RTC

前言

RTC（Real Time Clock，实时时钟）是个常用的外设，通过 RTC 我们可以知道日期和时间信息，因此在需要记录时间的场合就需要实时时钟。
可以使用专用的实时时钟芯片来完成此功能，如 DS1302，不过有些 SOC 内部就已经自带了 RTC 外设，比如 IMX6U。
RTC 本质上就是一个定时器，只要给它提供时钟，它就会一直运行。寄存器保存着秒数，直接读取寄存器值就知道过了多长时间了，一般以 1970 年 1 月 1 日为起点。
RTC 也是带有闹钟功能的，往相应寄存器写入值，当时钟值和闹钟值匹配时就会产生时钟中断。

驱动

step1：
内核启动打印

...
snvs_rtc 20cc000.snvs:snvs-rtc-lp: rtc core: registered 20cc000.snvs:snvs-r as rtc0
...
snvs_rtc 20cc000.snvs:snvs-rtc-lp: setting system clock to 2021-07-28 10:40:23 UTC (1627468823)
...

得知 rtc 的驱动为 20cc000.snvs

step2：
查看设备树

snvs: snvs@020cc000 {
     compatible = "fsl,sec-v4.0-mon", "syscon", "simple-mfd";
     reg = <0x020cc000 0x4000>;


     snvs_rtc: snvs-rtc-lp {
         compatible = "fsl,sec-v4.0-mon-rtc-lp";
         regmap = <&snvs>;
         offset = <0x34>;
         interrupts = <GIC_SPI 19 IRQ_TYPE_LEVEL_HIGH>, <GIC_SPI 20 IRQ_TYPE_LEVEL_HIGH>;
     };


     snvs_poweroff: snvs-poweroff {
         compatible = "syscon-poweroff";
         regmap = <&snvs>;
         offset = <0x38>;
         mask = <0x61>;
     };


     snvs_pwrkey: snvs-powerkey {
         compatible = "fsl,sec-v4.0-pwrkey";
         regmap = <&snvs>;
         interrupts = <GIC_SPI 4 IRQ_TYPE_LEVEL_HIGH>;
         linux,keycode = <KEY_POWER>;
         wakeup;
     };
 };

可知：rtc 是 snvs（Secure Non-Volatile Storage）的一个子功能，和其并列的还有 poweroff、pwrkey
目前我们只关注 snvs_rtc

snvs_rtc: snvs-rtc-lp {
    compatible = "fsl,sec-v4.0-mon-rtc-lp";
    regmap = <&snvs>;
    offset = <0x34>;
    interrupts = <GIC_SPI 19 IRQ_TYPE_LEVEL_HIGH>, <GIC_SPI 20 IRQ_TYPE_LEVEL_HIGH>;
};

step3：
在 C 文件中搜索 "fsl,sec-v4.0-mon-rtc-lp"，用来找到驱动

// drivers/rtc/rtc-snvs.c

static const struct of_device_id snvs_dt_ids[] = {
    { .compatible = "fsl,sec-v4.0-mon-rtc-lp", },
    { /* sentinel */ }
};
MODULE_DEVICE_TABLE(of, snvs_dt_ids);

进而找到对应的 probe() 函数

static struct platform_driver snvs_rtc_driver = {
    .driver = {
        .name    = "snvs_rtc",
        .pm    = SNVS_RTC_PM_OPS,
        .of_match_table = snvs_dt_ids,
    },
    .probe        = snvs_rtc_probe,
};
module_platform_driver(snvs_rtc_driver);    // 疑问：为什么是平台总线？

step4：
分析驱动注册（初始化）流程

static int snvs_rtc_probe(struct platform_device *pdev)
{
    struct snvs_rtc_data *data;
    struct resource *res;
    int ret;
    void __iomem *mmio;

    data = devm_kzalloc(&pdev->dev, sizeof(*data), GFP_KERNEL);
    if (!data)
        return -ENOMEM;

    data->regmap = syscon_regmap_lookup_by_phandle(pdev->dev.of_node, "regmap");

    if (IS_ERR(data->regmap)) {
        dev_warn(&pdev->dev, "snvs rtc: you use old dts file, please update it\n");
        res = platform_get_resource(pdev, IORESOURCE_MEM, 0);

        mmio = devm_ioremap_resource(&pdev->dev, res);
        if (IS_ERR(mmio))
            return PTR_ERR(mmio);

        data->regmap = devm_regmap_init_mmio(&pdev->dev, mmio, &snvs_rtc_config);
    } else {
        data->offset = SNVS_LPREGISTER_OFFSET;
        of_property_read_u32(pdev->dev.of_node, "offset", &data->offset);
    }

    if (!data->regmap) {
        dev_err(&pdev->dev, "Can't find snvs syscon\n");
        return -ENODEV;
    }


    data->irq = platform_get_irq(pdev, 0);
    if (data->irq < 0)
        return data->irq;

    data->clk = devm_clk_get(&pdev->dev, "snvs-rtc");
    if (IS_ERR(data->clk)) {
        data->clk = NULL;
    } else {
        ret = clk_prepare_enable(data->clk);
        if (ret) {
            dev_err(&pdev->dev,
                "Could not prepare or enable the snvs clock\n");
            return ret;
        }
    }

    platform_set_drvdata(pdev, data);

