Kernel Development

The kernel is built by Buildroot like any other package. Iterating on it without a full rebuild is the most common loop.

Develop the kernel out-of-tree

For larger kernel changes it is more convenient to keep a separate Linux checkout and have Buildroot pick it up instead of the tarball it would normally download.

Clone the kernel next to the Buildroot tree:

git clone https://github.com/torvalds/linux.git build/linux
git -C build/linux checkout v6.7

Tell Buildroot to use it via a local override file. Create mds_external/local.mk (gitignored, see local.mk.example):
```
LINUX_OVERRIDE_SRCDIR = $(TOPDIR)/../../build/linux
```
Re-run make build. Buildroot will rsync from build/linux/ instead of fetching the upstream tarball.

The same pattern works for any package — set <PACKAGE>_OVERRIDE_SRCDIR in local.mk. See the overrides reference for the full mechanism.

Build only the kernel

make build-linux-rebuild

This runs make linux-rebuild inside the Buildroot output for MACHINE. Outputs of interest:

build/output/<machine>/images/uImage — the kernel image used by U-Boot
build/output/<machine>/target/lib/modules/<ver>/ — built-as-modules drivers staged for the rootfs

Copy to a running target

A running board with USB Ethernet gadget shows up at 192.168.2.2. Push the new kernel:

scp -O /workspace/build/output/network_player/images/uImage \
    root@192.168.2.2:/boot

Push a single module (example: SPDIF driver):

scp -O \
  /workspace/build/output/network_player/target/lib/modules/6.7.2/kernel/sound/soc/sunxi/sun4i-spdif.ko \
  root@192.168.2.2:/usr/lib/modules/6.7.2/kernel/sound/soc/sunxi/sun4i-spdif.ko

Replace 6.7.2 with the actual kernel version of your build. Use uname -r on the device to check.

After copying, reload the module or reboot.

Change the kernel config

make build-linux-menuconfig
make build-linux-savedefconfig
cp build/output/network_player/build/linux-*/defconfig \
   mds_external/board/mds_network_player/kernel_defconfig

The defconfig file is what ships in version control — your menuconfig changes only persist when copied back into kernel_defconfig.

Kernel patches

Patches under mds_external/board/mds_network_player/linux-patches/ are applied in lexicographic order by Buildroot before the build. Add a new patch file there, give it a numeric prefix (e.g. 0002-fix-spdif.patch), and trigger a clean kernel rebuild:

make build-linux-dirclean
make build-linux

Useful peripherals notes

GPIO numbering: the f1c200s pinctrl uses banks A..E. The Linux GPIO number for PD6 (the user button on the Respeaker 2 mics HAT) is 4 * 32 - 1 + 6 = 102. Validate with:
```
gpio-event-mon -n gpiochip0 -o 102 -r -f -b 10000
```
DMA: not supported on f1c200s in mainline. Out-of-tree patches exist; SPI and audio peripherals currently run without DMA acceleration on this board.
I²S audio: the f1c200s I²S driver is not in mainline. SPDIF is used instead.