There is some “out of tree” source code referenced within the kernel using a relative path. You may have found a purely kernel tree download, but need the out of tree code as well. For the release you are using, within the driver package (the “Linux_for_Tegra/” directory) look for “source_sync.sh”. If your release is R28.2.1, then this should be how you download (it’ll produce a “source” directory, and within that both the out of tree content and the kernel content):