    /* Initialize glitch detect */
    regmap_write(data->regmap, data->offset + SNVS_LPPGDR, SNVS_LPPGDR_INIT);

    /* Clear interrupt status */
    regmap_write(data->regmap, data->offset + SNVS_LPSR, 0xffffffff);

    /* Enable RTC */
    snvs_rtc_enable(data, true);

    device_init_wakeup(&pdev->dev, true);

    ret = devm_request_irq(&pdev->dev, data->irq, snvs_rtc_irq_handler,
                   IRQF_SHARED, "rtc alarm", &pdev->dev);
    if (ret) {
        dev_err(&pdev->dev, "failed to request irq %d: %d\n",
            data->irq, ret);
        goto error_rtc_device_register;
    }

    data->rtc = devm_rtc_device_register(&pdev->dev, pdev->name,
                    &snvs_rtc_ops, THIS_MODULE); // 设备注册
    if (IS_ERR(data->rtc)) {
        ret = PTR_ERR(data->rtc);
        dev_err(&pdev->dev, "failed to register rtc: %d\n", ret);
        goto error_rtc_device_register;
    }

    return 0;

error_rtc_device_register:
    if (data->clk)
        clk_disable_unprepare(data->clk);

    return ret;
}

step5：
先分析结构体

struct snvs_rtc_data {
    struct rtc_device *rtc;
    struct regmap *regmap;
    int offset;
    int irq;    // 中断号，初始化过程中，register_irq()
    struct clk *clk;
};

struct rtc_device
{
    struct device dev;
    struct module *owner;


    int id;
    char name[RTC_DEVICE_NAME_SIZE];   // 初始化过程中，肯定有个函数来对其进行赋值，猜测赋值 “rtc0”


    const struct rtc_class_ops *ops;    // file operations，行为核心
    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
};

struct rtc_class_ops {
    int (*open)(struct device *);    // 打开
    void (*release)(struct device *);    // 关闭
    int (*ioctl)(struct device *, unsigned int, unsigned long);    // ioctl
    int (*read_time)(struct device *, struct rtc_time *);    // 读时间
    int (*set_time)(struct device *, struct rtc_time *);    // 设置时间
    int (*read_alarm)(struct device *, struct rtc_wkalrm *);    // 读闹钟时间
    int (*set_alarm)(struct device *, struct rtc_wkalrm *);    // 设置闹钟时间
    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);
};

static const struct file_operations rtc_dev_fops = {
    .owner        = THIS_MODULE,
    .llseek        = no_llseek,
    .read        = rtc_dev_read,
    .poll        = rtc_dev_poll,
    .unlocked_ioctl    = rtc_dev_ioctl, // case RTC_RD_TIME: rtc->ops->read_time(rtc->dev.parent, tm);
    .open        = rtc_dev_open,        // rtc->ops->open()
    .release    = rtc_dev_release,      // rtc->ops->release()
    .fasync        = rtc_dev_fasync,
};

关键函数

data->rtc = devm_rtc_device_register(&pdev->dev, pdev->name,
                    &snvs_rtc_ops, THIS_MODULE); // 设备注册

devm_rtc_device_register(,&snvs_rtc_ops,) // 设备注册
        rtc_device_register()
                rtc_dev_prepare()
                        cdev_init(&rtc->char_dev, &rtc_dev_fops);

驱动初始化的核心是上面这个函数 devm_rtc_device_register()，这个函数的核心是挂载 snvs_rtc_ops 和 rtc_dev_fops 这两个 operations。
其中 rtc_dev_fops 为 /dev/rtc0 对应的 operations，即在应用层对 /dev/rtc0 执行 open() 系统调用，则最终会执行 rtc_dev_open()，其它函数类似。
而 snvs_rtc_ops 是作为 RTC 功能实现的函数，作为 rtc_dev_fops 的 RTC 功能支撑。比如，应用层调用 open() 系统调用后，会调 file_operations 的 rtc_dev_open() 函数，而在这个函数里面又会去调用 RTC 功能的 snvs_rtc_ops 函数。
总结就是，file_operations 看名字带个 file 就知道，操作 /dev/rtc 文件，就会最终调用其 hook 函数。
而在 file_operations 的各个 hook 函数中，我们想要实现的是 RTC 功能，所以就要借助 RTC 相关的功能函数来完成任务，这些 RTC 的功能函数就是 snvs_rtc_ops。

open() --> rtc_dev_open() --> rtc->ops->open()
read() --> rtc_dev_read()
ioctl(fd, RTC_RD_TIME,) --> rtc_dev_ioctl: case RTC_RD_TIME: rtc->ops->read_time(); --> read_time()
close() --> rtc_dev_release() --> rtc->ops->release()

