[PATCH v3 1/2] watchdog_core: Add watchdog registration deferral mechanism

[PATCH v3 1/2] watchdog_core: Add watchdog registration deferral mechanism

[Jean-Baptiste Theou]

Currently, watchdog subsystem require the misc subsystem to
register a watchdog. This may not be the case in case of an
early registration of a watchdog, which can be required when
the watchdog cannot be disabled.

This patch introduces a deferral mechanism to remove this requirement.

Signed-off-by: Jean-Baptiste Theou <jtheou@adeneo-embedded.us>
---
Changes in v3:
 - Adjust patch based on Guenter feedbacks
 - Fix watchdog_deferred_registration_del
 - Fix typos

 Documentation/watchdog/watchdog-kernel-api.txt |   7 ++
 drivers/watchdog/watchdog_core.c               | 114 +++++++++++++++++++++----
 include/linux/watchdog.h                       |   3 +
 3 files changed, 106 insertions(+), 18 deletions(-)

diff --git a/Documentation/watchdog/watchdog-kernel-api.txt b/Documentation/watchdog/watchdog-kernel-api.txt
index a0438f3..e83518c 100644
--- a/Documentation/watchdog/watchdog-kernel-api.txt
+++ b/Documentation/watchdog/watchdog-kernel-api.txt
@@ -36,6 +36,10 @@ The watchdog_unregister_device routine deregisters a registered watchdog timer
 device. The parameter of this routine is the pointer to the registered
 watchdog_device structure.
 
+The watchdog subsystem includes an registration deferral mechanism,
+which allows you to register an watchdog as early as you wish during
+the boot process.
+
 The watchdog device structure looks like this:
 
 struct watchdog_device {
@@ -52,6 +56,7 @@ struct watchdog_device {
 	void *driver_data;
 	struct mutex lock;
 	unsigned long status;
+	struct list_head deferred_registration;
 };
 
 It contains following fields:
@@ -80,6 +85,8 @@ It contains following fields:
   information about the status of the device (Like: is the watchdog timer
   running/active, is the nowayout bit set, is the device opened via
   the /dev/watchdog interface or not, ...).
+* deferred_registration: entry in deferred_registration_list which is used to
+  register early initialized watchdogs.
 
 The list of watchdog operations is defined as:
 
diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
index cec9b55..7dba8d9 100644
--- a/drivers/watchdog/watchdog_core.c
+++ b/drivers/watchdog/watchdog_core.c
@@ -43,6 +43,43 @@
 static DEFINE_IDA(watchdog_ida);
 static struct class *watchdog_class;
 
+/*
+ * Deferred Registration infrastructure.
+ *
+ * Sometimes watchdog drivers needs to be loaded as soon as possible,
+ * for example when it's impossible to disable it. To do so,
+ * raising the initcall level of the watchdog driver is a solution.
+ * But in such case, the miscdev is maybe not ready (subsys_initcall), and
+ * watchdog_core need miscdev to register the watchdog as a char device.
+ *
+ * The deferred registration infrastructure offer a way for the watchdog
+ * subsystem to register a watchdog properly, even before miscdev is ready.
+ */
+
+static DEFINE_MUTEX(watchdog_deferred_registration_mutex);
+static LIST_HEAD(watchdog_deferred_registration_pending_list);
+static bool watchdog_deferred_registration_done;
+
+static int watchdog_deferred_registration_add(struct watchdog_device *wdd)
+{
+	list_add_tail(&wdd->deferred_registration,
+		 &watchdog_deferred_registration_pending_list);
+	return 0;
+}
+
+static void watchdog_deferred_registration_del(struct watchdog_device *wdd)
+{
+	struct list_head *p, *n;
+	struct watchdog_device *wdd_tmp;
+
+	list_for_each_safe(p, n, &watchdog_deferred_registration_pending_list) {
+		wdd_tmp = list_entry(p, struct watchdog_device,
+				     deferred_registration);
+		if (wdd_tmp == wdd)
+			list_del(&wdd_tmp->deferred_registration);
+	}
+}
+
 static void watchdog_check_min_max_timeout(struct watchdog_device *wdd)
 {
 	/*
@@ -98,17 +135,7 @@ int watchdog_init_timeout(struct watchdog_device *wdd,
 }
 EXPORT_SYMBOL_GPL(watchdog_init_timeout);
 
