drivers/base/power/qos.c - kernel/msm-4.9 - Gitiles

 /*
  * Devices PM QoS constraints management
  *
  * Copyright (C) 2011 Texas Instruments, Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  *
  *
  * This module exposes the interface to kernel space for specifying
  * per-device PM QoS dependencies. It provides infrastructure for registration
  * of:
  *
  * Dependents on a QoS value : register requests
  * Watchers of QoS value : get notified when target QoS value changes
  *
  * This QoS design is best effort based. Dependents register their QoS needs.
  * Watchers register to keep track of the current QoS needs of the system.
  * Watchers can register different types of notification callbacks:
  *  . a per-device notification callback using the dev_pm_qos_*_notifier API.
  *    The notification chain data is stored in the per-device constraint
  *    data struct.
  *  . a system-wide notification callback using the dev_pm_qos_*_global_notifier
  *    API. The notification chain data is stored in a static variable.
  *
  * Note about the per-device constraint data struct allocation:
  * . The per-device constraints data struct ptr is tored into the device
  *    dev_pm_info.
  * . To minimize the data usage by the per-device constraints, the data struct
  *   is only allocated at the first call to dev_pm_qos_add_request.
  * . The data is later free'd when the device is removed from the system.
  *  . A global mutex protects the constraints users from the data being
  *     allocated and free'd.
  */

 #include <linux/pm_qos.h>
 #include <linux/spinlock.h>
 #include <linux/slab.h>
 #include <linux/device.h>
 #include <linux/mutex.h>
 #include <linux/export.h>

 #include "power.h"

 static DEFINE_MUTEX(dev_pm_qos_mtx);

 static BLOCKING_NOTIFIER_HEAD(dev_pm_notifiers);

 /**
  * __dev_pm_qos_read_value - Get PM QoS constraint for a given device.
  * @dev: Device to get the PM QoS constraint value for.
  *
  * This routine must be called with dev->power.lock held.
  */
 s32 __dev_pm_qos_read_value(struct device *dev)
 {
 	struct pm_qos_constraints *c = dev->power.constraints;

 	return c ? pm_qos_read_value(c) : 0;
 }

 /**
  * dev_pm_qos_read_value - Get PM QoS constraint for a given device (locked).
  * @dev: Device to get the PM QoS constraint value for.
  */
 s32 dev_pm_qos_read_value(struct device *dev)
 {
 	unsigned long flags;
 	s32 ret;

 	spin_lock_irqsave(&dev->power.lock, flags);
 	ret = __dev_pm_qos_read_value(dev);
 	spin_unlock_irqrestore(&dev->power.lock, flags);

 	return ret;
 }

 /*
  * apply_constraint
  * @req: constraint request to apply
  * @action: action to perform add/update/remove, of type enum pm_qos_req_action
  * @value: defines the qos request
  *
  * Internal function to update the constraints list using the PM QoS core
  * code and if needed call the per-device and the global notification
  * callbacks
  */
 static int apply_constraint(struct dev_pm_qos_request *req,
 			    enum pm_qos_req_action action, int value)
 {
 	int ret, curr_value;

 	ret = pm_qos_update_target(req->dev->power.constraints,
 				   &req->node, action, value);

 	if (ret) {
 		/* Call the global callbacks if needed */
 		curr_value = pm_qos_read_value(req->dev->power.constraints);
 		blocking_notifier_call_chain(&dev_pm_notifiers,
 					     (unsigned long)curr_value,
 					     req);
 	}

 	return ret;
 }

 /*
  * dev_pm_qos_constraints_allocate
  * @dev: device to allocate data for
  *
  * Called at the first call to add_request, for constraint data allocation
  * Must be called with the dev_pm_qos_mtx mutex held
  */
 static int dev_pm_qos_constraints_allocate(struct device *dev)
 {
 	struct pm_qos_constraints *c;
 	struct blocking_notifier_head *n;

 	c = kzalloc(sizeof(*c), GFP_KERNEL);
 	if (!c)
 		return -ENOMEM;

 	n = kzalloc(sizeof(*n), GFP_KERNEL);
 	if (!n) {
 		kfree(c);
 		return -ENOMEM;
 	}
 	BLOCKING_INIT_NOTIFIER_HEAD(n);

 	plist_head_init(&c->list);
 	c->target_value = PM_QOS_DEV_LAT_DEFAULT_VALUE;
 	c->default_value = PM_QOS_DEV_LAT_DEFAULT_VALUE;
 	c->type = PM_QOS_MIN;
 	c->notifiers = n;

 	spin_lock_irq(&dev->power.lock);
 	dev->power.constraints = c;
 	spin_unlock_irq(&dev->power.lock);

 	return 0;
 }

 /**
  * dev_pm_qos_constraints_init - Initalize device's PM QoS constraints pointer.
  * @dev: target device
  *
  * Called from the device PM subsystem during device insertion under
  * device_pm_lock().
  */
 void dev_pm_qos_constraints_init(struct device *dev)
 {
 	mutex_lock(&dev_pm_qos_mtx);
 	dev->power.constraints = NULL;
 	dev->power.power_state = PMSG_ON;
 	mutex_unlock(&dev_pm_qos_mtx);
 }

