable state (the DMA stream is disabled by hardware when transfer
   is complete). //数据流使能状态
3. Use DMA Stream transfer counter value (the counter value is decremented when
   transfer is ongoing and is equal to 0 at the transfer end). //数据流传输计数
4. Use DMA Transfer Complete flag (polling mode). //输出完成标志，仅适用于轮询模式