-/**
- * watchdog_register_device() - register a watchdog device
- * @wdd: watchdog device
- *
- * Register a watchdog device with the kernel so that the
- * watchdog timer can be accessed from userspace.
- *
- * A zero is returned on success and a negative errno code for
- * failure.
- */
-int watchdog_register_device(struct watchdog_device *wdd)
+static int __watchdog_register_device(struct watchdog_device *wdd)
 {
 	int ret, id, devno;
 
@@ -164,16 +191,33 @@ int watchdog_register_device(struct watchdog_device *wdd)
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(watchdog_register_device);
 
 /**
- * watchdog_unregister_device() - unregister a watchdog device
- * @wdd: watchdog device to unregister
+ * watchdog_register_device() - register a watchdog device
+ * @wdd: watchdog device
  *
- * Unregister a watchdog device that was previously successfully
- * registered with watchdog_register_device().
+ * Register a watchdog device with the kernel so that the
+ * watchdog timer can be accessed from userspace.
+ *
+ * A zero is returned on success and a negative errno code for
+ * failure.
  */
-void watchdog_unregister_device(struct watchdog_device *wdd)
+
+int watchdog_register_device(struct watchdog_device *wdd)
+{
+	int ret;
+
+	mutex_lock(&watchdog_deferred_registration_mutex);
+	if (watchdog_deferred_registration_done)
+		ret = __watchdog_register_device(wdd);
+	else
+		ret = watchdog_deferred_registration_add(wdd);
+	mutex_unlock(&watchdog_deferred_registration_mutex);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(watchdog_register_device);
+
+static void __watchdog_unregister_device(struct watchdog_device *wdd)
 {
 	int ret;
 	int devno;
@@ -189,8 +233,41 @@ void watchdog_unregister_device(struct watchdog_device *wdd)
 	ida_simple_remove(&watchdog_ida, wdd->id);
 	wdd->dev = NULL;
 }
+
+/**
+ * watchdog_unregister_device() - unregister a watchdog device
+ * @wdd: watchdog device to unregister
+ *
+ * Unregister a watchdog device that was previously successfully
+ * registered with watchdog_register_device().
+ */
+
+void watchdog_unregister_device(struct watchdog_device *wdd)
+{
+	mutex_lock(&watchdog_deferred_registration_mutex);
+	if (watchdog_deferred_registration_done)
+		__watchdog_unregister_device(wdd);
+	else
+		watchdog_deferred_registration_del(wdd);
+	mutex_unlock(&watchdog_deferred_registration_mutex);
+}
+
 EXPORT_SYMBOL_GPL(watchdog_unregister_device);
 
+static int __init watchdog_deferred_registration(void)
+{
+	mutex_lock(&watchdog_deferred_registration_mutex);
+	watchdog_deferred_registration_done = true;
+	while (!list_empty(&watchdog_deferred_registration_pending_list)) {
+		struct watchdog_device *wdd = list_first_entry(&watchdog_deferred_registration_pending_list,
+							       struct watchdog_device, deferred_registration);
+		list_del(&wdd->deferred_registration);
+		__watchdog_register_device(wdd);
+	}
+	mutex_unlock(&watchdog_deferred_registration_mutex);
+	return 0;
+}
+
 static int __init watchdog_init(void)
 {
 	int err;
@@ -207,6 +284,7 @@ static int __init watchdog_init(void)
 		return err;
 	}
 