 /**
  * dev_pm_qos_constraints_destroy
  * @dev: target device
  *
  * Called from the device PM subsystem on device removal under device_pm_lock().
  */
 void dev_pm_qos_constraints_destroy(struct device *dev)
 {
 	struct dev_pm_qos_request *req, *tmp;
 	struct pm_qos_constraints *c;

 	/*
 	 * If the device's PM QoS resume latency limit has been exposed to user
 	 * space, it has to be hidden at this point.
 	 */
 	dev_pm_qos_hide_latency_limit(dev);

 	mutex_lock(&dev_pm_qos_mtx);

 	dev->power.power_state = PMSG_INVALID;
 	c = dev->power.constraints;
 	if (!c)
 		goto out;

 	/* Flush the constraints list for the device */
 	plist_for_each_entry_safe(req, tmp, &c->list, node) {
 		/*
 		 * Update constraints list and call the notification
 		 * callbacks if needed
 		 */
 		apply_constraint(req, PM_QOS_REMOVE_REQ, PM_QOS_DEFAULT_VALUE);
 		memset(req, 0, sizeof(*req));
 	}

 	spin_lock_irq(&dev->power.lock);
 	dev->power.constraints = NULL;
 	spin_unlock_irq(&dev->power.lock);

 	kfree(c->notifiers);
 	kfree(c);

  out:
 	mutex_unlock(&dev_pm_qos_mtx);
 }

 /**
  * dev_pm_qos_add_request - inserts new qos request into the list
  * @dev: target device for the constraint
  * @req: pointer to a preallocated handle
  * @value: defines the qos request
  *
  * This function inserts a new entry in the device constraints list of
  * requested qos performance characteristics. It recomputes the aggregate
  * QoS expectations of parameters and initializes the dev_pm_qos_request
  * handle.  Caller needs to save this handle for later use in updates and
  * removal.
  *
  * Returns 1 if the aggregated constraint value has changed,
  * 0 if the aggregated constraint value has not changed,
  * -EINVAL in case of wrong parameters, -ENOMEM if there's not enough memory
  * to allocate for data structures, -ENODEV if the device has just been removed
  * from the system.
  */
 int dev_pm_qos_add_request(struct device *dev, struct dev_pm_qos_request *req,
 			   s32 value)
 {
 	int ret = 0;

 	if (!dev || !req) /*guard against callers passing in null */
 		return -EINVAL;

 	if (WARN(dev_pm_qos_request_active(req),
 		 "%s() called for already added request\n", __func__))
 		return -EINVAL;

 	req->dev = dev;

 	mutex_lock(&dev_pm_qos_mtx);

 	if (!dev->power.constraints) {
 		if (dev->power.power_state.event == PM_EVENT_INVALID) {
 			/* The device has been removed from the system. */
 			req->dev = NULL;
 			ret = -ENODEV;
 			goto out;
 		} else {
 			/*
 			 * Allocate the constraints data on the first call to
 			 * add_request, i.e. only if the data is not already
 			 * allocated and if the device has not been removed.
 			 */
 			ret = dev_pm_qos_constraints_allocate(dev);
 		}
 	}

 	if (!ret)
 		ret = apply_constraint(req, PM_QOS_ADD_REQ, value);

  out:
 	mutex_unlock(&dev_pm_qos_mtx);

 	return ret;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_add_request);

 /**
  * dev_pm_qos_update_request - modifies an existing qos request
  * @req : handle to list element holding a dev_pm_qos request to use
  * @new_value: defines the qos request
  *
  * Updates an existing dev PM qos request along with updating the
  * target value.
  *
  * Attempts are made to make this code callable on hot code paths.
  *
  * Returns 1 if the aggregated constraint value has changed,
  * 0 if the aggregated constraint value has not changed,
  * -EINVAL in case of wrong parameters, -ENODEV if the device has been
  * removed from the system
  */
 int dev_pm_qos_update_request(struct dev_pm_qos_request *req,
 			      s32 new_value)
 {
 	int ret = 0;

 	if (!req) /*guard against callers passing in null */
 		return -EINVAL;

 	if (WARN(!dev_pm_qos_request_active(req),
 		 "%s() called for unknown object\n", __func__))
 		return -EINVAL;

 	mutex_lock(&dev_pm_qos_mtx);

 	if (req->dev->power.constraints) {
 		if (new_value != req->node.prio)
 			ret = apply_constraint(req, PM_QOS_UPDATE_REQ,
 					       new_value);
 	} else {
 		/* Return if the device has been removed */
 		ret = -ENODEV;
 	}

 	mutex_unlock(&dev_pm_qos_mtx);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_update_request);

