Linux Kernel for RTL8196E (6.18 + 7.1)¶

This directory contains everything needed to build a modern Linux kernel for the Realtek RTL8196E gateway.

Two kernel lines are shipped and selectable with the KERNEL environment variable (default 6.18):

`KERNEL`	Version	Sources
`6.18` (default)	Linux 6.18.41 — stable 6.18.x LTS family	`patches-6.18/`, `files-6.18/`, `config-6.18-realtek.txt`
`7.1`	Linux 7.1.7	`patches-7.1/`, `files-7.1/`, `config-7.1-realtek.txt`

The two lines coexist — each has its own patch/overlay/config triplet and its own pre-built images. A Lidl user who sets nothing builds and flashes the 6.18 line exactly as before.

Why Linux 6.18?¶

Linux 6.18 is an LTS release, maintained upstream for several years. It brings a modern kernel surface to the gateway while remaining practical to cross-compile against the project's Lexra MIPS toolchain. Compared to legacy 3.10 / 4.14 / 5.10 lines, 6.18 gives us current driver APIs, recent security hardening defaults, and the in-kernel UART↔TCP bridge (rtl8196e-uart-bridge) used for the Zigbee radio path.

The build_kernel.sh script pins a specific 6.18.x stable release via KERNEL_VERSION and applies our local patches-6.18/ on top. Bumping to a newer point release is a one-line edit — the patches, overlay and config are keyed on the 6.18 family, not the point version.

The Porting Challenge¶

Porting to modern Linux on the RTL8196E (Lexra RLX4181, MIPS-I class) was a significant undertaking:

Challenge	Description
Ethernet driver rewrite	The Realtek SDK driver used obsolete APIs. It was completely rewritten to comply with modern kernel networking standards (NAPI, phylib, devicetree).
Lexra CPU support	The Lexra MIPS variant lacks `ll/sc` in hardware. Required kernel-side emulation (`simulate_llsc`) plus atomics/memory-barrier patches.
Platform code cleanup	Realtek SDK platform code was heavily tied to old kernel internals. Board support was rewritten around modern devicetree and platform drivers.
Build system changes	Successive Kbuild generations moved enough code around that the overlay/patch set had to be regenerated against 6.x.

The result is a clean, maintainable kernel that can be updated to newer 6.18.x point releases with minimal friction.

Contents¶

Directory/File	Description
`patches-6.18/` · `patches-7.1/`	Patches to apply on vanilla Linux 6.18 / 7.1
`files-6.18/` · `files-7.1/`	New files to add to the kernel tree (Realtek platform support, custom drivers)
`config-6.18-realtek.txt` · `config-7.1-realtek.txt`	Kernel configuration (one per line)
`kernel-img/<board>/kernel-<line>.img`	Pre-built flashable images, one per (board, kernel) pair
`build_kernel.sh`	Build script
`tools/`	Optional on-gateway kernel diagnostic tools

Building¶

./build_kernel.sh [clean|menuconfig|olddefconfig|vmlinux]   # 6.18 / lidl (defaults)
KERNEL=7.1 ./build_kernel.sh                                # build the 7.1 line
BOARD=sengled-e39-g8c ./build_kernel.sh                     # build for the Sengled G4

Options¶

Option	Description
(none)	Normal build: download sources if needed, apply patches, compile, package
`clean`	Remove kernel source directory and rebuild from scratch
`menuconfig`	Run kernel menuconfig interactively
`olddefconfig`	Update `.config` non-interactively against the current Kconfig
`vmlinux`	Build `vmlinux` only (skip packaging)

Board and kernel selection¶

Two environment variables pick what build_kernel.sh produces; both default to the Lidl 6.18 build, and the output lands in kernel-img/<board>/kernel-<line>.img:

Variable	Default	Values
`BOARD`	`lidl`	`lidl`, `sengled-e39-g8c` (Sengled Smart Hub G4)
`KERNEL`	`6.18`	`6.18`, `7.1`

./build_kernel.sh                                  # lidl / 6.18  → kernel-img/lidl/kernel-6.18.img
KERNEL=7.1 ./build_kernel.sh                       # lidl / 7.1
BOARD=sengled-e39-g8c ./build_kernel.sh            # G4 / 6.18
BOARD=sengled-e39-g8c KERNEL=7.1 ./build_kernel.sh # G4 / 7.1

BOARD selects the device tree built into the image. Porting to another RTL8196E board means adding an rtl8196e-<board>.dts, one Kconfig entry and one Makefile line — the recipe is documented in files-<line>/arch/mips/boot/dts/realtek/Makefile and in the devicetree Kconfig choice (files-<line>/arch/mips/realtek/Kconfig).

Build process¶

The script will: 1. Download Linux 6.18.x source (if not present) 2. Apply all patches from patches-6.18/ 3. Overlay Realtek platform files from files-<line>/ 4. Compile the kernel 5. Package the compressed kernel image (zboot) into kernel-img/<board>/kernel-<line>.img, ready to flash

Requirements: Toolchain must be built first.

Output¶

kernel-img/<board>/kernel-<line>.img — Flashable kernel image with Realtek header (~1.4 MB). The four shipped pre-built images are kernel-img/{lidl,sengled-e39-g8c}/kernel-{6.18,7.1}.img.

Technical Details¶

Build process (zboot)¶

The kernel uses the in-tree arch/mips/boot/compressed/ (zboot) decompressor — no external LZMA tool or loader is needed.

Compile kernel → vmlinux
zboot compresses the kernel with LZMA and prepends a small decompressor → vmlinuz (ELF)
Strip to raw binary → vmlinuz.bin
Add Realtek header (cvimg) → kernel-img/<board>/kernel-<line>.img

Key patches¶

Lexra MIPS support (kernel-side ll/sc/sync emulation, atomics, memory barriers)
RTL8196E SoC and board support (devicetree, platform drivers)
SPI flash driver, GPIO bank, LEDs (gpio-leds + PWM brightness modes)
From-scratch Ethernet driver (rtl8196e-eth) targeting modern APIs
In-kernel UART↔TCP bridge (rtl8196e-uart-bridge) for the Zigbee radio path
8250-compatible UART (8250_rtl819x), hardware timer clocksource (timer-rtl819x)

🙏 Credits¶

The kernel platform support traces back to early work by Gaspare Bruno on the Realtek target: - ggbruno/openwrt — Realtek branch

Those patches (originally for Linux 4.14) were heavily reworked for 5.10, then regenerated against 6.18 as part of the v3.0 rebase.