Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Keil/AT32F407xx_v2/DMA1/DMA_SRC_SEL1#0x0
DMA channel source assignment register
CH5 SRC select
CH6 SRC select
CH7 SRC select
DMA FLEX Enable
https://github.com/cmsis-svd/cmsis-svd-data