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Linux内核裁剪

概述

Linux内核裁剪是嵌入式开发的重要环节，通过移除不需要的功能和驱动，可以显著减小内核体积、降低内存占用、提高启动速度。

裁剪目标

  1. 减小内核体积：从数百MB减小到几MB
  2. 降低内存占用：减少运行时内存消耗
  3. 加快启动速度：减少初始化时间和模块加载
  4. 提高安全性：移除不必要的功能减少攻击面
  5. 优化性能：针对性优化关键功能

获取内核源码

官方内核

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# 下载官方内核
wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.tar.xz
tar xvf linux-5.15.tar.xz
cd linux-5.15

厂商内核

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# 树莓派内核
git clone --depth=1 https://github.com/raspberrypi/linux.git

# 全志内核
git clone https://github.com/linux-sunxi/linux-sunxi.git

# NXP i.MX内核
git clone https://source.codeaurora.org/external/imx/linux-imx.git

内核配置方法

使用默认配置

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# 查看支持的架构
ls arch/

# 使用默认配置
make defconfig                        # 默认配置
make bcmrpi3_defconfig                # 树莓派3
make sun8i_h3_defconfig               # 全志H3
make imx_v7_defconfig                 # i.MX6/7

图形化配置

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# 文本界面配置
make menuconfig

# Qt图形界面配置
make xconfig

# GTK图形界面配置
make gconfig

# 保存配置
cp .config arch/arm/configs/myboard_defconfig

从现有配置修改

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# 加载现有配置
make myboard_defconfig

# 修改配置
make menuconfig

# 保存为新的默认配置
make savedefconfig
cp defconfig arch/arm/configs/myboard_defconfig

常用配置选项

通用设置

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General setup  --->
    [ ] Configure standard kernel features (expert users)  ---> 
    [ ] Enable loadable module support                      # 禁用模块支持
    [ ] Enable the slab memory cache                        # 使用slab分配器
    (256) Initial RAM filesystem disk capacity (kB)         # 减小initramfs大小

处理器类型

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System Type  --->
    [*] Support for the ARM EABI using binfmt_misc          # ARM EABI支持
    (3) Maximum number of CPUs (2-32)                       # CPU数量
    [*] SMP (Symmetric Multi-Processing) support            # 多核支持

调度器配置

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General setup  --->
    [*] CPU/Task time and stats accounting  --->
        [*]   Export task/pid/cgroup time to user via /proc
    [ ] Support for hot-pluggable devices                   # 禁用热插拔

内存管理

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General setup  --->
    [*] Enable VM event counters                           # VM事件计数
    [ ] Enable SLUB debugging support                      # 禁用SLUB调试
    [ ] Memory hotplug support                             # 禁用内存热插拔

驱动裁剪

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Device Drivers  --->
    [ ] Multiple devices driver support (RAID and LVM)      # 禁用RAID
    [ ] IEEE 1394 (FireWire) support                       # 禁用FireWire
    [ ] I2C support  --->                                  # 按需启用
    [ ] SPI support  --->                                  # 按需启用
    [ ] GPIO Support  --->                                 # 按需启用

网络功能裁剪

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Networking support  --->
    Networking options  --->
        [ ] Transformation user configuration interface     # 禁用IPsec
        [ ] TCP/IP networking                              # 按需启用
        [ ] The IPv6 protocol                              # 按需禁用IPv6

文件系统裁剪

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File systems  --->
    [ ] Second extended fs support                         # ext2按需
    [*] The Extended 3 (ext3) filesystem support           # ext3
    [ ] The Extended 4 (ext4) filesystem support           # ext4按需
    [*] FUSE (Filesystem in Userspace) support            # FUSE支持
    [*] Pseudo filesystems  --->
        [*]   /proc file system support
        [*]   sysfs file system support
        [*]   Tmpfs virtual memory file system support

调试选项裁剪

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Kernel hacking  --->
    [ ] Kernel debugging                                   # 禁用内核调试
    [ ] Magic SysRq keys                                   # 禁用SysRq
    [ ] Debug slab memory allocations                      # 禁用slab调试

