Linux的WDT(watchdog)驱动

第一部分： WDT驱动原理
WDT在内核中通常都实现为misc驱动。
WDT介绍
一个Watchdog Timer（WDT）是一个在软件出错的时候可以复位计算机系统的硬件电路。
通常一个用户空间守护进程会在正常的时间间隔内通过/dev/watchdog特殊设备文件来通知内核的watchdog驱动，用户空间仍然正常。当这样的一个通知发生时，驱动通常会告诉硬件watchdog一切正常，然后watchdog应该再等待一段时间来复位系统。如果用户空间出问题（RAM错误，内核bug等），则通知将会停止，然后硬件watchdog将在超时后复位系统。
Linux的watchdog API是一个相当特别的东西，不同的驱动实现是不同的，而且有时部分是不兼容的。这个文档正是要尝试着去说明已经出现的用法，并且使以后的驱动作者把它作为一份参考。
最简单的 API:
所有的设备驱动都支持的基本的操作模式，一旦/dev/watchdog被打开，则watchdog激活，并且除非喂狗，否则将在一段时间之后重启，这个时间被称为timeout或margin。最简单的喂狗方法就是写一些数据到设备。一个非常简单的watchdog守护进程看起来就像这个文件这样：
Documentation/watchdog/src/watchdog-simple.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(void)
{
    int fd = open("/dev/watchdog", O_WRONLY);
    int ret = 0;
    if (fd == -1) {
        perror("watchdog");
        exit(EXIT_FAILURE);
    }
    while (1) {
        ret = write(fd, "\0", 1);
        if (ret != 1) {
            ret = -1;
            break;
        }
        ret = fsync(fd);
        if (ret)
            break;
        sleep(10);
    }
    close(fd);
    return ret;
}

一个高级一些的驱动在喂狗之前，可能还会做一些其他的事情，比如说检查HTTP服务器是否依然可以相应。
当设备关闭的时候，除非支持"Magic Close"特性。否则watchdog被关闭。这并不总是一个好主意，比如watchdog守护进程出现了bug并且崩溃了，则系统将不会重启。因此，某些驱动支持"Disable watchdog shutdown on close", CONFIG_WATCHDOG_NOWAYOUT配置选项。当编译内核的时候这个选项被设置为Y，则一旦watchdog被启动，则将没有办法能够停止。这样，则当watchdog守护进程崩溃的时候，系统仍将在超时后重启。Watchdog设备常常也支持nowayout模块参数，这样这个选项就可以在运行时进行控制。
Magic Close 特性:
如果一个驱动支持"Magic Close"，则除非在关闭文件前，魔幻字符'V'被发送到/dev/watchdog，驱动将不停止watchdog。如果用户空间守护进程在关闭文件前没有发送这个字符，则驱动认为用户空间崩溃，并在关闭watchdog前停止喂狗。
这样的话，如果没有在一定的时间内重新打开watchdog，则将导致一个重启。
ioctl API:
所有标准的驱动也应该支持一个ioctl API。
喂狗使用一个ioctl：
所有的驱动都有一个ioctl接口支持至少一个ioctl命令，KEEPALIVE。这个 ioctl 做的事和一个写watchdog设备完全一样，所以，上面程序的主循环可以替换为：
while (1) {

      ioctl(fd, WDIOC_KEEPALIVE, 0);

      sleep(10);

