Android RTC 自下往上浅析 Android系统开发

1.首先搞清楚RTC在kernel内的作用: linux系统有两个时钟：一个是由主板电池驱动的“Real Time Clock”也叫做RTC或者叫CMOS时钟，硬件时钟。当操作系统关机的时候，用这个来记录时间，但是对于运行的系统是不用这个时间的。
另一个时间是 “System clock”也叫内核时钟或者软件时钟，是由软件根据时间中断来进行计数的，内核时钟在系统关机的情况下是不存在的，所以，当操作系统启动的时候，内核时钟是要读取RTC时间来进行时间同步。并且在系统关机的时候将系统时间写回RTC中进行同步。如前所述，Linux内核与RTC进行互操作的时机只有两个：
1) 内核在启动时从RTC中读取启动时的时间与日期；
2) 内核在需要时将时间与日期回写到RTC中。系统启动时，内核通过读取RTC来初始化内核时钟,又叫墙上时间，该时间放在xtime变量中。

The current time of day (the wall time) is defined in kernel/timer.c: struct timespec xtime; The timespec data structure is defined in as: struct timespec { time_t tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ };

最有可能读取RTC设置内核时钟的位置应该在arch/arm/kernel/time.c里的time_init函数内. time.c为系统的时钟驱动部分.
time_init函数会在系统初始化时,由init/main.c里的start_kernel函数内调用. ARM架构的time_init代码如下:
/* arch/arm/kernel/time.c */

void __init time_init(void) { system_timer = machine_desc->timer; system_timer->init(); #ifdef CONFIG_HAVE_SCHED_CLOCK sched_clock_postinit(); #endif }

2.RTC结构部分

static const struct rtc_class_ops hym8563_rtc_ops = { .read_time = hym8563_rtc_read_time, .set_time = hym8563_rtc_set_time, .read_alarm = hym8563_rtc_read_alarm, .set_alarm = hym8563_rtc_set_alarm, .ioctl= hym8563_rtc_ioctl, .proc= hym8563_rtc_proc }; static int __devinit hym8563_probe(struct i2c_client *client, const struct i2c_device_id *id) { int rc = 0; u8 reg = 0; struct hym8563 *hym8563; struct rtc_device *rtc = NULL; struct rtc_time tm_read, tm = { .tm_wday = 6, .tm_year = 111, .tm_mon = 0, .tm_mday = 1, .tm_hour = 12, .tm_min = 0, .tm_sec = 0, }; if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) return -ENODEV; hym8563 = kzalloc(sizeof(struct hym8563), GFP_KERNEL); if (!hym8563) { return -ENOMEM; } gClient = client; hym8563->client = client; mutex_init(&hym8563->mutex); wake_lock_init(&hym8563->wake_lock, WAKE_LOCK_SUSPEND, "rtc_hym8563"); INIT_WORK(&hym8563->work, hym8563_work_func); i2c_set_clientdata(client, hym8563); hym8563_init_device(client); // check power down hym8563_i2c_read_regs(client,RTC_SEC,?,1); if (reg&0x80) { dev_info(&client->dev, "clock/calendar information is no longer guaranteed\n"); hym8563_set_time(client, &tm); } hym8563_read_datetime(client, &tm_read); //read time from hym8563 if(((tm_read.tm_year < 70) | (tm_read.tm_year > 137 )) | (tm_read.tm_mon == -1) | (rtc_valid_tm(&tm_read) != 0)) //if the hym8563 haven't initialized { hym8563_set_time(client, &tm); //initialize the hym8563 } if(gpio_request(client->irq, "rtc gpio")) { dev_err(&client->dev, "gpio request fail\n"); gpio_free(client->irq); goto exit; } hym8563->irq = gpio_to_irq(client->irq); gpio_pull_updown(client->irq,GPIOPullUp); if (request_irq(hym8563->irq, hym8563_wakeup_irq, IRQF_TRIGGER_FALLING, client->dev.driver->name, hym8563) < 0) { printk("unable to request rtc irq\n"); goto exit; } enable_irq_wake(hym8563->irq); rtc = rtc_device_register(client->name, &client->dev, &hym8563_rtc_ops, THIS_MODULE); if (IS_ERR(rtc)) { rc = PTR_ERR(rtc); rtc = NULL; goto exit; } hym8563->rtc = rtc; return 0; exit: if (rtc) rtc_device_unregister(rtc); if (hym8563) kfree(hym8563); return rc; }

看这两个结构体，我认为就已经达到目的，第2个结构体是平台设备中的driver部分，也就是hym8563_probe,是个很重要的函数，在这里面，第1个结构体被顺利注册进rtc子系统。Rtc的所用到的结构体被定义在，LINUX/include/linux/rtc.h里面。
struct rtc_device这个结构体是核心部分，内核中就是靠它传递信息，不管在哪使用，都要靠它间接的调用底层信息。比如在alarm.c 中。
alarm_ioctl这个函数中，多次使用了rtc_set_time/rtc_get_time，这些函数虽然是定义在rtc目录下的interface.c 中，但实质还是rtc-hym8563.c中结构体 rtc_class_ops所指过去的函数。

