adsp-2156x_hwr and adsp-2159x-sc591-592-594-hrm.pdf don't explain everything about controller mode of SPI and Register-Based Linear-Buffer Stop Flow Mode of DMA.
For example, SPI transmit start only if TDR of SPI_TXCTL set non zero value.
According adsp-2156x_hwr and adsp-2159x-sc591-592-594-hrm.pdf TDR of SPI_TXCTL is direct the SPI to generate a transmit status interrupt request. But this request direct DMA to start transfer data from DMA to SPI.
I found out it to hack services\pdma and drivers\spi.
Best regards.