So why is this information relevant to our pursuits? Well, PCIe is significantly different to SDIO and USB in terms of isolation. Without going into the internals of each of the interfaces, SDIO simply allows the serial transfer of small command “packets” (on the CMD pin), potentially accompanied by data (on the DATA pins). The SDIO controller then decodes the command and responds appropriately. While SDIO may support DMA (for example, the host can set up a DMA engine to continually read data from the SD bus and transfer it to memory), this feature is not used on mobile devices, and is not an inherent part of SDIO. Furthermore, the low-level SDIO communication on the BCM SoC is handled by the “SDIOD” core. In order to craft special SDIO commands, we would most likely need to gain access to this controller first.

Likewise, USB (up to version 3.1) does not include support for DMA.