[PATCH v1 2/6] kernel: Add power off handler call chain API

[Dmitry Osipenko <digetx@gmail.com>]

SoC platforms often have multiple options for performing system's
power off. Meanwhile kernel is limited to a single option. Add power off
handler call chain API, which is inspired by the restart API. The old
pm_power_off method will be kept around till all drivers are converted
to the new API.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 include/linux/reboot.h |   5 ++
 kernel/reboot.c        | 111 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 115 insertions(+), 1 deletion(-)

diff --git a/include/linux/reboot.h b/include/linux/reboot.h
index af907a3d68d1..f72fb8729608 100644
--- a/include/linux/reboot.h
+++ b/include/linux/reboot.h
@@ -49,6 +49,11 @@ extern int register_restart_handler(struct notifier_block *);
 extern int unregister_restart_handler(struct notifier_block *);
 extern void do_kernel_restart(char *cmd);
 
+extern int register_poweroff_handler(struct notifier_block *);
+extern int unregister_poweroff_handler(struct notifier_block *);
+
+extern int devm_register_poweroff_handler(struct device *, struct notifier_block *);
+
 /*
  * Architecture-specific implementations of sys_reboot commands.
  */
diff --git a/kernel/reboot.c b/kernel/reboot.c
index f7440c0c7e43..6e90afec2bb8 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -280,6 +280,113 @@ void kernel_halt(void)
 }
 EXPORT_SYMBOL_GPL(kernel_halt);
 
+/*
+ *	Notifier list for kernel code which wants to be called
+ *	to power off the system.
+ */
+static BLOCKING_NOTIFIER_HEAD(poweroff_handler_list);
+
+/**
+ *	register_poweroff_handler - Register function to be called to power off
+ *				    the system
+ *	@nb: Info about handler function to be called
+ *	@nb->priority:	Handler priority. Handlers should follow the
+ *			following guidelines for setting priorities.
+ *			0:	Poweroff handler of last resort,
+ *				with limited poweroff capabilities
+ *			128:	Default poweroff handler; use if no other
+ *				poweroff handler is expected to be available,
+ *				and/or if poweroff functionality is
+ *				sufficient to poweroff the entire system
+ *			255:	Highest priority poweroff handler, will
+ *				preempt all other poweroff handlers
+ *
+ *	Registers a function with code to be called to power off the
+ *	system.
+ *
+ *	Registered functions will be called as last step of the poweroff
+ *	sequence.
+ *
+ *	Registered functions are expected to power off the system immediately.
+ *	If more than one function is registered, the poweroff handler priority
+ *	selects which function will be called first.
+ *
+ *	Poweroff handlers are expected to be registered from non-architecture
+ *	code, typically from drivers. A typical use case would be a system
+ *	where poweroff functionality is provided through a PMIC. Multiple
+ *	poweroff handlers may exist; for example, one poweroff handler might
+ *	turn off the entire system, while another only turns off part of
+ *	system. In such cases, the poweroff handler which only disables part
+ *	of the hardware is expected to register with low priority to ensure
+ *	that it only runs if no other means to power off the system is
+ *	available.
+ *
+ *	Currently always returns zero, as blocking_notifier_chain_register()
+ *	always returns zero.
+ */
+int register_poweroff_handler(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&poweroff_handler_list, nb);
+}
+EXPORT_SYMBOL(register_poweroff_handler);
+
+/**
+ *	unregister_poweroff_handler - Unregister previously registered
+ *				      poweroff handler
+ *	@nb: Hook to be unregistered
+ *
+ *	Unregisters a previously registered poweroff handler function.
+ *
+ *	Returns zero on success, or %-ENOENT on failure.
+ */
+int unregister_poweroff_handler(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&poweroff_handler_list, nb);
+}
+EXPORT_SYMBOL(unregister_poweroff_handler);
+
+static void devm_unregister_poweroff_handler(struct device *dev, void *res)
+{
+	WARN_ON(unregister_poweroff_handler(*(struct notifier_block **)res));
+}
+
+int devm_register_poweroff_handler(struct device *dev, struct notifier_block *nb)
+{
+	struct notifier_block **rcnb;
+	int ret;
+
+	rcnb = devres_alloc(devm_unregister_poweroff_handler,
+			    sizeof(*rcnb), GFP_KERNEL);
+	if (!rcnb)
+		return -ENOMEM;
+
+	ret = register_poweroff_handler(nb);
+	if (!ret) {
+		*rcnb = nb;
+		devres_add(dev, rcnb);
+	} else {
+		devres_free(rcnb);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(devm_register_poweroff_handler);
+
+/**
+ *	do_kernel_power_off - Execute kernel poweroff handler call chain
+ *
+ *	Calls functions registered with register_poweroff_handler.
+ *
+ *	Expected to be called as last step of the poweroff sequence.
+ *
+ *	Powers off the system immediately if a poweroff handler function has
+ *	been registered. Otherwise does nothing.
+ */
+static void do_kernel_power_off(void)
+{
+	blocking_notifier_call_chain(&poweroff_handler_list, 0, NULL);
+}
+
 /**
  *	kernel_power_off - power_off the system
  *
@@ -295,6 +402,7 @@ void kernel_power_off(void)
 	pr_emerg("Power down\n");
 	kmsg_dump(KMSG_DUMP_SHUTDOWN);
 	machine_power_off();
+	do_kernel_power_off();
 }
 EXPORT_SYMBOL_GPL(kernel_power_off);
 
@@ -339,7 +447,8 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 	/* Instead of trying to make the power_off code look like
 	 * halt when pm_power_off is not set do it the easy way.
 	 */
-	if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)
+	if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off &&
+	    blocking_notifier_call_chain_empty(&poweroff_handler_list))
 		cmd = LINUX_REBOOT_CMD_HALT;
 
 	mutex_lock(&system_transition_mutex);
-- 
2.32.0