关于C6747的SPI读写问题的解决办法？

下载了 spiflash_writer_dsp 这个程序，运行正确，仔细分析了代码，有个疑惑的地方：
spi_xfer这个函数：
int spi_xfer(struct spi_slave *slave, unsigned int bitlen,
const void *dout, void *din, unsigned long flags)
[
struct davinci_spi_slave *ds = to_davinci_spi(slave);
unsigned int len;
int i;
const Uint8 *txp = dout;
Uint8 *rxp = din;
if (bitlen == 0)
/* Finish any previously submitted transfers */
goto out;

/*
* It's not clear how non-8-bit-aligned transfers are supposed to be
* represented as a stream of bytes...this is a limitation of
* the current SPI interface - here we terminate on receiving such a
* transfer request.
*/
if (bitlen % 8) [
/* Errors always terminate an ongoing transfer */
flags |= SPI_XFER_END;
goto out;
]
len = bitlen / 8;
/* do an empty read to clear the current contents */
SPI_SPIBUF;
/* keep writing and reading 1 byte until done */
for (i = 0; i < len; i++)
[
/* wait till TXFULL is asserted */
while( SPI_SPIBUF & 0x20000000 );
/* write the data */
data1_reg_val &= ~0xFFFF;
if(txp) [
data1_reg_val |= *txp & 0xFF;
txp++;
]
/* write to DAT1 is required to keep the serial transfer going */
/* we just terminate when we reach the end */
if((i == (len -1)) && (flags & SPI_XFER_END)) [
SPI_SPIDAT1 = data1_reg_val & ~(1 << 28);
] else [
SPI_SPIDAT1 = data1_reg_val;
]
/* read the data - wait for data availability */
while ( SPI_SPIBUF & ( 0x80000000 ) );
if(rxp) [
*rxp = SPI_SPIBUF & 0xFF;
rxp++;
] else [
SPI_SPIBUF;
]
]
return 0;
out:
if (flags & SPI_XFER_END) [
SPI_SPIDAT1 = data1_reg_val & ~(1 << 28);
]
return 0;
]
有一个语句：
/* read the data - wait for data availability */
while ( SPI_SPIBUF & ( 0x80000000 ) );
为什么需要这个？winbond SPI有些命令，比如 0x06，写允许命令，并不返回数据，岂不是一直等待！
请 TI 专家解释解释。