也就是说在和内核层以上的交互是通过alarm-dev.c里面的alarm_ioctl及其余的函数交互，但是在这个文件里面的rtc_set_time/rtc_get_time操作是为了设置RTC时间等的操作是调用alarm.c里面的函数，但是alarm.c驱动本身和硬件没有关系，在这里屏蔽了RTC的硬件操作，比如HYM8563的时间I2C硬件驱动操作在rtc-HYM8563.c驱动里，只需要使用 rtc_class_ops进行注册就可以了，完整的实现了硬件对平台无关性的屏蔽。
那么我可以告诉你了，为什么多了一个alarm.c ，因为在android中它为了使得平台无关性提高，因此大量的增加过渡代码层，HAL就是这种性质的存在。alarm.c在用户空间中会多一个/dev/alarm 节点，而rtc-hym8563.c.c 会产生/dev/rtc这样的节点。
3.JNI层

namespace android {static jint android_server_AlarmManagerService_setKernelTimezone(JNIEnv* env, jobject obj, jint fd, jint minswest) { struct timezone tz; tz.tz_minuteswest = minswest; tz.tz_dsttime = 0; int result = settimeofday(NULL, &tz); if (result < 0) { LOGE("Unable to set kernel timezone to %d: %s\n", minswest, strerror(errno)); return -1; } else { LOGD("Kernel timezone updated to %d minutes west of GMT\n", minswest); }return 0; }static jint android_server_AlarmManagerService_init(JNIEnv* env, jobject obj) { return open("/dev/alarm", O_RDWR); }static void android_server_AlarmManagerService_close(JNIEnv* env, jobject obj, jint fd) { close(fd); }static void android_server_AlarmManagerService_set(JNIEnv* env, jobject obj, jint fd, jint type, jlong seconds, jlong nanoseconds) { struct timespec ts; ts.tv_sec = seconds; ts.tv_nsec = nanoseconds; int result = ioctl(fd, ANDROID_ALARM_SET(type), &ts); if (result < 0) { LOGE("Unable to set alarm to %lld.%09lld: %s\n", seconds, nanoseconds, strerror(errno)); } }static jint android_server_AlarmManagerService_waitForAlarm(JNIEnv* env, jobject obj, jint fd) { int result = 0; do { result = ioctl(fd, ANDROID_ALARM_WAIT); } while (result < 0 && errno == EINTR); if (result < 0) { LOGE("Unable to wait on alarm: %s\n", strerror(errno)); return 0; }return result; }static JNINativeMethod sMethods[] = { /* name, signature, funcPtr */ {"init", "()I", (void*)android_server_AlarmManagerService_init}, {"close", "(I)V", (void*)android_server_AlarmManagerService_close}, {"set", "(IIJJ)V", (void*)android_server_AlarmManagerService_set}, {"waitForAlarm", "(I)I", (void*)android_server_AlarmManagerService_waitForAlarm}, {"setKernelTimezone", "(II)I", (void*)android_server_AlarmManagerService_setKernelTimezone}, }; int register_android_server_AlarmManagerService(JNIEnv* env) { return jniRegisterNativeMethods(env, "com/android/server/AlarmManagerService", sMethods, NELEM(sMethods)); }} /* namespace android */

其实在JNI层这里RTC就和其余的模块一样，直接去通过打开/关闭/设置/等待等来操作节点/dev/alarm和底层进行通信，不仔细解释。
4、 framework层

frameworks/base/services/java/com/android/server/AlarmManagerService.java
frameworks/base/core/java/android/app/AlarmManager.java
下面的是直接提供给app层的API接口，它是AlarmManagerService.java的一个封装。
这里只是简单的解释下service到底在此做什么了。
其实也没做什么，仅仅是把上面分析的JNI拿来在此调用一下而已。然后包装一下，将功能实现得更完美些。
5.App层

package android.app; import android.content.Context; import android.content.Intent; import android.os.RemoteException; import android.os.ServiceManager; public class AlarmManager { public static final int RTC_WAKEUP = 0; public static final int RTC = 1; public static final int ELAPSED_REALTIME_WAKEUP = 2; public static final int ELAPSED_REALTIME = 3; private final IAlarmManager mService; AlarmManager(IAlarmManager service) { mService = service; }public void set(int type, long triggerAtTime, PendingIntent operation) { try { mService.set(type, triggerAtTime, operation); } catch (RemoteException ex) { } }public void setRepeating(int type, long triggerAtTime, long interval, PendingIntent operation) { try { mService.setRepeating(type, triggerAtTime, interval, operation); } catch (RemoteException ex) { } }public static final long INTERVAL_FIFTEEN_MINUTES = 15 * 60 * 1000; public static final long INTERVAL_HALF_HOUR = 2*INTERVAL_FIFTEEN_MINUTES; public static final long INTERVAL_HOUR = 2*INTERVAL_HALF_HOUR; public static final long INTERVAL_HALF_DAY = 12*INTERVAL_HOUR; public static final long INTERVAL_DAY = 2*INTERVAL_HALF_DAY; public void setInexactRepeating(int type, long triggerAtTime, long interval, PendingIntent operation) { try { mService.setInexactRepeating(type, triggerAtTime, interval, operation); } catch (RemoteException ex) { } }public void cancel(PendingIntent operation) { try { mService.remove(operation); } catch (RemoteException ex) { } }public void setTime(long millis) { try { mService.setTime(millis); } catch (RemoteException ex) { } }public void setTimeZone(String timeZone) { try { mService.setTimeZone(timeZone); } catch (RemoteException ex) { } } }

frameworks\base\core\java\android\app 这个目录下，就是系统自带定时器的源代码，比如Alarms.java 中：第一个导入的包就是 import android.app.AlarmManager。
【Android RTC 自下往上浅析】