 /**
  * dev_pm_qos_remove_request - modifies an existing qos request
  * @req: handle to request list element
  *
  * Will remove pm qos request from the list of constraints and
  * recompute the current target value. Call this on slow code paths.
  *
  * Returns 1 if the aggregated constraint value has changed,
  * 0 if the aggregated constraint value has not changed,
  * -EINVAL in case of wrong parameters, -ENODEV if the device has been
  * removed from the system
  */
 int dev_pm_qos_remove_request(struct dev_pm_qos_request *req)
 {
 	int ret = 0;

 	if (!req) /*guard against callers passing in null */
 		return -EINVAL;

 	if (WARN(!dev_pm_qos_request_active(req),
 		 "%s() called for unknown object\n", __func__))
 		return -EINVAL;

 	mutex_lock(&dev_pm_qos_mtx);

 	if (req->dev->power.constraints) {
 		ret = apply_constraint(req, PM_QOS_REMOVE_REQ,
 				       PM_QOS_DEFAULT_VALUE);
 		memset(req, 0, sizeof(*req));
 	} else {
 		/* Return if the device has been removed */
 		ret = -ENODEV;
 	}

 	mutex_unlock(&dev_pm_qos_mtx);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_remove_request);

 /**
  * dev_pm_qos_add_notifier - sets notification entry for changes to target value
  * of per-device PM QoS constraints
  *
  * @dev: target device for the constraint
  * @notifier: notifier block managed by caller.
  *
  * Will register the notifier into a notification chain that gets called
  * upon changes to the target value for the device.
  *
  * If the device's constraints object doesn't exist when this routine is called,
  * it will be created (or error code will be returned if that fails).
  */
 int dev_pm_qos_add_notifier(struct device *dev, struct notifier_block *notifier)
 {
 	int ret = 0;

 	mutex_lock(&dev_pm_qos_mtx);

 	if (!dev->power.constraints)
 		ret = dev->power.power_state.event != PM_EVENT_INVALID ?
 			dev_pm_qos_constraints_allocate(dev) : -ENODEV;

 	if (!ret)
 		ret = blocking_notifier_chain_register(
 				dev->power.constraints->notifiers, notifier);

 	mutex_unlock(&dev_pm_qos_mtx);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_add_notifier);

 /**
  * dev_pm_qos_remove_notifier - deletes notification for changes to target value
  * of per-device PM QoS constraints
  *
  * @dev: target device for the constraint
  * @notifier: notifier block to be removed.
  *
  * Will remove the notifier from the notification chain that gets called
  * upon changes to the target value.
  */
 int dev_pm_qos_remove_notifier(struct device *dev,
 			       struct notifier_block *notifier)
 {
 	int retval = 0;

 	mutex_lock(&dev_pm_qos_mtx);

 	/* Silently return if the constraints object is not present. */
 	if (dev->power.constraints)
 		retval = blocking_notifier_chain_unregister(
 				dev->power.constraints->notifiers,
 				notifier);

 	mutex_unlock(&dev_pm_qos_mtx);
 	return retval;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_remove_notifier);

 /**
  * dev_pm_qos_add_global_notifier - sets notification entry for changes to
  * target value of the PM QoS constraints for any device
  *
  * @notifier: notifier block managed by caller.
  *
  * Will register the notifier into a notification chain that gets called
  * upon changes to the target value for any device.
  */
 int dev_pm_qos_add_global_notifier(struct notifier_block *notifier)
 {
 	return blocking_notifier_chain_register(&dev_pm_notifiers, notifier);
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_add_global_notifier);

 /**
  * dev_pm_qos_remove_global_notifier - deletes notification for changes to
  * target value of PM QoS constraints for any device
  *
  * @notifier: notifier block to be removed.
  *
  * Will remove the notifier from the notification chain that gets called
  * upon changes to the target value for any device.
  */
 int dev_pm_qos_remove_global_notifier(struct notifier_block *notifier)
 {
 	return blocking_notifier_chain_unregister(&dev_pm_notifiers, notifier);
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_remove_global_notifier);

 /**
  * dev_pm_qos_add_ancestor_request - Add PM QoS request for device's ancestor.
  * @dev: Device whose ancestor to add the request for.
  * @req: Pointer to the preallocated handle.
  * @value: Constraint latency value.
  */
 int dev_pm_qos_add_ancestor_request(struct device *dev,
 				    struct dev_pm_qos_request *req, s32 value)
 {
 	struct device *ancestor = dev->parent;
 	int error = -ENODEV;

 	while (ancestor && !ancestor->power.ignore_children)
 		ancestor = ancestor->parent;

 	if (ancestor)
 		error = dev_pm_qos_add_request(ancestor, req, value);

 	if (error)
 		req->dev = NULL;

 	return error;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_add_ancestor_request);

