The radio's firmware, not the Linux OS running on the application processor. All 802.11ac radios have closed-source firmware even when there are open-source Linux drivers to communicate with the NIC. The 802.11n chipsets by Atheros didn't use proprietary firmware and exposed a fairly low level interface to the host system. This led to the open-source ath9k Linux driver being the platform of choice for people trying to fix WiFi in general or improve the Linux WiFi stack in specific.