1 前言 

在前一篇文章提到，从源代码树下载下来的最新Android源代码，是不包括内核代码的，也就是Android源代码工程默认不包含Linux Kernel代码，而是使用预先编译好的内核，也就是prebuilt/android-arm/kernel/kernel-qemu文件。那么，如何才能DIY自己的内核呢？这篇文章一一道来。

2 下载Linux Kernel for Android源代码然后编译

参考谷歌官网http://source.android.com/source/building-kernels.html

2.1  下载源码

谷歌官网上列举了所有类型Kernel源码的下载地址和说明：

Depending on which kernel you want,

$ git clone https://android.googlesource.com/kernel/common.git

$ git clone https://android.googlesource.com/kernel/x86_64.git

$ git clone https://android.googlesource.com/kernel/exynos.git

$ git clone https://android.googlesource.com/kernel/goldfish.git

$ git clone https://android.googlesource.com/kernel/msm.git

$ git clone https://android.googlesource.com/kernel/omap.git

$ git clone https://android.googlesource.com/kernel/samsung.git

$ git clone https://android.googlesource.com/kernel/tegra.git

• The goldfish project contains the kernel sources for the emulated platforms.

• The msm project has the sources for ADP1, ADP2, Nexus One, Nexus 4, Nexus 5, Nexus 6, and can be used as a starting point for work on Qualcomm MSM chipsets.

• The omap project is used for PandaBoard and Galaxy Nexus, and can be used as a starting point for work on TI OMAP chipsets.

• The samsung project is used for Nexus S, and can be used as a starting point for work on Samsung Hummingbird chipsets.

• The tegra project is for Xoom, Nexus 7, Nexus 9, and can be used as a starting point for work on NVIDIA Tegra chipsets.

• The exynos project has the kernel sources for Nexus 10, and can be used as a starting point for work on Samsung Exynos chipsets.

• The x86_64 project has the kernel sources for Nexus Player, and can be used as a starting point for work on Intel x86_64 chipsets.

我下载的是goldfish，因此我运行如下命令下载内核源码(必须先用vpn进行翻墙)：

cd /Volumes/android/

mkdir kernel

cd kernel

git clone https://android.googlesource.com/kernel/goldfish.git

下载完成后切换到，会看见当前目录下多了一个goldfish目录。

2.2  配置编译环境

$ export PATH=$PATH:/Volumes/android/android-4.4.4_r2/prebuilts/gcc/darwin-x86/arm/arm-eabi-4.6/bin/

$ export ARCH=arm

$ export SUBARCH=arm

$ export CROSS_COMPILE=arm-eabi-

第一个命令是将交叉编译工具所在目录添加到$PATH环境变量中去，中间两个命令设置体系结构为arm，第四个命令是设置交叉编译工具链前缀。

2.3 编译源码

2.3.1 首先要确定我要编译那个版本的内核源码，执行如下命令启动模拟器(也就是我上一篇博客中编译android源代码生成的模拟器)：

$ cd /Volumes/android/android-4.4.4_r2/

$ emulator -partition-size 1024 &

$ adb shell 

# cd proc

# cat version

Linux version 3.4.0-gd853d22 (nnk@nnk.mtv.corp.google.com) (gcc version 4.6.x-google 20120106 (prerelease) (GCC) ) #1 PREEMPT Tue Jul 9 17:46:46 PDT 2013

由上面命令运行的结果可以得知android4.4.4_r2用的Linux内核源码版本是3.4.0

因此首先我们执行如下命令来切换到对应版本：

cd /Volumes/android/kernel/goldfish/

chenyangdeMacBook-Pro:goldfish chenyang$ git branch -a

* master

  remotes/origin/HEAD -> origin/master

  remotes/origin/android-3.10

  remotes/origin/android-3.4

  remotes/origin/android-goldfish-2.6.29

  remotes/origin/android-goldfish-3.10

  remotes/origin/android-goldfish-3.10-m-dev

  remotes/origin/android-goldfish-3.4

  remotes/origin/linux-goldfish-3.0-wip

  remotes/origin/master

git checkout android-goldfish-3.4

2.3.2 将上一步启动的模拟器中的config.gz文件放到/Volumes/android/kernel/goldfish/目录中，因为我要指定该模拟器运行我编译的Linux内核，执行如下命令：

adb pull proc/config.gz

gunzip config.gz

mv config .config

2.3.4 执行如下命令开始编译

make

如果最后输出如下两句话，说明编译成功：

OBJCOPY arch/arm/boot/zImage

Kernel: arch/arm/boot/zImage is ready

3 启动模拟器并且指定其运行我编译的Linux内核

emulator -kernel ../kernel/goldfish/arch/arm/boot/zImage -partition-size 1024 &

用adb工具连接模拟器，查看内核版本信息，看看模拟器上跑的内核是不是我们刚才编译出来的内核：

$ adb shell

root@generic:/ # cd proc

root@generic:/proc # cat v

version      vmallocinfo  vmstat       

root@generic:/proc # cat version                                               

Linux version 3.4.67-g49ef260 (chenyang@chenyangdeMacBook-Pro.local) (gcc version 4.7 (GCC) ) #1 PREEMPT Sun Aug 9 21:50:04 CST 2015

 从机器名chenyang@chenyangdeMacBook-Pro.local和日期 Aug 9 21:50:04 CST 2015可以看出，模拟器使用的内核即为刚刚编译出来的内核。

4 遇到的问题

执行make命令后提示如下错误

scripts/mod/mk_elfconfig.c:4:10: fatal error: 'elf.h' file not found

#include <elf.h>

         ^

1 error generated.

解决方法：有错误可以很明显的看出mk_elfconfig.c文件中要引用elf.h文件，但是elf.sh文件不存在，其实这个文件可以从android源代码中得到，然后将其放到/usr/include/文件夹中即可，执行如下命令：

sudo cp /Volumes/android/android-4.4.4_r2/external/elfutils/libelf/elf.h /usr/include/