    }

ioctl的参数被忽略。
设置和获得超时值：
对于某些驱动来说，在上层使用SETTIMEOUT ioctl命令改变watchdog的超时值是可能的，那些驱动在他们的选项与中有WDIOF_SETTIMEOUT标志。参数是一个代表以秒为单位的超时值，驱动将在同一个变量中返回实际使用的超时值，这个超时值可能由于硬件的限制，而不同于所请求的超时值
    int timeout = 45;
    ioctl(fd, WDIOC_SETTIMEOUT, &timeout);
    printf("The timeout was set to %d seconds\n", timeout);
如果设备的超时值的粒度只能到分钟，则这个例子可能实际打印"The timeout was set to 60 seconds"。
自从Linux 2.4.18内核，通过GETTIMEOUT ioctl命令查询当前超时值也是可能的：
    ioctl(fd, WDIOC_GETTIMEOUT, &timeout);
    printf("The timeout was is %d seconds\n", timeout);
预处理：
Pretimeouts:
一些watchdog定时器，可以被设置为，在他们实际复位系统前，有一个触发。这可能通过一个NMI，中断，或其他机制。这将允许在它复位系统前Linux去记录一些有用的信息（比如panic信息和内核转储）。
    pretimeout = 10;
    ioctl(fd, WDIOC_SETPRETIMEOUT, &pretimeout);
注意，预超时值应该是一个相对于超时值提前的秒数。而不是直到预超时的秒数。
比如，如果你设置超时值为60秒，预超时值为10秒，那么预超时将在50秒后到达。设置为0则是禁用它。预超时还有一个get功能：
    ioctl(fd, WDIOC_GETPRETIMEOUT, &timeout);
    printf("The pretimeout was is %d seconds\n", timeout);
不是所有的watchdog驱动都支持一个预超时的。
获得重启前的秒数
一些watchdog驱动有一个报告在重启前的剩余时间的功能。WDIOC_GETTIMELEFT就是返回重启前的秒数的ioctl命令。
    ioctl(fd, WDIOC_GETTIMELEFT, &timeleft);
    printf("The timeout was is %d seconds\n", timeleft);
环境监视：
Environmental monitoring:
所有的watchdog驱动都被要求返回更多关于系统的信息，有些返回温度，风扇和功率水平监测，依稀可以告诉你上一次重启系统的原因。GETSUPPORT ioctl可以用来查询设备可以做什么：
    struct watchdog_info ident;
    ioctl(fd, WDIOC_GETSUPPORT, &ident);
ident结构中返回的字段是：
        identity    一个标识watchdog驱动的字符串
    firmware_version 如果可用的话，就是卡的固件版本
    options          一个描述设备支持什么的标志
options字段可以有下面的位集，和描述GET_STATUS 和 GET_BOOT_STATUS ioctls可以返回什么种类的信息。
第二部分： WDT驱动源码
驱动架构比较简单，由于kernel启动时，定义并加入了watchdog的platform_device,所以驱动定义并注册watchdog 的platform_driver
/* linux/drivers/char/watchdog/s3c2410_wdt.c
 *
 * Copyright (c) 2004 Simtec Electronics
 * Ben Dooks <[email protected]>
 *
 * S3C2410 Watchdog Timer Support
 *
 * Based on, softdog.c by Alan Cox,
 * (c) Copyright 1996 Alan Cox <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/

#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/types.h>
#include <linux/timer.h>
#include <linux/miscdevice.h>
#include <linux/watchdog.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/platform_device.h>
#include <linux/interrupt.h>
#include <linux/clk.h>
#include <linux/uaccess.h>
#include <linux/io.h>

#include <mach/map.h>

#undef S3C_VA_WATCHDOG
#define S3C_VA_WATCHDOG (0)

#include <plat/regs-watchdog.h>

#define PFX "s3c2410-wdt: "

#define CONFIG_S3C2410_WATCHDOG_ATBOOT (0)
#define CONFIG_S3C2410_WATCHDOG_DEFAULT_TIME (15)

static int nowayout = WATCHDOG_NOWAYOUT;
static int tmr_margin = CONFIG_S3C2410_WATCHDOG_DEFAULT_TIME;
static int tmr_atboot = CONFIG_S3C2410_WATCHDOG_ATBOOT;
static int soft_noboot;
static int debug;

module_param(tmr_margin, int, 0);
module_param(tmr_atboot, int, 0);
module_param(nowayout, int, 0);
module_param(soft_noboot, int, 0);
module_param(debug, int, 0);