+	watchdog_deferred_registration();
 	return 0;
 }
 
@@ -217,7 +295,7 @@ static void __exit watchdog_exit(void)
 	ida_destroy(&watchdog_ida);
 }
 
-subsys_initcall(watchdog_init);
+subsys_initcall_sync(watchdog_init);
 module_exit(watchdog_exit);
 
 MODULE_AUTHOR("Alan Cox <alan@lxorguk.ukuu.org.uk>");
diff --git a/include/linux/watchdog.h b/include/linux/watchdog.h
index a746bf5..f9bb943 100644
--- a/include/linux/watchdog.h
+++ b/include/linux/watchdog.h
@@ -65,6 +65,8 @@ struct watchdog_ops {
  * @driver-data:Pointer to the drivers private data.
  * @lock:	Lock for watchdog core internal use only.
  * @status:	Field that contains the devices internal status bits.
+ * @deferred_registration: entry in deferred_registration_list which is used to
+ *			   register early initialized watchdogs.
  *
  * The watchdog_device structure contains all information about a
  * watchdog timer device.
@@ -95,6 +97,7 @@ struct watchdog_device {
 #define WDOG_ALLOW_RELEASE	2	/* Did we receive the magic char ? */
 #define WDOG_NO_WAY_OUT		3	/* Is 'nowayout' feature set ? */
 #define WDOG_UNREGISTERED	4	/* Has the device been unregistered */
+	struct list_head deferred_registration;
 };
 
 #define WATCHDOG_NOWAYOUT		IS_BUILTIN(CONFIG_WATCHDOG_NOWAYOUT)
-- 
2.4.2

[PATCH v3 2/2] gpio_wdt: Add option for early registration

[Jean-Baptiste Theou]

In some situation, mainly when it's not possible to disable a
watchdog, you may want the watchdog driver to be started as soon
as possible.

Adding GPIO_WATCHDOG_ARCH_INITCALL to raise initcall from
module_init to arch_initcall.

This patch require watchdog registration deferral mechanism

Signed-off-by: Jean-Baptiste Theou <jtheou@adeneo-embedded.us>
---
Changes in v3:
 - Simplify changes, since the modification is for built-in only

 drivers/watchdog/Kconfig    | 12 ++++++++++++
 drivers/watchdog/gpio_wdt.c |  8 ++++++++
 2 files changed, 20 insertions(+)

diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
index e5e7c55..7a5c912 100644
--- a/drivers/watchdog/Kconfig
+++ b/drivers/watchdog/Kconfig
@@ -1,3 +1,4 @@
+
 #
 # Watchdog device configuration
 #
@@ -104,6 +105,17 @@ config GPIO_WATCHDOG
 	  If you say yes here you get support for watchdog device
 	  controlled through GPIO-line.
 
+config GPIO_WATCHDOG_ARCH_INITCALL
+	bool "Register the watchdog as early as possible"
+	depends on GPIO_WATCHDOG=y
+	help
+	  In some situations, the default initcall level (module_init)
+	  in not early enough in the boot process to avoid the watchdog
+	  to be triggered.
+	  If you say yes here, the initcall level would be raised to
+	  arch_initcall.
+	  If in doubt, say N.
+
 config MENF21BMC_WATCHDOG
 	tristate "MEN 14F021P00 BMC Watchdog"
 	depends on MFD_MENF21BMC
diff --git a/drivers/watchdog/gpio_wdt.c b/drivers/watchdog/gpio_wdt.c
index cbc313d..f9149a2 100644
--- a/drivers/watchdog/gpio_wdt.c
+++ b/drivers/watchdog/gpio_wdt.c
@@ -267,7 +267,15 @@ static struct platform_driver gpio_wdt_driver = {
 	.probe	= gpio_wdt_probe,
 	.remove	= gpio_wdt_remove,
 };
+#ifdef GPIO_WATCHDOG_ARCH_INITCALL
+static int __init gpio_wdt_init(void)
+{
+	return platform_driver_register(&gpio_wdt_driver);
+}
+arch_initcall(gpio_wdt_init);
+#else
 module_platform_driver(gpio_wdt_driver);
+#endif
 
