详解linux 看门狗驱动编写

看门狗是linux驱动的一个重要环节。某些特殊的设备，有时候需要放在一些环境恶劣的地方，比如电信设备。但是，任何软件都不可能100%没有bug。如何保证软件在遇到严重bug、死机的时候也能正常运行呢，那么看门狗就是有效的一种方法。看门狗一般要求用户定时喂狗，如果一段时间没有喂狗的话，那么系统就会自动重启。今天，我们就来看看这个看门狗驱动怎么编写？

1、代码目录

drivers/watchdog 

2、阅读目录下的Kconfig，可以找一个s3c模块macro

config HAVE_S3C2410_WATCHDOG
  bool
  help
   This will include watchdog timer support for Samsung SoCs. If
   you want to include watchdog support for any machine, kindly
   select this in the respective mach-XXXX/Kconfig file. 

config S3C2410_WATCHDOG
  tristate "S3C2410 Watchdog"
  depends on HAVE_S3C2410_WATCHDOG || COMPILE_TEST
  select WATCHDOG_CORE
  select MFD_SYSCON if ARCH_EXYNOS
  help
   Watchdog timer block in the Samsung SoCs. This will reboot
   the system when the timer expires with the watchdog enabled. 

   The driver is limited by the speed of the system's PCLK
   signal, so with reasonably fast systems (PCLK around 50-66MHz)
   then watchdog intervals of over approximately 20seconds are
   unavailable. 

   The driver can be built as a module by choosing M, and will
   be called s3c2410_wdt

3、S3C2410_WATCHDOG主要依赖WATCHDOG_CORE，可以继续跟踪Makefile

obj-$(CONFIG_S3C2410_WATCHDOG) += s3c2410_wdt.o 

4、macro只依赖一个s3c2410_wdt.c文件，继续查看

static SIMPLE_DEV_PM_OPS(s3c2410wdt_pm_ops, s3c2410wdt_suspend,
      s3c2410wdt_resume); 

static struct platform_driver s3c2410wdt_driver = {
  .probe   = s3c2410wdt_probe,
  .remove   = s3c2410wdt_remove,
  .shutdown  = s3c2410wdt_shutdown,
  .id_table  = s3c2410_wdt_ids,
  .driver   = {
    .name  = "s3c2410-wdt",
    .pm = &s3c2410wdt_pm_ops,
    .of_match_table = of_match_ptr(s3c2410_wdt_match),
  },
}; 

module_platform_driver(s3c2410wdt_driver); 

5、确认driver为platform类型，继续在probe函数中查找有用的code

ret = watchdog_register_device(&wdt->wdt_device);
if (ret) {
  dev_err(dev, "cannot register watchdog (%d)\n", ret);
  goto err_cpufreq;
}

6、网上继续查找，寻找到和watchdog有关的数据结构

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

static const struct watchdog_ops s3c2410wdt_ops = {
  .owner = THIS_MODULE,
  .start = s3c2410wdt_start,
  .stop = s3c2410wdt_stop,
  .ping = s3c2410wdt_keepalive,
  .set_timeout = s3c2410wdt_set_heartbeat,
  .restart = s3c2410wdt_restart,
}; 

static const struct watchdog_device s3c2410_wdd = {
  .info = &s3c2410_wdt_ident,
  .ops = &s3c2410wdt_ops,
  .timeout = S3C2410_WATCHDOG_DEFAULT_TIME,
};

7、找到设备注册函数、函数结构基本就算结束了，当然有中断的话，也可以确认一下

ret = devm_request_irq(dev, wdt_irq->start, s3c2410wdt_irq, 0,
      pdev->name, pdev);
if (ret != 0) {
  dev_err(dev, "failed to install irq (%d)\n", ret);
  goto err_cpufreq;
}

8、有兴趣的话，可以找一个函数阅读一下。比如下面这个重启函数，可以和spec对比者来看

static int s3c2410wdt_restart(struct watchdog_device *wdd, unsigned long action,
         void *data)
{
  struct s3c2410_wdt *wdt = watchdog_get_drvdata(wdd);
  void __iomem *wdt_base = wdt->reg_base; 

  /* disable watchdog, to be safe */
  writel(0, wdt_base + S3C2410_WTCON); 

  /* put initial values into count and data */
  writel(0x80, wdt_base + S3C2410_WTCNT);
  writel(0x80, wdt_base + S3C2410_WTDAT); 

  /* set the watchdog to go and reset... */
  writel(S3C2410_WTCON_ENABLE | S3C2410_WTCON_DIV16 |
    S3C2410_WTCON_RSTEN | S3C2410_WTCON_PRESCALE(0x20),
    wdt_base + S3C2410_WTCON); 

  /* wait for reset to assert... */
  mdelay(500); 

  return 0;
}

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