MODULE_PARM_DESC(tmr_margin, "Watchdog tmr_margin in seconds. default="
        __MODULE_STRING(CONFIG_S3C2410_WATCHDOG_DEFAULT_TIME) ")");
MODULE_PARM_DESC(tmr_atboot,
        "Watchdog is started at boot time if set to 1, default="
            __MODULE_STRING(CONFIG_S3C2410_WATCHDOG_ATBOOT));
MODULE_PARM_DESC(nowayout, "Watchdog cannot be stopped once started (default="
            __MODULE_STRING(WATCHDOG_NOWAYOUT) ")");
MODULE_PARM_DESC(soft_noboot, "Watchdog action, set to 1 to ignore reboots, "
            "0 to reboot (default depends on ONLY_TESTING)");
MODULE_PARM_DESC(debug, "Watchdog debug, set to >1 for debug, (default 0)");

static unsigned long open_lock;
static struct device *wdt_dev; /* platform device attached to */
static struct resource *wdt_mem;
static struct resource *wdt_irq;
static struct clk *wdt_clock;
static void __iomem *wdt_base;
static unsigned int wdt_count;
static char expect_close;
static DEFINE_SPINLOCK(wdt_lock);

/* watchdog control routines */

#define DBG(msg...) do { \
    if (debug) \
        printk(KERN_INFO msg); \
    } while (0)
/* functions */

static void s3c2410wdt_keepalive(void)
{
    spin_lock(&wdt_lock);
    writel(wdt_count, wdt_base + S3C2410_WTCNT);
    spin_unlock(&wdt_lock);
}

static void __s3c2410wdt_stop(void)
{
    unsigned long wtcon;

    wtcon = readl(wdt_base + S3C2410_WTCON);
    wtcon &= ~(S3C2410_WTCON_ENABLE | S3C2410_WTCON_RSTEN);
    writel(wtcon, wdt_base + S3C2410_WTCON);
}

static void s3c2410wdt_stop(void)
{
    spin_lock(&wdt_lock);
    __s3c2410wdt_stop();
    spin_unlock(&wdt_lock);
}

static void s3c2410wdt_start(void)
{
    unsigned long wtcon;

    spin_lock(&wdt_lock);

    __s3c2410wdt_stop();

    wtcon = readl(wdt_base + S3C2410_WTCON);
    wtcon |= S3C2410_WTCON_ENABLE | S3C2410_WTCON_DIV128;

    if (soft_noboot) {
        wtcon |= S3C2410_WTCON_INTEN;
        wtcon &= ~S3C2410_WTCON_RSTEN;
    } else {
        wtcon &= ~S3C2410_WTCON_INTEN;
        wtcon |= S3C2410_WTCON_RSTEN;
    }

    DBG("%s: wdt_count=0x%08x, wtcon=%08lx\n",
        __func__, wdt_count, wtcon);

    writel(wdt_count, wdt_base + S3C2410_WTDAT);
    writel(wdt_count, wdt_base + S3C2410_WTCNT);
    writel(wtcon, wdt_base + S3C2410_WTCON);
    spin_unlock(&wdt_lock);
}

static int s3c2410wdt_set_heartbeat(int timeout)
{
    unsigned int freq = clk_get_rate(wdt_clock);
    unsigned int count;
    unsigned int divisor = 1;
    unsigned long wtcon;
    if (timeout < 1)
        return -EINVAL;

    freq /= 128;
    count = timeout * freq;

    DBG("%s: count=%d, timeout=%d, freq=%d\n",
        __func__, count, timeout, freq);

    /* if the count is bigger than the watchdog register,
      then work out what we need to do (and if) we can
      actually make this value
    */

    if (count >= 0x10000) {
        for (divisor = 1; divisor <= 0x100; divisor++) {
            if ((count / divisor) < 0x10000)
                break;
        }

        if ((count / divisor) >= 0x10000) {
            dev_err(wdt_dev, "timeout %d too big\n", timeout);
            return -EINVAL;
        }
    }

    tmr_margin = timeout;

