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Kernel Strategy

Current state

The current build uses bcm2711_defconfig from the RPi kernel fork (rpi-6.12.y) with a small fragment overlay adding overlayfs, initramfs, and USB gadget support.

This defconfig is designed for all Raspberry Pi models (Pi 3, 3+, 4, 400, Zero 2W, CM3, CM4) and includes support for a huge range of peripherals. The resulting kernel has:

  • 3814 enabled options (1916 built-in, 1898 modules)
  • 85 network vendor drivers (Intel, Mellanox, Broadcom enterprise, Amazon, etc.)
  • 307 media/video drivers (V4L2 capture, DVB tuners, webcam drivers)
  • 157 sound drivers (HiFiBerry, JustBoom, IQAudio, USB audio, dozens of I2C codecs)
  • 73 input drivers (touchscreens, joysticks, keyboards, mice)
  • 126 crypto algorithms (most unused)
  • Pi 4/5 specific hardware (PCIe, BCM2711 thermal, BCM2712 IOMMU, V3D, GENET)
  • Unused buses (CAN, ATA/SATA, NFC, SCSI, Gameport)
  • All wireless vendors (Atheros, Intel, MediaTek, Realtek, Marvell; only Broadcom is needed)
  • Enterprise storage (NFS, CIFS, CEPH, GFS2, OCFS2, Btrfs, ReiserFS, JFS, XFS)

On a Pi Zero 2W with 512MB RAM, every unnecessary module wastes build time, image space, and kernel memory for metadata. The full module tree is ~100MB+ uncompressed.

What we actually need

SoC & boot essentials (built-in, =y)

  • BCM2835 mailbox, power, thermal, watchdog
  • MMC: BCM2835, SDHCI_IPROC (must be built-in for initramfs to find /dev/mmcblk0)
  • Serial: SERIAL_8250 + BCM2835AUX (mini UART/ttyS0), SERIAL_AMBA_PL011 (BT/ttyAMA0)
  • GPIO, pinctrl, clocks (pulled in by SoC selection)

Overlayfs & initramfs (built-in)

  • OVERLAY_FS, BLK_DEV_INITRD, RD_GZIP

USB gadget (built-in)

  • USB_DWC2, USB_GADGET, USB_CONFIGFS, USB_LIBCOMPOSITE
  • Functions: USB_F_ACM, USB_F_ECM, USB_F_SERIAL, USB_U_SERIAL, USB_U_ETHER
  • Keep USB_CONFIGFS_MASS_STORAGE (useful for exposing SD card partitions to host)

WiFi (module)

  • CFG80211, MAC80211 (wireless stack)
  • BRCMUTIL, BRCMFMAC, BRCMFMAC_SDIO (only driver we need)

Bluetooth (module)

  • BT, BT_BREDR, BT_LE, BT_RFCOMM, BT_HIDP, BT_BNEP
  • BT_HCIUART + BT_HCIUART_BCM + BT_HCIUART_H4 + BT_HCIUART_SERDEV
  • BT_BCM (BCM43xx firmware loading)

Audio (module)

  • SND_BCM2835 (HDMI + headphone jack audio via VCHIQ)
  • SND_SOC + SND_BCM2835_SOC_I2S (if I2S DAC hats needed later)
  • SND_USB_AUDIO (USB speakers/DACs; keep for portable service use cases)
  • Drop all 30+ HiFiBerry/IQAudio/JustBoom board-specific codec drivers

GPU/Display (module)

  • DRM_VC4 + DRM_VC4_HDMI_CEC (vc4 is the Pi Zero 2W GPU)
  • DRM_FBDEV_EMULATION (console on HDMI)
  • Drop DRM_V3D (Pi 4/5 only), all panel drivers, DRM_UDL, DRM_GUD

Filesystems

  • EXT4_FS (rootfs, data partition)
  • VFAT_FS, FAT_FS, MSDOS_FS (boot partition)
  • OVERLAY_FS (core feature)
  • SQUASHFS + SQUASHFS_ZSTD (portable services)
  • CONFIGFS_FS (USB gadget)
  • PROC_FS, SYSFS, TMPFS, DEVTMPFS
  • Drop: ReiserFS, JFS, XFS, GFS2, OCFS2, Btrfs, Bcachefs, NILFS2, F2FS, NTFS3, exFAT, HFS, HFSPLUS, JFFS2, UBIFS, EROFS, NFS, CEPH, 9P, ISO9660, UDF, eCryptFS

Networking

  • INET, IPV6 (basic stack)
  • NETFILTER basics (for future firewall rules)
  • BRIDGE only if needed for container networking (can defer)
  • Drop: all 85 NET_VENDOR_* drivers, CAN, NFC, SCSI, ATA, InfiniBand

Crypto

  • Keep only what the kernel, WiFi (WPA), and Bluetooth need
  • CRYPTO_AES, CRYPTO_SHA256, CRYPTO_HMAC, CRYPTO_CCM, CRYPTO_GCM
  • CRYPTO_ECB, CRYPTO_CBC (needed by various subsystems)
  • Drop the other ~100 algorithms

Implementation plan

Phase 0.5: Kernel trimming (after first successful boot)

Approach: expanded kernel fragment that disables categories of drivers.

