#Drivers in the configuration: what the kernel is built with

A Linux kernel builds a driver when its Kconfig symbol is on: drivers/input/touchscreen/Kconfig declares TOUCHSCREEN_GOODIX, the directory's Makefile says obj-$(CONFIG_TOUCHSCREEN_GOODIX) += goodix.o, and a device's defconfig (arch/arm64/configs/vendor/<device>_defconfig in an Android kernel) turns on what that phone has. Every phone vendor's tree is that, with its own drivers added the same way.

This kernel is Apple's XNU, which has the same idea under other names, much older than Kconfig: config/MASTER declares the options (options APFS # <apfs>), a component's conf/files lists its files as standard or optional <option>, and a configuration (DEVELOPMENT = [ ... ] in MASTER.<arch>.<platform>) is the list of what is on; SETUP/config/doconf and config(8) turn that into the list of objects and -D<OPTION> for the compiler. The drivers of this port are XNU options of that kind. Nothing here imitates Kconfig's files; what a driver is, is written once per subsystem, in drivers/<subsystem>/conf.

drivers/<subsystem>/conf      the drivers of a subsystem: their files, what they require, what they are
configs/*_defconfig           CONFIG_HAKIOS_<NAME>=y or `# CONFIG_HAKIOS_<NAME> is not set`, beside the boot-argument symbols
tools/hakios-config.py kernel the choice of the .config put into the kernel tree (make kernel runs it)

#drivers/<subsystem>/conf

standard bsd/dev/hakios.input.c
	help    /dev/hakios-touch, the queue of touches and buttons, and where the phone's drivers are started from

driver GOODIX
	file    bsd/dev/hakios.goodix.c
	help    the touch screen: a Goodix GT9916 or a FocalTech FT3680 on SPI (touchscreen=1) ...

driver SOUND
	file    bsd/dev/hakios.sound.c
	file    bsd/dev/hakios.guard.c
	requires ADSP SPEAKER
	help    the speakers playing (sound=N) ...

Every .c and .cpp of drivers/map is built by exactly one standard line or driver, of the conf of its own subsystem; tools/drivers-check.py (CI) checks that, and a file that none builds would be in the tree and never compiled.

The 25 drivers there are now, by subsystem:

tools/hakios-config.py help HAKIOS_SOUND says the same of one of them, and what requires it.

#A device's defconfig chooses its drivers

A driver nobody names is built (default y): every driver was built before there were these symbols, and a .config made then still gives the same kernel. A device leaves out what it does not have:

# configs/qemu_defconfig (in the tree): QEMU's virt machine, the phone's drivers left out of the kernel
#include common_defconfig
#include open.config
#include virt.config
CONFIG_DEVICE="qemu"
# CONFIG_HAKIOS_GOODIX is not set
# CONFIG_HAKIOS_ADRENO is not set
...

make qemu_defconfig        # .config: 4 drivers built in (the virtio ones), 21 left out
make config                # says which are built and which are not
make kernel                # builds that kernel

A port to another phone starts the same way: its configs/<name>.config leaves out each driver whose hardware it has not ported yet (the addresses and pins in a driver are the POCO X5 Pro's, and its head says where each came from), and turns one on when it has been made to work there. A phone with another touch controller adds a driver, not a branch: its file in drivers/input/, a line in drivers/map, a driver block in drivers/input/conf, and CONFIG_HAKIOS_<IT>=y in its defconfig.

#What is checked

• # CONFIG_HAKIOS_X is not set means left out, not "the default": a fragment can take a driver out of a configuration that an earlier file put it in, as in Linux.
• A driver built without one it requires is refused: CONFIG_HAKIOS_SOUND requires CONFIG_HAKIOS_ADSP (drivers/audio/conf:13).
• A boot-argument that only left-out drivers read is refused, with the driver named: CONFIG_TOUCHSCREEN gives touchscreen=, which only hakios.buttons.c, hakios.goodix.c read, and they are not built (CONFIG_HAKIOS_BUTTONS, CONFIG_HAKIOS_GOODIX). The kernel would not complain: the argument would just never be looked at. (The readers are found the way tools/boot-args.py finds them, in the source.)

#How the choice reaches XNU's build

tools/hakios-config.py kernel (run by make kernel through tools/make-kernel.sh, and by xnu/apply.sh) writes, into the kernel tree, only what has changed:

and what the patch carries once: SETUP/config/doconf reads MASTER.hakios after MASTER and MASTER.arm64.bcm2837; the kernel configurations of MASTER.arm64.bcm2837 (RELEASE, DEVELOPMENT, DEBUG, KASAN) include HAKIOS_DRIVERS; each component's conf/Makefile makes its configuration again when MASTER.hakios changes. XNU's build passes the options as -DHAKIOS_<NAME> in the compiler's command line rather than as a file an object depends on, so when the choice changes, kernel also removes the objects of the files that ask about it (#ifdef HAKIOS_*): they are built again.

Linux's ath11k object (tools/mk-ath11k.sh) is linked in only with CONFIG_HAKIOS_WIFI (xnu-build.sh): its calls are answered by hakios.lx.c.

#Leaving a driver out without breaking the others

The rest of the kernel calls the drivers in a few places: where they are started (hakios.input.c, hakios.audio.c, bsd_init.c), where a run ends or a panic leaves (loader_return.c, the thermal and battery guards, machine_routines.c), and a few questions on the way ("is this device one of the storage's?", "how warm is the battery?"). Those calls stay as they are. For a driver that is left out, drivers/core/hakios.stubs.c (XNU keeps bsd/kern/bsd_stubs.c for the same purpose) has what each such call then does: nothing, and the answer "no", "not known" or "not there". With every driver built it compiles to nothing, so the phone's kernel is the one it was.

What several drivers share was moved where none of them owns it, drivers/core/hakios.platform.c: the PMIC arbiter's lock (from hakios.buttons.c), the locks of the always-on processor's mailbox, the shared memory's allocation, RPMh and the SMMU (from hakios.wifi.c), and the kern.hakios sysctl node (from hakios.battery.c). Before, the camera could not be linked without the Wi-Fi driver, and every driver's sysctls without the battery's. The camera's and the motor's firmware is read with hakios_image_file() directly instead of through the Wi-Fi driver.

#What was checked

On a Mac (Apple clang, the toolchain of xnu-deps/), 27 October 2026: the kernel built and linked with every driver (make kernel without a .config), and with make qemu_defconfig, four drivers of 25. The QEMU kernel was not booted yet, and nothing here was run on the phone. tools/hakios-config-test.py and tools/drivers-check.py pass.