    DBG("%s: timeout=%d, divisor=%d, count=%d (%08x)\n",
        __func__, timeout, divisor, count, count/divisor);

    count /= divisor;
    wdt_count = count;

    /* update the pre-scaler */
    wtcon = readl(wdt_base + S3C2410_WTCON);
    wtcon &= ~S3C2410_WTCON_PRESCALE_MASK;
    wtcon |= S3C2410_WTCON_PRESCALE(divisor-1);

    writel(count, wdt_base + S3C2410_WTDAT);
    writel(wtcon, wdt_base + S3C2410_WTCON);

    return 0;
}

/*
 * /dev/watchdog handling
 */

static int s3c2410wdt_open(struct inode *inode, struct file *file)
{
    if (test_and_set_bit(0, &open_lock))
        return -EBUSY;

    if (nowayout)
        __module_get(THIS_MODULE);

    expect_close = 0;

    /* start the timer */
    s3c2410wdt_start();
    return nonseekable_open(inode, file);
}

static int s3c2410wdt_release(struct inode *inode, struct file *file)
{
    /*
    * Shut off the timer.
    * Lock it in if it's a module and we set nowayout
    */

    if (expect_close == 42)
        s3c2410wdt_stop();
    else {
        dev_err(wdt_dev, "Unexpected close, not stopping watchdog\n");
        s3c2410wdt_keepalive();
    }
    expect_close = 0;
    clear_bit(0, &open_lock);
    return 0;
}

static ssize_t s3c2410wdt_write(struct file *file, const char __user *data,
                size_t len, loff_t *ppos)
{
    /*
    * Refresh the timer.
    */
    if (len) {
        if (!nowayout) {
            size_t i;

            /* In case it was set long ago */
            expect_close = 0;

            for (i = 0; i != len; i++) {
                char c;

                if (get_user(c, data + i))
                    return -EFAULT;
                if (c == 'V')
                    expect_close = 42;
            }
        }
        s3c2410wdt_keepalive();
    }
    return len;
}

#define OPTIONS (WDIOF_SETTIMEOUT | WDIOF_KEEPALIVEPING | WDIOF_MAGICCLOSE)

static const struct watchdog_info s3c2410_wdt_ident = {
    .options = OPTIONS,
    .firmware_version = 0,
    .identity = "S3C2410 Watchdog",
};

static long s3c2410wdt_ioctl(struct file *file, unsigned int cmd,
                            unsigned long arg)
{
    void __user *argp = (void __user *)arg;
    int __user *p = argp;
    int new_margin;

    switch (cmd) {
    case WDIOC_GETSUPPORT:
        return copy_to_user(argp, &s3c2410_wdt_ident,
            sizeof(s3c2410_wdt_ident)) ? -EFAULT : 0;
    case WDIOC_GETSTATUS:
    case WDIOC_GETBOOTSTATUS:
        return put_user(0, p);
    case WDIOC_KEEPALIVE:
        s3c2410wdt_keepalive();
        return 0;
    case WDIOC_SETTIMEOUT:
        if (get_user(new_margin, p))
            return -EFAULT;
        if (s3c2410wdt_set_heartbeat(new_margin))
            return -EINVAL;
        s3c2410wdt_keepalive();
        return put_user(tmr_margin, p);
    case WDIOC_GETTIMEOUT:
        return put_user(tmr_margin, p);
    default:
        return -ENOTTY;
    }
}

/* kernel interface */

static const struct file_operations s3c2410wdt_fops = {
    .owner = THIS_MODULE,
    .llseek = no_llseek,
    .write = s3c2410wdt_write,
    .unlocked_ioctl = s3c2410wdt_ioctl,
    .open = s3c2410wdt_open,
    .release = s3c2410wdt_release,
};

static struct miscdevice s3c2410wdt_miscdev = {
    .minor = WATCHDOG_MINOR,
    .name = "watchdog",
    .fops = &s3c2410wdt_fops,
};