  1. Boot the current image, run lsmod to see what's actually loaded
  2. Save the running config via /proc/config.gz
  3. Build expanded fragment file that disables unused categories:
  4. All NET_VENDOR_* except Broadcom
  5. All WLAN_VENDOR_* except Broadcom
  6. All SND_BCM2708_SOC_* hat drivers
  7. All DRM_PANEL_* except RASPBERRYPI_TOUCHSCREEN (maybe)
  8. All non-Broadcom Bluetooth drivers (BT_ATH3K, BT_INTEL, BT_MTK, BT_MRVL, BT_RTL)
  9. All USB audio except SND_USB_AUDIO
  10. DRM_V3D, DRM_GUD, DRM_UDL, DRM_SSD130X
  11. BCM2711_THERMAL, BCM2712_*, PCIE_BRCMSTB (Pi 4/5 only)
  12. CAN, NFC, ATA, GAMEPORT, SCSI (as module)
  13. Unused filesystems
  14. Unused crypto

Expected result: ~1500-2000 enabled options (down from 3814), faster builds, smaller module directory (~20-30MB vs 100MB+), lower RAM overhead.

Phase 1: Custom defconfig

Approach: replace bcm2711_defconfig + fragment with a full custom defconfig.

  1. Take the trimmed .config from Phase 0.5
  2. Run make savedefconfig to produce a minimal defconfig
  3. Store as br2-external/configs/offlinelab_pi_zero_2w_defconfig (replace current)
  4. Remove the kernel fragment file (everything is in the defconfig now)
  5. Test boot, WiFi, BT, USB gadget, audio, HDMI

Benefits: single source of truth, no fragment merge surprises, easier to review.

Phase 1+: Power-saving patches

Approach: patch series on top of RPi fork.

Candidates for power-saving: - CPU frequency governor tuning (schedutil with aggressive downscaling) - CONFIG_CPU_IDLE + CONFIG_ARM_CPUIDLE (C-states for Cortex-A53) - CONFIG_SUSPEND + CONFIG_PM for system suspend-to-RAM - USB autosuspend for when gadget is not connected - WiFi power save (CONFIG_CFG80211_DEFAULT_PS=y, already enabled) - Runtime PM for unused peripherals (HDMI, I2C buses, SPI) - Disable kernel features that consume power: CONFIG_NO_HZ_FULL, tickless idle

Patch location: br2-external/boards/rpi/pi-zero-2w/patches/linux/

Why not mainline kernel

  • BCM2710 (Pi Zero 2W SoC) has quirks handled by RPi fork:
  • Mini UART clock tied to GPU core clock (RPi-specific clock driver)
  • Firmware-based WiFi/BT initialization sequence
  • DWC2 OTG errata workarounds
  • VCHIQ kernel module for GPU communication (audio, camera, video decode)
  • vc4 driver patches not yet upstream
  • Mainline support works but requires more debugging effort for less hardware coverage
  • RPi fork tracks mainline closely (rebased regularly), so we get upstream fixes anyway
  • If we ever need a mainline feature, cherry-pick into the fork

Sizing estimates

Config Options Modules dir Build time RAM savings
bcm2711 (current) ~3800 ~100MB baseline —
Trimmed fragment ~2000 ~30MB -30-40% ~10-15MB
Full custom defconf ~1200 ~15MB -50-60% ~15-20MB
Tiny + addback ~800 ~8MB -70% ~20-25MB

On 512MB RAM, 15–25MB saved by the kernel is significant. That is 3–5% of total RAM freed for portable services.

Decision log

  • RPi fork over mainline: hardware support maturity, less debugging, no blocker
  • Fragment-first over immediate custom defconfig: need a working boot before trimming
  • Keep modules over built-in for WiFi/BT/audio/GPU: allows unloading when unused, smaller kernel image for faster boot, runtime PM can suspend module hardware
  • Squashfs as module: only loaded when portable services are attached