1.SPI做从，偶尔会掉中断，数据已经到FIFO了，
2. 不连SS，能收到数据，但有1的问题，连上反而收不到了
3.接收FIFO RR中断必须4字节才触发，我想设成1字节触发，burstlength已经设成了7，即1字节触发
Currently, our BSP is not support spi slave mode.
Only kernel 4.15 and 4.16 from mainline supports it.
In order to use SPI slave in these kernels the "spi-slave" property needs to be passed in the devicetree as explained at:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/Documentation/devicetre...
For your reference, there is a patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/patch/?id=71abd29057cb17b6b95324218...
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