 MODULE_AUTHOR("Alexander Shiyan <shc_work@mail.ru>");
 MODULE_DESCRIPTION("GPIO Watchdog");
-- 
2.4.2

Re: [PATCH v3 2/2] gpio_wdt: Add option for early registration

[Paul Bolle]

On Sat, 2015-06-06 at 00:46 -0700, Jean-Baptiste Theou wrote:
> --- a/drivers/watchdog/Kconfig
> +++ b/drivers/watchdog/Kconfig

> +config GPIO_WATCHDOG_ARCH_INITCALL
> +	bool "Register the watchdog as early as possible"
> +	depends on GPIO_WATCHDOG=y
> +	help
> +	  In some situations, the default initcall level (module_init)
> +	  in not early enough in the boot process to avoid the watchdog
> +	  to be triggered.
> +	  If you say yes here, the initcall level would be raised to
> +	  arch_initcall.
> +	  If in doubt, say N.

> --- a/drivers/watchdog/gpio_wdt.c
> +++ b/drivers/watchdog/gpio_wdt.c

> +#ifdef GPIO_WATCHDOG_ARCH_INITCALL

You meant
    #ifdef CONFIG_GPIO_WATCHDOG_ARCH_INITCALL

Please use scripts/checkkconfigsymbols.py.

> +static int __init gpio_wdt_init(void)
> +{
> +	return platform_driver_register(&gpio_wdt_driver);
> +}
> +arch_initcall(gpio_wdt_init);
> +#else
>  module_platform_driver(gpio_wdt_driver);
> +#endif

The entire patch is basically an elaborate NOP. How did this pass your
testing?


Paul Bolle

Re: [PATCH v3 2/2] gpio_wdt: Add option for early registration

[Valentin Rothberg]

On Sat, Jun 06, 2015 at 12:03:06PM +0200, Paul Bolle wrote:
> On Sat, 2015-06-06 at 00:46 -0700, Jean-Baptiste Theou wrote:
> > --- a/drivers/watchdog/Kconfig
> > +++ b/drivers/watchdog/Kconfig
> 
> > +config GPIO_WATCHDOG_ARCH_INITCALL
> > +	bool "Register the watchdog as early as possible"
> > +	depends on GPIO_WATCHDOG=y
> > +	help
> > +	  In some situations, the default initcall level (module_init)
> > +	  in not early enough in the boot process to avoid the watchdog
> > +	  to be triggered.
> > +	  If you say yes here, the initcall level would be raised to
> > +	  arch_initcall.
> > +	  If in doubt, say N.
> 
> > --- a/drivers/watchdog/gpio_wdt.c
> > +++ b/drivers/watchdog/gpio_wdt.c
> 
> > +#ifdef GPIO_WATCHDOG_ARCH_INITCALL
> 
> You meant
>     #ifdef CONFIG_GPIO_WATCHDOG_ARCH_INITCALL
> 
> Please use scripts/checkkconfigsymbols.py.

checkkconfigsymbols.py only checks CONFIG_ prefix symbols.  Since this
prefix is missing here, the script would not complain.  But that's
an interesting case and worth checking for.

