Some quick notes on SDIO. It seems like it's common for low power / low speed embedded linux. It's derived from SD cards for like... palm pilots to add features via SD card. There are [brcm80211](https://wireless.docs.kernel.org/en/latest/en/users/drivers/brcm80211.html) drivers for broadcom SDIO that ship with the kernel. This [tutorial](https://www.apriorit.com/dev-blog/645-lin-linux-wi-fi-driver-tutorial-how-to-write-simple-linux-wireless-driver-prototype) looks really interesting on the topic