Android 内核简单分析

Google的Android操作系统已经成功面市,因为Android操作系统是基于Linux-2.6.25内核的,并且目前所有的源代码也都全部开放,因此可以简单地加以分析以方便以后向其他平台的移植工作.

首先需要了解的就是Android的一些基本情况,并下载Android的源代码以方便分析. Android的官方站点: http://www.android.com/ Android Source 的官方安装方法: http://source.android.com/download Android的中文站点: http://www.androidin.com/ 源代码包的相应联结地址: http://www.androidin.com/
我们可以采用Android官方站点推荐方式来下载Android的源代码,也可以从Android中文站点直接下载源代码程序包,随后再更新和同步. 当采用Android官方推荐方式下载或者需要更新你的源代码时,都需要git工具支持, 当你的系统没有安装git或者git版本过低时,建议从下面的地址更新到最新的git包.(Android要求git版本必须大于1.5.4) http://git.or.cz/ 给git自动升级的方式: git clone git://git.kernel.org/pub/scm/git/git.git 如果采用下载源代码包的方式,下载并展开后,在 .repo/repo 目录下有同步执行的文件repo, 你可以将其拷贝到你的PATH所在路径以备使用,例如/usr/bin等等. 或者也可以从Android站点得到最新的repo文件并复制到PATH所在路径. wget http://android.git.kernel.org/repo 源代码包解开后,其主目录下存在repo_sync.sh文件, 其主要是执行repo sync以实现和Android站点同步代码包的目的, 当执行出错后将再一次运行repo sync命令直到同步正常结束,但原文件包含错误,建议改成下面的程序:(原文件中少了一个空格) #!/bin/bash
echo "============start repo sync================="
repo sync
while [ $? = 1 ]; do
echo "============sync failed, re-sync again ====="
sleep 3
repo sync
done
这样,运行sh repo_sync.sh就可以自动实现本地代码包的同步和更新动作了. 我在Fedora Core 6上经过同步之后, du -h ./mydroid, 占用空间大约是2.1G
以下为Android官方公布的架构图： Android 内核简单分析_第1张图片

在kernel子目录下存放的就是Android的Linux Kernel了, 通过和标准的Linux 2.6.25 Kernel的对比,我们可以发现,其主要增加了以下的内容: 1. 基于ARM架构增加Gold-Fish平台,相应增加的目录如下: kernel/arch/arm/mach-goldfish kernel/include/asm-arm/arch-goldfish 目前Google的基于Gold-Fish的手机为G1手机，明年G2手机将面世。
该手机采用QualComm公司的高性能处理芯片MSM7201A，该芯片以ARM 11作为Application应用处理器，以ARM926T作为BaseBand主处理器(主要承载GSM／GPRS／EDGE／3G等协议栈处理)，支持JAVA硬加速(ARM本身也自带JAVA硬件处理)，包含GPS Processor, 支持2D/3D图形加速(每秒可处理4百万个多边形),另外还支持最大800万像素的Sensor, 支持MPEG 4/H.263/H.264/Real Media等多种Codec。

其中ARM926T采用的主频为274M HZ, ARM11的主频为528MHZ. (MSM7200的ARM11的主频为400M HZ)

因为没有找到MSM7201A的详细DataSheet, 因此可以简单地参考MSM7200的规格: http://blogimg.chinaunix.net/blog/upfile2/081222113141.pdf 下面为MSM7200架构图:
Android 内核简单分析_第2张图片

2. 增加了yaffs2 FLASH文件系统,相应增加的目录为: kernel/fs/yaffs2 实际上，Android包经过编译后生成的system.img和ramdisk.img文件就是yaffs2格式的包.
3. 增加了Android的相关Driver,相应目录为:

kernel/drivers/android

主要分为:

Android IPC系统: Binder (binder.c)

Android 日志系统: Logger (logger.c)
Android 电源管理: Power (power.c)
Android 闹钟管理: Alarm (alarm.c)

Android 内存控制台: Ram_console (ram_console.c)

Android 时钟控制的gpio: Timed_gpio (timed_gpio.c)

4. 增加了switch处理, 相应的目录为:

kernel/drivers/switch/

5. 增加了一种新的共享内存处理方式, 相应增加的文件为:

kernel/mm/ashmem.c

6. 其他为Linux-2.6.25内核所做的补丁等等，例如BlueTooth, 在此不做详细分析

另外GoldFish平台相关的驱动文件如下:

