The Android Open Source Project is where manufacturers can stay secure. manufacturers start when they want to port Android to a new device. Manufacturers add in the drivers (and other hardware-specific code), as well as perhaps custom UI overlays. If this all passes Google’s muster (see box on previous page), the device can be shipped with Android pre-installed. Those wanting to compile custom ROMs for that device may find that the manufacturer has open sourced the code they added in (namely device trees, hardware libraries and things that end up in the vendor partitions), but this is rarely the case. Even if they provide something, drivers for the GPU, cameras, GPS and the baseband modem itself will more than likely only exist as proprietary blobs. Those are no use for compiling AOSP, so custom ROM developers must find another way.