应用程序

rtc_test.c

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

static const char default_rtc[] = "/dev/rtc0";

int main(int argc, char **argv)
{
	int i, fd, retval, irqcount = 0;
	unsigned long tmp, data;
	struct rtc_time rtc_tm;
	const char *rtc = default_rtc;

	switch (argc) {
	case 2:
		rtc = argv[1];
		/* FALLTHROUGH */
	case 1:
		break;
	default:
		fprintf(stderr, "usage:  rtctest [rtcdev]\n");
		return 1;
	}

	fd = open(rtc, O_RDONLY);

	if (fd == -1) {
		perror(rtc);
		exit(errno);
	}

	fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");

	/* Turn on update interrupts (one per second) */
	retval = ioctl(fd, RTC_UIE_ON, 0);
	if (retval == -1) {
		if (errno == ENOTTY) {
			fprintf(stderr, "\n...Update IRQs not supported.\n");
			goto test_READ;
		}
		perror("RTC_UIE_ON ioctl");
		exit(errno);
	}

	fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:\n", rtc);
	fflush(stderr);
	for (i = 1; i < 6; i++) {
		/* This read will block */
		retval = read(fd, &data, sizeof(unsigned long));
		if (retval == -1) {
			perror("read");
			exit(errno);
		}
		fprintf(stderr, "%d ", i);
		fflush(stderr);
		irqcount++;
	}

	fprintf(stderr, "\nAgain, from using select(2) on %s:\n", rtc);
	fflush(stderr);
	for (i = 1; i < 6; i++) {
		struct timeval tv = {5, 0}; /* 5 second timeout on select */
		fd_set readfds;

		FD_ZERO(&readfds);
		FD_SET(fd, &readfds);
		/* The select will wait until an RTC interrupt happens. */
		retval = select(fd + 1, &readfds, NULL, NULL, &tv);
		if (retval == -1) {
			perror("select");
			exit(errno);
		}
		/* This read won't block unlike the select-less case above. */
		retval = read(fd, &data, sizeof(unsigned long));
		if (retval == -1) {
			perror("read");
			exit(errno);
		}
		fprintf(stderr, "%d ", i);
		fflush(stderr);
		irqcount++;
	}

	/* Turn off update interrupts */
	retval = ioctl(fd, RTC_UIE_OFF, 0);
	if (retval == -1) {
		perror("RTC_UIE_OFF ioctl");
		exit(errno);
	}

test_READ:
	/* Read the RTC time/date */
	retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
	if (retval == -1) {
		perror("RTC_RD_TIME ioctl");
		exit(errno);
	}

	fprintf(stderr, "\n\nCurrent RTC date/time is :\n%d-%d-%d, %02d:%02d:%02d\n", rtc_tm.tm_mday,
			rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900, rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);

	/* Set the alarm to 5 sec in the future, and check for rollover */
	rtc_tm.tm_sec += 5;
	if (rtc_tm.tm_sec >= 60) {
		rtc_tm.tm_sec %= 60;
		rtc_tm.tm_min++;
	}
	if (rtc_tm.tm_min == 60) {
		rtc_tm.tm_min = 0;
		rtc_tm.tm_hour++;
	}
	if (rtc_tm.tm_hour == 24)
		rtc_tm.tm_hour = 0;

	retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
	if (retval == -1) {
		if (errno == ENOTTY) {
			fprintf(stderr, "\n...Alarm IRQs not supported.\n");
			goto test_PIE;
		}
		perror("RTC_ALM_SET ioctl");
		exit(errno);
	}

	/* Read the current alarm settings */
	retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
	if (retval == -1) {
		perror("RTC_ALM_READ ioctl");
		exit(errno);
	}

	fprintf(stderr, "Alarm time now set to :\n%02d:%02d:%02d\n", rtc_tm.tm_hour, rtc_tm.tm_min,
			rtc_tm.tm_sec);

	/* Enable alarm interrupts */
	retval = ioctl(fd, RTC_AIE_ON, 0);
	if (retval == -1) {
		perror("RTC_AIE_ON ioctl");
		exit(errno);
	}

	fprintf(stderr, "Waiting 5 seconds for alarm...\n");
	fflush(stderr);
	/* This blocks until the alarm ring causes an interrupt */
	retval = read(fd, &data, sizeof(unsigned long));
	if (retval == -1) {
		perror("read");
		exit(errno);
	}
	irqcount++;
	fprintf(stderr, "okay. Alarm rang.\n");