1. 字符输出设备:

kernel/drivers/char/goldfish_tty.c

2. 图象显示设备: (Frame Buffer)

kernel/drivers/video/goldfishfb.c

3. 键盘输入设备:

kernel/drivers/input/keyboard/goldfish_events.c

4. RTC设备: (Real Time Clock)

kernel/drivers/rtc/rtc-goldfish.c

5. USB Device设备:

kernel/drivers/usb/gadget/android_adb.c

6. SD卡设备:

kernel/drivers/mmc/host/goldfish.c

7. FLASH设备:

kernel/drivers/mtd/devices/goldfish_nand.c

kernel/drivers/mtd/devices/goldfish_nand_reg.h

8. LED设备:

kernel/drivers/leds/ledtrig-sleep.c

9. 电源设备:

kernel/drivers/power/goldfish_battery.c

10. 音频设备:

kernel/arch/arm/mach-goldfish/audio.c

11. 电源管理:

kernel/arch/arm/mach-goldfish/pm.c

12. 时钟管理:

kernel/arch/arm/mach-goldfish/timer.c

下面的链接为Android Kernel和标准Linux-2.6.25包相比较所产生的PATCH包，其中比较小的other patch包是除了上面列出的增加文件之外所修改的文件细节，供大家参考。

http://blogimg.chinaunix.net/blog/upfile2/090102095410.gz

以下为Android内核启动信息：

Uncompressing Linux.................................................................................... done, booting the kernel.
emulator: emulator window was out of view and was recentred

