This code example does most of what you need:
However, it doesn't use DMA to access the SMIF.
You could configure the SMIF to run in XIP mode, so you could use the external memory the same way you use the internal memory. Refer to this code example:
For the DMA part, refer to this code example:
SMIF in XIP + DMA should be very similar to SPI + DMA.
Your reply is very helpful to me,Thanks.