Kind regards,
 Valentin

> 
> > +static int __init gpio_wdt_init(void)
> > +{
> > +	return platform_driver_register(&gpio_wdt_driver);
> > +}
> > +arch_initcall(gpio_wdt_init);
> > +#else
> >  module_platform_driver(gpio_wdt_driver);
> > +#endif
> 
> The entire patch is basically an elaborate NOP. How did this pass your
> testing?
> 
> 
> Paul Bolle

Re: [PATCH v3 2/2] gpio_wdt: Add option for early registration

[Paul Bolle]

On Sat, 2015-06-06 at 14:56 +0200, Valentin Rothberg wrote:
> On Sat, Jun 06, 2015 at 12:03:06PM +0200, Paul Bolle wrote:
> > On Sat, 2015-06-06 at 00:46 -0700, Jean-Baptiste Theou wrote:
> > > +#ifdef GPIO_WATCHDOG_ARCH_INITCALL
> > 
> > You meant
> >     #ifdef CONFIG_GPIO_WATCHDOG_ARCH_INITCALL
> > 
> > Please use scripts/checkkconfigsymbols.py.
> 
> checkkconfigsymbols.py only checks CONFIG_ prefix symbols.  Since this
> prefix is missing here, the script would not complain.

Of course. Thanks for correcting me.

> But that's an interesting case and worth checking for.

Perhaps. Depends on the amount of false positives that such checks would
trigger, I'd say.


Paul Bolle

Re: [PATCH v3 2/2] gpio_wdt: Add option for early registration

[Jean-Baptiste Theou]

Thanks for the feedback.

I tested a version before cleanup, and the mistake was introduce during it.
It's my mistake, sorry about that. Will fix it on v4.


On Sat, 6 Jun 2015 16:11:18 +0200
Paul Bolle <pebolle@tiscali.nl> wrote:

> On Sat, 2015-06-06 at 14:56 +0200, Valentin Rothberg wrote:
> > On Sat, Jun 06, 2015 at 12:03:06PM +0200, Paul Bolle wrote:
> > > On Sat, 2015-06-06 at 00:46 -0700, Jean-Baptiste Theou wrote:
> > > > +#ifdef GPIO_WATCHDOG_ARCH_INITCALL
> > > 
> > > You meant
> > >     #ifdef CONFIG_GPIO_WATCHDOG_ARCH_INITCALL
> > > 
> > > Please use scripts/checkkconfigsymbols.py.
> > 
> > checkkconfigsymbols.py only checks CONFIG_ prefix symbols.  Since this
> > prefix is missing here, the script would not complain.
> 
> Of course. Thanks for correcting me.
> 
> > But that's an interesting case and worth checking for.
> 
> Perhaps. Depends on the amount of false positives that such checks would
> trigger, I'd say.
> 
> 
> Paul Bolle
> 


-- 
Jean-Baptiste Theou <jtheou@adeneo-embedded.us>

Re: [PATCH v3 2/2] gpio_wdt: Add option for early registration

[Guenter Roeck]

On 06/06/2015 12:46 AM, Jean-Baptiste Theou wrote:
> In some situation, mainly when it's not possible to disable a
> watchdog, you may want the watchdog driver to be started as soon
> as possible.
>
> Adding GPIO_WATCHDOG_ARCH_INITCALL to raise initcall from
> module_init to arch_initcall.
>
> This patch require watchdog registration deferral mechanism
>
> Signed-off-by: Jean-Baptiste Theou <jtheou@adeneo-embedded.us>
> ---
> Changes in v3:
>   - Simplify changes, since the modification is for built-in only
>
>   drivers/watchdog/Kconfig    | 12 ++++++++++++
>   drivers/watchdog/gpio_wdt.c |  8 ++++++++
>   2 files changed, 20 insertions(+)
>
> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
> index e5e7c55..7a5c912 100644
> --- a/drivers/watchdog/Kconfig
> +++ b/drivers/watchdog/Kconfig
> @@ -1,3 +1,4 @@
> +
>   #
>   # Watchdog device configuration
>   #
> @@ -104,6 +105,17 @@ config GPIO_WATCHDOG
>   	  If you say yes here you get support for watchdog device
>   	  controlled through GPIO-line.
>
> +config GPIO_WATCHDOG_ARCH_INITCALL
> +	bool "Register the watchdog as early as possible"
> +	depends on GPIO_WATCHDOG=y
> +	help
> +	  In some situations, the default initcall level (module_init)
> +	  in not early enough in the boot process to avoid the watchdog
> +	  to be triggered.
> +	  If you say yes here, the initcall level would be raised to
> +	  arch_initcall.
> +	  If in doubt, say N.
> +
>   config MENF21BMC_WATCHDOG
>   	tristate "MEN 14F021P00 BMC Watchdog"
>   	depends on MFD_MENF21BMC
> diff --git a/drivers/watchdog/gpio_wdt.c b/drivers/watchdog/gpio_wdt.c
> index cbc313d..f9149a2 100644
> --- a/drivers/watchdog/gpio_wdt.c
> +++ b/drivers/watchdog/gpio_wdt.c
> @@ -267,7 +267,15 @@ static struct platform_driver gpio_wdt_driver = {
>   	.probe	= gpio_wdt_probe,
>   	.remove	= gpio_wdt_remove,
>   };