/* interrupt handler code */

static irqreturn_t s3c2410wdt_irq(int irqno, void *param)
{
    dev_info(wdt_dev, "watchdog timer expired (irq)\n");

    s3c2410wdt_keepalive();
    return IRQ_HANDLED;
}
/* device interface */

static int __devinit s3c2410wdt_probe(struct platform_device *pdev)
{
    struct resource *res;
    struct device *dev;
    unsigned int wtcon;
    int started = 0;
    int ret;
    int size;

    DBG("%s: probe=%p\n", __func__, pdev);

    dev = &pdev->dev;
    wdt_dev = &pdev->dev;

    /* get the memory region for the watchdog timer -- flags is IORESOURCE_MEM */
    res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
    if (res == NULL) {
        dev_err(dev, "no memory resource specified\n");
        return -ENOENT;
    }

    size = (res->end - res->start) + 1;

    //请求分配指定的I/O内存资源
    wdt_mem = request_mem_region(res->start, size, pdev->name);
    if (wdt_mem == NULL) {
        dev_err(dev, "failed to get memory region\n");
        ret = -ENOENT;
        goto err_req;
    }

    //将一个IO地址空间映射到内核的虚拟地址空间上去，便于访问
    wdt_base = ioremap(res->start, size);
    if (wdt_base == NULL) {
        dev_err(dev, "failed to ioremap() region\n");
        ret = -EINVAL;
        goto err_req;
    }

    DBG("probe: mapped wdt_base=%p\n", wdt_base);

    /* get the memory region for the watchdog timer -- flags is IORESOURCE_IRQ */
    wdt_irq = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
    if (wdt_irq == NULL) {
        dev_err(dev, "no irq resource specified\n");
        ret = -ENOENT;
        goto err_map;
    }

    //注册中断服务函数s3c2410wdt_irq()
    ret = request_irq(wdt_irq->start, s3c2410wdt_irq, 0, pdev->name, pdev);
    if (ret != 0) {
        dev_err(dev, "failed to install irq (%d)\n", ret);
        goto err_map;
    }

    //从平台时钟队列中获取clk
    wdt_clock = clk_get(&pdev->dev, "watchdog");
    if (IS_ERR(wdt_clock)) {
        dev_err(dev, "failed to find watchdog clock source\n");
        ret = PTR_ERR(wdt_clock);
        goto err_irq;
    }

    //inform the system when the clock source should be running
    clk_enable(wdt_clock);

    /* see if we can actually set the requested timer margin, and if
    * not, try the default value */

    if (s3c2410wdt_set_heartbeat(tmr_margin)) {
        started = s3c2410wdt_set_heartbeat(
                    CONFIG_S3C2410_WATCHDOG_DEFAULT_TIME);

        if (started == 0)
            dev_info(dev,
              "tmr_margin value out of range, default %d used\n",
                  CONFIG_S3C2410_WATCHDOG_DEFAULT_TIME);
        else
            dev_info(dev, "default timer value is out of range, "
                            "cannot start\n");
    }

    ret = misc_register(&s3c2410wdt_miscdev);
    if (ret) {
        dev_err(dev, "cannot register miscdev on minor=%d (%d)\n",
            WATCHDOG_MINOR, ret);
        goto err_clk;
    }

    if (tmr_atboot && started == 0) {
        dev_info(dev, "starting watchdog timer\n");
        s3c2410wdt_start();
    } else if (!tmr_atboot) {
        /* if we're not enabling the watchdog, then ensure it is
        * disabled if it has been left running from the bootloader
        * or other source */

        s3c2410wdt_stop();
    }

    /* print out a statement of readiness */

    wtcon = readl(wdt_base + S3C2410_WTCON);

    dev_info(dev, "watchdog %sactive, reset %sabled, irq %sabled\n",
        (wtcon & S3C2410_WTCON_ENABLE) ? "" : "in",
        (wtcon & S3C2410_WTCON_RSTEN) ? "" : "dis",
        (wtcon & S3C2410_WTCON_INTEN) ? "" : "en");

    return 0;