驱动精简策略

按需选择驱动

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# 只选择需要的驱动
Device Drivers  --->
    Character devices  --->
        [*] Serial drivers  --->
            [*] 8250/16550 serial support
    Block devices  --->
        [*] Loopback device support                        # 按需
    Network device support  --->
        [*] Ethernet driver support                        # 只选择需要网卡

移除不需要的驱动

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# 移除不支持的硬件驱动
Device Drivers  --->
    [ ] Sound card support                                 # 无音频硬件
    [ ] USB support                                        # 无USB接口
    [ ] Bluetooth support                                  # 无蓝牙
    [ ] Multimedia support                                 # 无多媒体

内核编译

交叉编译设置

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# 设置交叉编译工具链
export ARCH=arm
export CROSS_COMPILE=arm-linux-gnueabihf-

# 或使用make变量
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- menuconfig

编译流程

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# 清理
make distclean

# 配置
make xxx_defconfig

# 编译内核
make zImage -j$(nproc)

# 编译设备树
make dtbs

# 编译模块（如果启用）
make modules -j$(nproc)
make modules_install INSTALL_MOD_PATH=/path/to/rootfs

输出文件

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# 内核镜像
arch/arm/boot/zImage                      # 压缩内核
arch/arm/boot/uImage                      # U-Boot格式

# 设备树
arch/arm/boot/dts/*.dtb                   # 编译后的设备树

# 模块
/lib/modules/$(uname -r)/                 # 内核模块

内核大小优化

使用压缩

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General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
    (gzip) Built-in initramfs compression mode
        ( ) gzip
        ( ) bzip2
        (X) LZMA                          # LZMA压缩率更高
        ( ) XZ
        ( ) lzo
        ( ) lz4

优化内核镜像

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# 使用更小的内核格式
make xipImage                             # XIP内核（片上执行）

# 禁用符号表
Kernel hacking  --->
    [ ] Include debugging information in kernel binary

# 使用strip
arm-linux-gnueabihf-strip arch/arm/boot/zImage

设备树配置

设备树文件结构

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/dts-v1/;
/plugin/;

/ {
    compatible = "vendor,board";
    model = "My Board";
    
    memory@40000000 {
        device_type = "memory";
        reg = <0x40000000 0x20000000>;    /* 512MB内存 */
    };
    
    chosen {
        bootargs = "console=ttyS0,115200 root=/dev/mmcblk0p2";
    };
};

编译设备树

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# 编译dts为dtb
dtc -I dts -O dtb -o board.dtb board.dts

# 反编译dtb为dts
dtc -I dtb -O dts -o board.dts board.dtb

启动优化

减少启动时间

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General setup  --->
    (0) Default boot command line arguments
    [ ] Support for hot-pluggable devices
    [ ] Mark the kernel read-only in rodata section

Power management options  --->
    [ ] Suspend to RAM and standby                          # 禁用休眠
    [ ] Hibernation (aka 'suspend to disk')

控制台优化

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Device Drivers  --->
    Character devices  --->
        Serial drivers  --->
            [*] Console on 8250/16550 serial port
            (115200) Default Baud Rate

内核配置保存与恢复

保存配置

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# 保存当前配置
make savedefconfig
cp defconfig arch/arm/configs/myboard_defconfig

# 或直接复制
cp .config arch/arm/configs/myboard_defconfig

恢复配置

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# 从保存的配置恢复
make myboard_defconfig

调试与验证

检查内核大小

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# 查看内核大小
ls -lh arch/arm/boot/zImage

# 查看内核符号大小
nm --size-sort vmlinux | tail -20

# 查看内核段大小
arm-linux-gnueabihf-size vmlinux

检查模块依赖

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# 查看模块信息
modinfo xxx.ko

# 检查模块依赖
depmod -a

常见问题解决

编译错误处理

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# 配置错误
make oldconfig                           # 更新配置
make menuconfig                          # 手动配置

# 依赖问题
make clean && make mrproper              # 清理

驱动加载问题

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# 查看内核消息
dmesg | grep -i error

# 查看模块加载情况
lsmod
cat /proc/modules

参考资料