一、RTC时间查看与设置
1. 内核启动日志
查看Linux内核启动时与RTC相关的日志：
dmesg | grep rtc
2. 查看与设置当前系统时间
（1）查看当前时间：
date
（2）设置当前时间
date -s "2022-07-02 13:47:00"
3. 设置当前时间到RTC外设
hwclock -w
使用此命令设置时间到RTC外设，有纽扣电池的情况下，时间掉电不丢失。
二、Linux内核提供的RTC驱动
1. rtc_device结构体
RTC设备驱动是一个标准的字符驱动设备，应用程序通过open、release、read、write和ioctl接口完成对RTC设备的操作。
Linux内核将RTC设备抽象为rtc_device结构体，因此RTC设备驱动就是申请并初始化rtc_device，最后将rtc_device注册到Linux内核里面，这样内核中就有了一个rtc设备节点。
rtc_device结构体定义在文件include/linux/rtc.h中，如下：
struct rtc_device
{
struct device dev;
struct module *owner;
int id;
char name[RTC_DEVICE_NAME_SIZE];
const struct rtc_class_ops *ops;
struct mutex ops_lock;
struct cdev char_dev;
unsigned long flags;
unsigned long irq_data;
spinlock_t irq_lock;
wait_queue_head_t irq_queue;
struct fasync_struct *async_queue;
struct rtc_task *irq_task;
spinlock_t irq_task_lock;
int irq_freq;
int max_user_freq;
struct timerqueue_head timerqueue;
struct rtc_timer aie_timer;
struct rtc_timer uie_rtctimer;
struct hrtimer pie_timer; /* sub second exp, so needs hrtimer */
int pie_enabled;
struct work_struct irqwork;
/* Some hardware can't support UIE mode */
int uie_unsupported;
#ifdef CONFIG_RTC_INTF_DEV_UIE_EMUL
struct work_struct uie_task;
struct timer_list uie_timer;
/* Those fields are protected by rtc->irq_lock */
unsigned int oldsecs;
unsigned int uie_irq_active:1;
unsigned int stop_uie_polling:1;
unsigned int uie_task_active:1;
unsigned int uie_timer_active:1;
#endif
};
2. 对下向驱动适配——rtc_class_ops
rtc_device结构体中ops成员变量是rtc_classs_ops类型，定义在``文件中，如下：
/*
* For these RTC methods the device parameter is the physical device
* on whatever bus holds the hardware (I2C, Platform, SPI, etc), which
* was passed to rtc_device_register(). Its driver_data normally holds
* device state, including the rtc_device pointer for the RTC.
*
* Most of these methods are called with rtc_device.ops_lock held,
* through the rtc_*(struct rtc_device *, ...) calls.
*
* The (current) exceptions are mostly filesystem hooks:
* - the proc() hook for procfs
* - non-ioctl() chardev hooks: open(), release(), read_callback()
*
* REVISIT those periodic irq calls *do* have ops_lock when they're
* issued through ioctl() ...
*/
struct rtc_class_ops {
int (*open)(struct device *);
void (*release)(struct device *);
int (*ioctl)(struct device *, unsigned int, unsigned long);
int (*read_time)(struct device *, struct rtc_time *);
int (*set_time)(struct device *, struct rtc_time *);
int (*read_alarm)(struct device *, struct rtc_wkalrm *);
int (*set_alarm)(struct device *, struct rtc_wkalrm *);
int (*proc)(struct device *, struct seq_file *);
int (*set_mmss64)(struct device *, time64_t secs);
int (*set_mmss)(struct device *, unsigned long secs);
int (*read_callback)(struct device *, int data);
int (*alarm_irq_enable)(struct device *, unsigned int enabled);
};
需要注意，这些函数是提供给底层驱动适配的RTC设备操作函数，并不是提供给应用层的操作函数。
3. 对上向应用提供操作接口
Linux内核提供了一个RTC通用字符设备驱动文件：drivers/rtc/rtc-dev.c。
该文件中提供了 file_operations 函数操作集：
其中最重要的为 rtc_dev_ioctl API，该函数中实现了很多对应命令的操作，实现如下：
通过该API可以完成对硬件RTC的操作，命令定义在rtc.h中，如下：
/*
* ioctl calls that are permitted to the /dev/rtc interface, if
* any of the RTC drivers are enabled.
*/
#define RTC_AIE_ON _IO('p', 0x01) /* Alarm int. enable on */
#define RTC_AIE_OFF _IO('p', 0x02) /* ... off */
#define RTC_UIE_ON _IO('p', 0x03) /* Update int. enable on */
#define RTC_UIE_OFF _IO('p', 0x04) /* ... off */
#define RTC_PIE_ON _IO('p', 0x05) /* Periodic int. enable on */
#define RTC_PIE_OFF _IO('p', 0x06) /* ... off */
#define RTC_WIE_ON _IO('p', 0x0f) /* Watchdog int. enable on */
#define RTC_WIE_OFF _IO('p', 0x10) /* ... off */
#define RTC_ALM_SET _IOW('p', 0x07, struct rtc_time) /* Set alarm time */
#define RTC_ALM_READ _IOR('p', 0x08, struct rtc_time) /* Read alarm time */
#define RTC_RD_TIME _IOR('p', 0x09, struct rtc_time) /* Read RTC time */
#define RTC_SET_TIME _IOW('p', 0x0a, struct rtc_time) /* Set RTC time */
#define RTC_IRQP_READ _IOR('p', 0x0b, unsigned long) /* Read IRQ rate */
#define RTC_IRQP_SET _IOW('p', 0x0c, unsigned long) /* Set IRQ rate */
#define RTC_EPOCH_READ _IOR('p', 0x0d, unsigned long) /* Read epoch */
#define RTC_EPOCH_SET _IOW('p', 0x0e, unsigned long) /* Set epoch */
#define RTC_WKALM_SET _IOW('p', 0x0f, struct rtc_wkalrm)/* Set wakeup alarm*/
#define RTC_WKALM_RD _IOR('p', 0x10, struct rtc_wkalrm)/* Get wakeup alarm*/
#define RTC_PLL_GET _IOR('p', 0x11, struct rtc_pll_info) /* Get PLL correction */
#define RTC_PLL_SET _IOW('p', 0x12, struct rtc_pll_info) /* Set PLL correction */
#define RTC_VL_READ _IOR('p', 0x13, int) /* Voltage low detector */
#define RTC_VL_CLR _IO('p', 0x14) /* Clear voltage low information */
三、NXP原厂提供的RTC外设驱动
1. 设备树描述
RTC外设属于SOC芯片级外设，所以在imx6ull通用描述文件arch/arm/boot/dts/imx6ull.dtsi中寻找：
2. 驱动源码
根据兼容性"fsl,sec-v4.0-mon-rtc-lp"寻找对应的驱动源码：
grep -nR "fsl,sec-v4.0-mon-rtc-lp" *
找到驱动文件为：drivers/rtc/rtc-snvs.c。
3. 驱动文件浅析
可以看到RTC外设驱动也是按照platform平台驱动框架所写：
（1）在probe函数中，驱动通过使用regmap机制完成对硬件寄存器的操作。
（2）之后向内核申请了一个中断作为rtc闹钟中断：
中断处理函数如下：
（3）向内核注册一个rtc设备：
注册的操作函数指针如下：
这些实际操作的函数指针中，完成对RTC外设的操作，以snvs_rtc_set_time为例：
四、RTC驱动整体调用流程