Linux version 2.6.25-00350-g40fff9a (android-build@apa27.mtv.corp.google.com) (gcc version 4.2.1) #1 Wed Jul 23 18:10:44 PDT 2008
CPU: ARM926EJ-S [41069265] revision 5 (ARMv5TEJ), cr=00003137
Machine: Goldfish
Memory policy: ECC disabled, Data cache writeback
CPU0: D VIVT write-through cache
CPU0: I cache: 4096 bytes, associativity 4, 32 byte lines, 32 sets
CPU0: D cache: 65536 bytes, associativity 4, 32 byte lines, 512 sets
Built 1 zonelists in Zone order, mobility grouping on. Total pages: 24384
Kernel command line: qemu=1 console=ttyS0 android.checkjni=1 android.qemud=ttyS1 android.ndns=2
Unknown boot option `android.checkjni=1': ignoring
Unknown boot option `android.qemud=ttyS1': ignoring
Unknown boot option `android.ndns=2': ignoring
PID hash table entries: 512 (order: 9, 2048 bytes)
Console: colour dummy device 80x30
Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
Memory: 96MB = 96MB total
Memory: 94268KB available (2380K code, 445K data, 100K init)
Mount-cache hash table entries: 512
CPU: Testing write buffer coherency: ok
net_namespace: 152 bytes
android_power_init
android_power_init done
NET: Registered protocol family 16
NET: Registered protocol family 2
IP route cache hash table entries: 1024 (order: 0, 4096 bytes)
TCP established hash table entries: 4096 (order: 3, 32768 bytes)
TCP bind hash table entries: 4096 (order: 2, 16384 bytes)
TCP: Hash tables configured (established 4096 bind 4096)
TCP reno registered
checking if image is initramfs... it is
Freeing initrd memory: 136K
goldfish_new_pdev goldfish_interrupt_controller at ff000000 irq -1
goldfish_new_pdev goldfish_device_bus at ff001000 irq 1
goldfish_new_pdev goldfish_timer at ff003000 irq 3
goldfish_new_pdev goldfish_rtc at ff010000 irq 10
goldfish_new_pdev goldfish_tty at ff002000 irq 4
goldfish_new_pdev goldfish_tty at ff011000 irq 11
goldfish_new_pdev smc91x at ff012000 irq 12
goldfish_new_pdev goldfish_fb at ff013000 irq 13
goldfish_new_pdev goldfish_audio at ff004000 irq 14
goldfish_new_pdev goldfish_memlog at ff006000 irq -1
goldfish_new_pdev goldfish-battery at ff014000 irq 15
goldfish_new_pdev goldfish_events at ff015000 irq 16
goldfish_new_pdev goldfish_nand at ff016000 irq -1
goldfish_new_pdev goldfish-switch at ff017000 irq 17
goldfish_new_pdev goldfish-switch at ff018000 irq 18
goldfish_pdev_worker registered goldfish-switch
goldfish_pdev_worker registered goldfish-switch
goldfish_pdev_worker registered goldfish_nand
goldfish_pdev_worker registered goldfish_events
goldfish_pdev_worker registered goldfish-battery
goldfish_pdev_worker registered goldfish_memlog
goldfish_audio_probe
goldfish_pdev_worker registered goldfish_audio
goldfish_pdev_worker registered goldfish_fb
goldfish_pdev_worker registered smc91x
goldfish_pdev_worker registered goldfish_tty
goldfish_pdev_worker registered goldfish_tty
goldfish_pdev_worker registered goldfish_rtc
goldfish_pdev_worker registered goldfish_timer
goldfish_pdev_worker registered goldfish_device_bus
goldfish_pdev_worker registered goldfish_interrupt_controller
ashmem: initialized
Installing knfsd (copyright (C) 1996 okir@monad.swb.de).
yaffs Jul 23 2008 18:10:35 Installing.
io scheduler noop registered
io scheduler anticipatory registered (default)
io scheduler deadline registered
io scheduler cfq registered
allocating frame buffer 320 * 480, got ffc10000
Console: switching to colour frame buffer device 40x30
console [ttyS0] enabled
brd: module loaded
loop: module loaded
nbd: registered device at major 43
smc91x.c: v1.1, sep 22 2004 by Nicolas Pitre <nico@cam.org>
No IRQF_TRIGGER set_type function for IRQ 12 (goldfish)
eth0: SMC91C11xFD (rev 1) at c6800000 IRQ 12 [nowait]
eth0: Ethernet addr: 52:54:00:12:34:56
goldfish nand dev0: size 4000000, page 2048, extra 64, erase 131072
goldfish nand dev1: size 4000000, page 2048, extra 64, erase 131072
goldfish nand dev2: size 4000000, page 2048, extra 64, erase 131072
mice: PS/2 mouse device common for all mice
*** events probe ***
events_probe() addr=0xc6804000 irq=16
events_probe() keymap=qwerty2
input: qwerty2 as /class/input/input0
goldfish_rtc goldfish_rtc: rtc core: registered goldfish_rtc as rtc0
logger: created 64K log 'log_main'
logger: created 64K log 'log_events'
logger: created 64K log 'log_radio'
IPv4 over IPv4 tunneling driver
GRE over IPv4 tunneling driver
TCP cubic registered
NET: Registered protocol family 1
NET: Registered protocol family 17
RPC: Registered udp transport module.
RPC: Registered tcp transport module.
802.1Q VLAN Support v1.8 Ben Greear <greearb@candelatech.com>
All bugs added by David S. Miller <davem@redhat.com>
VFP support v0.3: implementor 41 architecture 1 part 10 variant 9 rev 0
goldfish_rtc goldfish_rtc: setting system clock to 2009-01-02 07:43:01 UTC (1230882181)
Freeing init memory: 100K
init: cannot open '/initlogo.rle'
yaffs: dev is 32505856 name is "mtdblock0"
yaffs: passed flags ""
yaffs: Attempting MTD mount on 31.0, "mtdblock0"
yaffs: dev is 32505857 name is "mtdblock1"
yaffs: passed flags ""
yaffs: Attempting MTD mount on 31.1, "mtdblock1"
yaffs: dev is 32505858 name is "mtdblock2"
yaffs: passed flags ""
yaffs: Attempting MTD mount on 31.2, "mtdblock2"
sh: can't access tty; job control turned off
# init: cannot find '/system/bin/playmp3', disabling 'bootsound'
eth0: link up
warning: `rild' uses 32-bit capabilities (legacy support in use)
init: sys_prop: mis-match msg size recieved: -1 expected: 128

从启动信息可以看出，其主CPU为ARM926EJ-S,并非ARM11 CPU, 说明下载的Emulator内核并非是针对G1手机的，估计只是实现了对ARM926EJ-S CPU的模拟。

以上为Android内核的大致分析，希望能给有兴趣的人员以简单帮助。

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