🛠 Développement noyau : extlinux sur FAT32 et contournement de l'erratum i2329

Objectives

Iterate quickly on a custom kernel for the BeaglePlay (TI AM625): the boot partition is a FAT32 (mmcblk0p1) read by U-Boot via extlinux, allowing you to swap kernels from any PC without reflashing. Two notable constraints: the boot chain expects an uncompressed Image kernel, and the AM62x family silicon erratum i2329 requires driving MDIO via GPIO bit-banging instead of the hardware MDIO controller, depending on the Ethernet port configuration. We integrate the workaround via an overlay.

Hardware

• A BeaglePlay board booting from SD or eMMC
• An aarch64 cross-build PC with kernel build packages
• Reliable serial or SSH access (risk of kernel panic)

Complete Code

# 1. Monter la partition de boot FAT32
sudo mkdir -p /mnt/boot && sudo mount /dev/mmcblk0p1 /mnt/boot
cat /mnt/boot/extlinux/extlinux.conf

# 2. Config et build (noyau non compressé)
make ARCH=arm64 defconfig
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j$(nproc) Image dtbs

# 3. Déployer sur la FAT
sudo cp arch/arm64/boot/Image /mnt/boot/Image-dev
sudo cp arch/arm64/boot/dts/ti/k3-am625-beagleplay.dtb /mnt/boot/dtbs/

# 4. Nouveau label dans extlinux.conf
# label dev
#   linux /Image-dev
#   fdtdir /dtbs/
#   fdtoverlays /overlays/k3-am625-beagleplay-mdio-bitbang.dtbo  # erratum i2329
#   append root=PARTUUID=... rootwait console=ttyS2,115200n8

sudo umount /mnt/boot && sudo reboot

Expected Verification

In the extlinux menu, the dev label boots the custom kernel: uname -a displays the local version, dmesg shows the MDIO probe using GPIO bit-banging instead of the hardware controller, and the Ethernet link comes up normally. Reverting to the factory kernel is done by selecting the other label — since the FAT partition is readable from the PC, no reflash is needed to roll back.

Common Errors

• Deploying a bare Image.gz: the boot chain does not decompress it, so nothing starts after selecting the label.
• Editing the extlinux config on the SD card while having rebooted on eMMC (or vice versa): the kernel does not change, leading you to believe the build is broken.
• Ignoring erratum i2329 on a configuration that triggers it: the PHY stops responding on the hardware MDIO, there is no Ethernet link, and hours are wasted debugging cables.
• Removing rootwait from the append line: the root filesystem does not have time to appear, causing an Unable to mount root panic.
• Working without a serial console: at the first oops, you have no way to understand why.