这里使用基于freescale的imx_r9.2 patch的android源码分析.
android中的recovery模式，作为android的开发者或者用户都应该清楚和了解。android利用recovery模式完成整个系统的升级和恢复出厂设置。
这里先讲讲怎么进入recovery模式，这个工作是由bootloader完成。freescale使用的uboot.
bootloader每次启动的时候都要执行./lib_arm/board.c的代码：
464 #ifdef CONFIG_ANDROID_RECOVERY
465 check_recovery_mode();
466 #endif
该函数位于./board/freescale/common/recovery.c，代码如下：
98 /* export to lib_arm/board.c */
99 void check_recovery_mode(void)
100 {
101 if (check_key_pressing())
102 setup_recovery_env();
103 else if (check_recovery_cmd_file()) {
104 puts("Recovery command file founded!/n");
105 setup_recovery_env();
106 }
107 }
line 101和line 103指出有两种方式可以执行setup_recovery_env()进入recovery模式。
我们接下来看看这三个函数到底干了些什么，
check_key_pressing也在board/freescale/common/recovery.c中，
32 int check_key_pressing(void)
33 {
...
38
39 puts("Detecting HOME+POWER key for recovery .../n");
...
45
46 for (i = 0; i < keys; i++) {
47 if (test_key(CONFIG_POWER_KEY , &key_info[i]))
48 state |= PRESSED_HOME;
49 else if (test_key(CONFIG_HOME_KE Y, &key_info[i]))
50 state |= PRESSED_POWER;
51 }
...
55 if ((state & RECOVERY_KEY_MASK) == RECOVERY_KEY_MASK)
56 return 1;
57
58 return 0;
59 }
该函数用于探测HOME+POWER键，若同时按下，函数返回1，进入recovery模式。
893 int check_recovery_cmd_file(void)
894 {
...
938 filelen = ext2fs_open(CONFIG_ANDROID_RECOVERY_CMD_FILE );
...
955 return (filelen > 0) ? 1 : 0;
956
957 }
该函数探测宏CONFIG_ANDROID_RECOVERY_CMD_FILE，表示的文件是否存在，若存在返回1，进入recovery模式。
该宏定义在./include/configs/mx51_bbg_android.h
115 #define CONFIG_ANDROID_RECOVERY_CMD_FILE "/recovery/command"
接下来看看setup_recovery_env()函数，做了些什么事情。
70 void setup_recovery_env(void)
71 {
72 char *env, *boot_args, *boot_cmd;
73 int bootdev = get_boot_device();
74
75 boot_cmd = supported_reco_envs[bootdev].cmd;
76 boot_args = supported_reco_envs[bootdev].args;
77
78 if (boot_cmd == NULL) {
79 printf("Unsupported bootup device for recovery/n");
80 return;
81 }
82
83 printf("setup env for recovery../n");
84
85 env = getenv("bootargs_android_recovery");
86 /* Set env to recovery mode */
87 /* Only set recovery env when these env not exist, give user a
88 * chance to change their recovery env */
89 if (!env)
90 setenv("bootargs_android_recovery", boot_args);
91
92 env = getenv("bootcmd_android_recovery");
93 if (!env)
94 setenv("bootcmd_android_recovery", boot_cmd);
95 setenv("bootcmd", "run bootcmd_android_recovery");
96 }
该函数主要用来设置bootargs_android_recovery和bootcmd_android_recovery环境变量，我们看看这两个环境变量到底设置成了什么，
请看代码：board/freescale/mx51_bbg/mx51_bbg.c
878 struct reco_envs supported_reco_envs[BOOT_DEV_NUM] = {
...
888 .cmd = CONFIG_ANDROID_RECOVERY_BOOTCMD_MMC ,
889 .args = CONFIG_ANDROID_RECOVERY_BOOTARGS_MMC,
...
891 };
上面line 888和line 889的宏定义在include/configs/mx51_bbg_android.h:
109 #define CONFIG_ANDROID_RECOVERY_BOOTARGS_MMC /
110 "setenv bootargs ${bootargs} init=/init root=/dev/mmcblk0p4 " /
111 "rootfs=ext4 di1_primary"
112 #define CONFIG_ANDROID_RECOVERY_BOOTCMD_MMC /
113 "run bootargs_base bootargs_android_recovery ;" /
114 "mmc read 0 ${loadaddr} 0x800 0x1800;bootm"
看到这里终于了解了，到底怎么进入recovery模式，实际上就是通过bootloader传递参数给内核，让kernel挂载recovery分区便启动了recovery模式。