Please add an empty line here.

Thanks,
Guenter

> +#ifdef GPIO_WATCHDOG_ARCH_INITCALL
> +static int __init gpio_wdt_init(void)
> +{
> +	return platform_driver_register(&gpio_wdt_driver);
> +}
> +arch_initcall(gpio_wdt_init);
> +#else
>   module_platform_driver(gpio_wdt_driver);
> +#endif
>
>   MODULE_AUTHOR("Alexander Shiyan <shc_work@mail.ru>");
>   MODULE_DESCRIPTION("GPIO Watchdog");
>

Re: [PATCH v3 1/2] watchdog_core: Add watchdog registration deferral mechanism

[Guenter Roeck]

On 06/06/2015 12:46 AM, Jean-Baptiste Theou wrote:
> Currently, watchdog subsystem require the misc subsystem to
> register a watchdog. This may not be the case in case of an
> early registration of a watchdog, which can be required when
> the watchdog cannot be disabled.
>
> This patch introduces a deferral mechanism to remove this requirement.
>
Couple of comments, but almost good.

Thanks,
Guenter

> Signed-off-by: Jean-Baptiste Theou <jtheou@adeneo-embedded.us>
> ---
> Changes in v3:
>   - Adjust patch based on Guenter feedbacks
>   - Fix watchdog_deferred_registration_del
>   - Fix typos
>
>   Documentation/watchdog/watchdog-kernel-api.txt |   7 ++
>   drivers/watchdog/watchdog_core.c               | 114 +++++++++++++++++++++----
>   include/linux/watchdog.h                       |   3 +
>   3 files changed, 106 insertions(+), 18 deletions(-)
>
> diff --git a/Documentation/watchdog/watchdog-kernel-api.txt b/Documentation/watchdog/watchdog-kernel-api.txt
> index a0438f3..e83518c 100644
> --- a/Documentation/watchdog/watchdog-kernel-api.txt
> +++ b/Documentation/watchdog/watchdog-kernel-api.txt
> @@ -36,6 +36,10 @@ The watchdog_unregister_device routine deregisters a registered watchdog timer
>   device. The parameter of this routine is the pointer to the registered
>   watchdog_device structure.
>
> +The watchdog subsystem includes an registration deferral mechanism,
> +which allows you to register an watchdog as early as you wish during
> +the boot process.
> +
>   The watchdog device structure looks like this:
>
>   struct watchdog_device {
> @@ -52,6 +56,7 @@ struct watchdog_device {
>   	void *driver_data;
>   	struct mutex lock;
>   	unsigned long status;
> +	struct list_head deferred_registration;
>   };
>
>   It contains following fields:
> @@ -80,6 +85,8 @@ It contains following fields:
>     information about the status of the device (Like: is the watchdog timer
>     running/active, is the nowayout bit set, is the device opened via
>     the /dev/watchdog interface or not, ...).
> +* deferred_registration: entry in deferred_registration_list which is used to
> +  register early initialized watchdogs.
>
>   The list of watchdog operations is defined as:
>
> diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
> index cec9b55..7dba8d9 100644
> --- a/drivers/watchdog/watchdog_core.c
> +++ b/drivers/watchdog/watchdog_core.c
> @@ -43,6 +43,43 @@
>   static DEFINE_IDA(watchdog_ida);
>   static struct class *watchdog_class;
>
> +/*
> + * Deferred Registration infrastructure.
> + *
> + * Sometimes watchdog drivers needs to be loaded as soon as possible,
> + * for example when it's impossible to disable it. To do so,
> + * raising the initcall level of the watchdog driver is a solution.
> + * But in such case, the miscdev is maybe not ready (subsys_initcall), and
> + * watchdog_core need miscdev to register the watchdog as a char device.
> + *
> + * The deferred registration infrastructure offer a way for the watchdog
> + * subsystem to register a watchdog properly, even before miscdev is ready.
> + */
> +
> +static DEFINE_MUTEX(watchdog_deferred_registration_mutex);
> +static LIST_HEAD(watchdog_deferred_registration_pending_list);
> +static bool watchdog_deferred_registration_done;
> +
> +static int watchdog_deferred_registration_add(struct watchdog_device *wdd)
> +{
> +	list_add_tail(&wdd->deferred_registration,
> +		 &watchdog_deferred_registration_pending_list);