	/* Disable alarm interrupts */
	retval = ioctl(fd, RTC_AIE_OFF, 0);
	if (retval == -1) {
		perror("RTC_AIE_OFF ioctl");
		exit(errno);
	}

test_PIE:
	/* Read periodic IRQ rate */
	retval = ioctl(fd, RTC_IRQP_READ, &tmp);
	if (retval == -1) {
		/* not all RTCs support periodic IRQs */
		if (errno == ENOTTY) {
			fprintf(stderr, "\nNo periodic IRQ support\n");
			goto done;
		}
		perror("RTC_IRQP_READ ioctl");
		exit(errno);
	}
	fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);

	fprintf(stderr, "Counting 20 interrupts at:");
	fflush(stderr);

	/* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
	for (tmp = 2; tmp <= 64; tmp *= 2) {

		retval = ioctl(fd, RTC_IRQP_SET, tmp);
		if (retval == -1) {
			/* not all RTCs can change their periodic IRQ rate */
			if (errno == ENOTTY) {
				fprintf(stderr, "\n...Periodic IRQ rate is fixed\n");
				goto done;
			}
			perror("RTC_IRQP_SET ioctl");
			exit(errno);
		}

		fprintf(stderr, "\n%ldHz:\t", tmp);
		fflush(stderr);

		/* Enable periodic interrupts */
		retval = ioctl(fd, RTC_PIE_ON, 0);
		if (retval == -1) {
			perror("RTC_PIE_ON ioctl");
			exit(errno);
		}

		for (i = 1; i < 21; i++) {
			/* This blocks */
			retval = read(fd, &data, sizeof(unsigned long));
			if (retval == -1) {
				perror("read");
				exit(errno);
			}
			fprintf(stderr, " %d", i);
			fflush(stderr);
			irqcount++;
		}

		/* Disable periodic interrupts */
		retval = ioctl(fd, RTC_PIE_OFF, 0);
		if (retval == -1) {
			perror("RTC_PIE_OFF ioctl");
			exit(errno);
		}
	}

done:
	fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");

	close(fd);

	return 0;
}

Makefile

all:
	arm-linux-gnueabihf-gcc rtc_test.c -o rtc_test.out

install:
	# cp rtc_test.out ~/tftp/
	cp rtc_test.out ~/project/board/IMX6ULL/rootfs/home/root/

clean:
	rm *.out

主要讲下 ioctl(fd, RTC_UIE_ON, 0)，在应用程序调用 ioctl() 最终会执行驱动程序中的 rtc_dev_ioctl()。
内核中那么多 ioctl，怎么知道是要执行 rtc_dev_ioctl() 这个 ioctl 的呢，是根据 fd 匹配的。
在 rtc_dev_ioctl() 中解析 cmd，根据不同命令，执行不同操作，如读取时间、设置时间、中断使能和关闭等。

static long rtc_dev_ioctl(struct file *file,
		unsigned int cmd, unsigned long arg)
{
	int err = 0;
	struct rtc_device *rtc = file->private_data;
	const struct rtc_class_ops *ops = rtc->ops;
	struct rtc_time tm;
	struct rtc_wkalrm alarm;
	void __user *uarg = (void __user *) arg;

	err = mutex_lock_interruptible(&rtc->ops_lock);
	if (err)
		return err;

	switch (cmd) {
	case RTC_RD_TIME:
		mutex_unlock(&rtc->ops_lock);

		err = rtc_read_time(rtc, &tm);
		if (err < 0)
			return err;

		if (copy_to_user(uarg, &tm, sizeof(tm)))
			err = -EFAULT;
		return err;

	case RTC_SET_TIME:
		mutex_unlock(&rtc->ops_lock);

		if (copy_from_user(&tm, uarg, sizeof(tm)))
			return -EFAULT;

		return rtc_set_time(rtc, &tm);

	case RTC_UIE_ON:
		mutex_unlock(&rtc->ops_lock);
		return rtc_update_irq_enable(rtc, 1);

	case RTC_UIE_OFF:
		mutex_unlock(&rtc->ops_lock);
		return rtc_update_irq_enable(rtc, 0);

	default:
		/* Finally try the driver's ioctl interface */
		if (ops->ioctl) {
			err = ops->ioctl(rtc->dev.parent, cmd, arg);
			if (err == -ENOIOCTLCMD)
				err = -ENOTTY;
		} else
			err = -ENOTTY;
		break;
	}

done:
	mutex_unlock(&rtc->ops_lock);
	return err;
}

运行