BF533芯片指导手册原话：

When using DMA for SPI transmit, the DMA_DONE interrupt signifies

that the DMA FIFO is empty. However, at this point there

may still be data in the SPI DMA FIFO waiting to be transmitted.

Therefore, software needs to poll TXS in the SPI_STAT register until

it goes low for 2 successive reads, at which point the SPI DMA

FIFO will be empty.

请问：这2successive reads是什么意思呢？