Please align the continuation line with '('.

> +	return 0;
> +}
> +
> +static void watchdog_deferred_registration_del(struct watchdog_device *wdd)
> +{
> +	struct list_head *p, *n;
> +	struct watchdog_device *wdd_tmp;
> +
> +	list_for_each_safe(p, n, &watchdog_deferred_registration_pending_list) {
> +		wdd_tmp = list_entry(p, struct watchdog_device,
> +				     deferred_registration);
> +		if (wdd_tmp == wdd)
> +			list_del(&wdd_tmp->deferred_registration);

You can break out of the loop here. No need to check the rest of the entries.

> +	}
> +}
> +
>   static void watchdog_check_min_max_timeout(struct watchdog_device *wdd)
>   {
>   	/*
> @@ -98,17 +135,7 @@ int watchdog_init_timeout(struct watchdog_device *wdd,
>   }
>   EXPORT_SYMBOL_GPL(watchdog_init_timeout);
>
> -/**
> - * watchdog_register_device() - register a watchdog device
> - * @wdd: watchdog device
> - *
> - * Register a watchdog device with the kernel so that the
> - * watchdog timer can be accessed from userspace.
> - *
> - * A zero is returned on success and a negative errno code for
> - * failure.
> - */
> -int watchdog_register_device(struct watchdog_device *wdd)
> +static int __watchdog_register_device(struct watchdog_device *wdd)
>   {
>   	int ret, id, devno;
>
> @@ -164,16 +191,33 @@ int watchdog_register_device(struct watchdog_device *wdd)
>
>   	return 0;
>   }
> -EXPORT_SYMBOL_GPL(watchdog_register_device);
>
>   /**
> - * watchdog_unregister_device() - unregister a watchdog device
> - * @wdd: watchdog device to unregister
> + * watchdog_register_device() - register a watchdog device
> + * @wdd: watchdog device
>    *
> - * Unregister a watchdog device that was previously successfully
> - * registered with watchdog_register_device().
> + * Register a watchdog device with the kernel so that the
> + * watchdog timer can be accessed from userspace.
> + *
> + * A zero is returned on success and a negative errno code for
> + * failure.
>    */
> -void watchdog_unregister_device(struct watchdog_device *wdd)
> +
> +int watchdog_register_device(struct watchdog_device *wdd)
> +{
> +	int ret;
> +
> +	mutex_lock(&watchdog_deferred_registration_mutex);
> +	if (watchdog_deferred_registration_done)
> +		ret = __watchdog_register_device(wdd);
> +	else
> +		ret = watchdog_deferred_registration_add(wdd);
> +	mutex_unlock(&watchdog_deferred_registration_mutex);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(watchdog_register_device);
> +
> +static void __watchdog_unregister_device(struct watchdog_device *wdd)
>   {
>   	int ret;
>   	int devno;
> @@ -189,8 +233,41 @@ void watchdog_unregister_device(struct watchdog_device *wdd)
>   	ida_simple_remove(&watchdog_ida, wdd->id);
>   	wdd->dev = NULL;
>   }
> +
> +/**
> + * watchdog_unregister_device() - unregister a watchdog device
> + * @wdd: watchdog device to unregister
> + *
> + * Unregister a watchdog device that was previously successfully
> + * registered with watchdog_register_device().
> + */
> +
> +void watchdog_unregister_device(struct watchdog_device *wdd)
> +{
> +	mutex_lock(&watchdog_deferred_registration_mutex);
> +	if (watchdog_deferred_registration_done)
> +		__watchdog_unregister_device(wdd);
> +	else
> +		watchdog_deferred_registration_del(wdd);
> +	mutex_unlock(&watchdog_deferred_registration_mutex);
> +}
> +
>   EXPORT_SYMBOL_GPL(watchdog_unregister_device);
>
> +static int __init watchdog_deferred_registration(void)
> +{
> +	mutex_lock(&watchdog_deferred_registration_mutex);
> +	watchdog_deferred_registration_done = true;
> +	while (!list_empty(&watchdog_deferred_registration_pending_list)) {
> +		struct watchdog_device *wdd = list_first_entry(&watchdog_deferred_registration_pending_list,
> +							       struct watchdog_device, deferred_registration);

