Comment 5 for bug 1936771

Ah, I wasn't seeing the connection between DMA and MD RAID, but Kconfig clarifies:

menuconfig DMADEVICES
        bool "DMA Engine support"
        depends on HAS_DMA
        help
          DMA engines can do asynchronous data transfers without
          involving the host CPU. Currently, this framework can be
          used to offload memory copies in the network stack and
          RAID operations in the MD driver. This menu only presents
          DMA Device drivers supported by the configured arch, it may
          be empty in some cases.