 err_clk:
    clk_disable(wdt_clock);
    clk_put(wdt_clock);

 err_irq:
    free_irq(wdt_irq->start, pdev);

 err_map:
    iounmap(wdt_base);

 err_req:
    release_resource(wdt_mem);
    kfree(wdt_mem);

    return ret;
}

static int __devexit s3c2410wdt_remove(struct platform_device *dev)
{
    release_resource(wdt_mem);
    kfree(wdt_mem);
    wdt_mem = NULL;

    free_irq(wdt_irq->start, dev);
    wdt_irq = NULL;

    clk_disable(wdt_clock);
    clk_put(wdt_clock);
    wdt_clock = NULL;

    iounmap(wdt_base);
    misc_deregister(&s3c2410wdt_miscdev);
    return 0;
}

static void s3c2410wdt_shutdown(struct platform_device *dev)
{
    s3c2410wdt_stop();
}

#ifdef CONFIG_PM

static unsigned long wtcon_save;
static unsigned long wtdat_save;

static int s3c2410wdt_suspend(struct platform_device *dev, pm_message_t state)
{
    /* Save watchdog state, and turn it off. */
    wtcon_save = readl(wdt_base + S3C2410_WTCON);
    wtdat_save = readl(wdt_base + S3C2410_WTDAT);

    /* Note that WTCNT doesn't need to be saved. */
    s3c2410wdt_stop();

    return 0;
}

static int s3c2410wdt_resume(struct platform_device *dev)
{
    /* Restore watchdog state. */

    writel(wtdat_save, wdt_base + S3C2410_WTDAT);
    writel(wtdat_save, wdt_base + S3C2410_WTCNT); /* Reset count */
    writel(wtcon_save, wdt_base + S3C2410_WTCON);

    printk(KERN_INFO PFX "watchdog %sabled\n",
          (wtcon_save & S3C2410_WTCON_ENABLE) ? "en" : "dis");

    return 0;
}
#else
#define s3c2410wdt_suspend NULL
#define s3c2410wdt_resume NULL
#endif /* CONFIG_PM */

/*
 *platform_driver s3c2410wdt_driver 与 platform_device s3c_device_wdt 对应
 *s3c_device_wdt 在arch/arm/plat-s3c24xx/devs.c中定义
 *两者的工作顺序是先定义platform_device -> 注册 platform_device->
 *在mini2440_machine_init()中完成
 *再定义 platform_driver-> 注册 platform_driver
 */
static struct platform_driver s3c2410wdt_driver = {
    .probe = s3c2410wdt_probe, //设备的检测，所以需要先注册设备
    .remove = __devexit_p(s3c2410wdt_remove), //删除该设备
    .shutdown = s3c2410wdt_shutdown, //关闭该设备
    .suspend = s3c2410wdt_suspend,
    .resume = s3c2410wdt_resume,
    .driver = { //设备驱动
        .owner = THIS_MODULE,
        /*
        *对应 struct platform_device s3c_device_wdt = {
        *    .name        = "s3c2410-wdt",
        *      ...
        *    };
        */
        .name = "s3c2410-wdt",
    },
};

static char banner[] __initdata =
    KERN_INFO "S3C2410 Watchdog Timer, (c) 2004 Simtec Electronics\n";

static int __init watchdog_init(void) //模块初始化
{
    printk(banner); //打印信息
    return platform_driver_register(&s3c2410wdt_driver); //注册设备的驱动程序
}

static void __exit watchdog_exit(void) //移除模块
{
    platform_driver_unregister(&s3c2410wdt_driver); //unregister a driver for platform-level devices
}

module_init(watchdog_init);
module_exit(watchdog_exit);

MODULE_AUTHOR("Ben Dooks , "
          "Dimitry Andric ");
MODULE_DESCRIPTION("S3C2410 Watchdog Device Driver");
MODULE_LICENSE("GPL");
MODULE_ALIAS_MISCDEV(WATCHDOG_MINOR);
MODULE_ALIAS("platform:s3c2410-wdt");