Please watch out for line length.

		struct watchdog_device *wdd;

		wdd = list_first_entry(&watchdog_deferred_registration_pending_list,
			               struct watchdog_device, deferred_registration);

might work better here.

> +		list_del(&wdd->deferred_registration);
> +		__watchdog_register_device(wdd);
> +	}
> +	mutex_unlock(&watchdog_deferred_registration_mutex);
> +	return 0;
> +}
> +
>   static int __init watchdog_init(void)
>   {
>   	int err;
> @@ -207,6 +284,7 @@ static int __init watchdog_init(void)
>   		return err;
>   	}
>
> +	watchdog_deferred_registration();
>   	return 0;
>   }
>
> @@ -217,7 +295,7 @@ static void __exit watchdog_exit(void)
>   	ida_destroy(&watchdog_ida);
>   }
>
> -subsys_initcall(watchdog_init);
> +subsys_initcall_sync(watchdog_init);
>   module_exit(watchdog_exit);
>
>   MODULE_AUTHOR("Alan Cox <alan@lxorguk.ukuu.org.uk>");
> diff --git a/include/linux/watchdog.h b/include/linux/watchdog.h
> index a746bf5..f9bb943 100644
> --- a/include/linux/watchdog.h
> +++ b/include/linux/watchdog.h
> @@ -65,6 +65,8 @@ struct watchdog_ops {
>    * @driver-data:Pointer to the drivers private data.
>    * @lock:	Lock for watchdog core internal use only.
>    * @status:	Field that contains the devices internal status bits.
> + * @deferred_registration: entry in deferred_registration_list which is used to
> + *			   register early initialized watchdogs.
>    *
>    * The watchdog_device structure contains all information about a
>    * watchdog timer device.
> @@ -95,6 +97,7 @@ struct watchdog_device {
>   #define WDOG_ALLOW_RELEASE	2	/* Did we receive the magic char ? */
>   #define WDOG_NO_WAY_OUT		3	/* Is 'nowayout' feature set ? */
>   #define WDOG_UNREGISTERED	4	/* Has the device been unregistered */
> +	struct list_head deferred_registration;
>   };
>
>   #define WATCHDOG_NOWAYOUT		IS_BUILTIN(CONFIG_WATCHDOG_NOWAYOUT)
>