 #ifdef CONFIG_PM_RUNTIME
 static void __dev_pm_qos_drop_user_request(struct device *dev)
 {
 	dev_pm_qos_remove_request(dev->power.pq_req);
 	dev->power.pq_req = NULL;
 }

 /**
  * dev_pm_qos_expose_latency_limit - Expose PM QoS latency limit to user space.
  * @dev: Device whose PM QoS latency limit is to be exposed to user space.
  * @value: Initial value of the latency limit.
  */
 int dev_pm_qos_expose_latency_limit(struct device *dev, s32 value)
 {
 	struct dev_pm_qos_request *req;
 	int ret;

 	if (!device_is_registered(dev) || value < 0)
 		return -EINVAL;

 	if (dev->power.pq_req)
 		return -EEXIST;

 	req = kzalloc(sizeof(*req), GFP_KERNEL);
 	if (!req)
 		return -ENOMEM;

 	ret = dev_pm_qos_add_request(dev, req, value);
 	if (ret < 0)
 		return ret;

 	dev->power.pq_req = req;
 	ret = pm_qos_sysfs_add(dev);
 	if (ret)
 		__dev_pm_qos_drop_user_request(dev);

 	return ret;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_expose_latency_limit);

 /**
  * dev_pm_qos_hide_latency_limit - Hide PM QoS latency limit from user space.
  * @dev: Device whose PM QoS latency limit is to be hidden from user space.
  */
 void dev_pm_qos_hide_latency_limit(struct device *dev)
 {
 	if (dev->power.pq_req) {
 		pm_qos_sysfs_remove(dev);
 		__dev_pm_qos_drop_user_request(dev);
 	}
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_hide_latency_limit);
 #endif /* CONFIG_PM_RUNTIME */
	/*
	* Devices PM QoS constraints management
	*
	* Copyright (C) 2011 Texas Instruments, Inc.
	*
	* This program is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License version 2 as
	* published by the Free Software Foundation.
	*
	*
	* This module exposes the interface to kernel space for specifying
	* per-device PM QoS dependencies. It provides infrastructure for registration
	* of:
	*
	* Dependents on a QoS value : register requests
	* Watchers of QoS value : get notified when target QoS value changes
	*
	* This QoS design is best effort based. Dependents register their QoS needs.
	* Watchers register to keep track of the current QoS needs of the system.
	* Watchers can register different types of notification callbacks:
	* . a per-device notification callback using the dev_pm_qos_*_notifier API.
	* The notification chain data is stored in the per-device constraint
	* data struct.
	* . a system-wide notification callback using the dev_pm_qos_*_global_notifier
	* API. The notification chain data is stored in a static variable.
	*
	* Note about the per-device constraint data struct allocation:
	* . The per-device constraints data struct ptr is tored into the device
	* dev_pm_info.
	* . To minimize the data usage by the per-device constraints, the data struct
	* is only allocated at the first call to dev_pm_qos_add_request.
	* . The data is later free'd when the device is removed from the system.
	* . A global mutex protects the constraints users from the data being
	* allocated and free'd.
	*/

	#include <linux/pm_qos.h>
	#include <linux/spinlock.h>
	#include <linux/slab.h>
	#include <linux/device.h>
	#include <linux/mutex.h>
	#include <linux/export.h>

	#include "power.h"

	static DEFINE_MUTEX(dev_pm_qos_mtx);

	static BLOCKING_NOTIFIER_HEAD(dev_pm_notifiers);

	/**
	* __dev_pm_qos_read_value - Get PM QoS constraint for a given device.
	* @dev: Device to get the PM QoS constraint value for.
	*
	* This routine must be called with dev->power.lock held.
	*/
	s32 __dev_pm_qos_read_value(struct device *dev)
	{
	struct pm_qos_constraints *c = dev->power.constraints;

	return c ? pm_qos_read_value(c) : 0;
	}

	/**
	* dev_pm_qos_read_value - Get PM QoS constraint for a given device (locked).
	* @dev: Device to get the PM QoS constraint value for.
	*/
	s32 dev_pm_qos_read_value(struct device *dev)
	{
	unsigned long flags;
	s32 ret;

	spin_lock_irqsave(&dev->power.lock, flags);
	ret = __dev_pm_qos_read_value(dev);
	spin_unlock_irqrestore(&dev->power.lock, flags);

	return ret;
	}

	/*
	* apply_constraint
	* @req: constraint request to apply
	* @action: action to perform add/update/remove, of type enum pm_qos_req_action
	* @value: defines the qos request
	*
	* Internal function to update the constraints list using the PM QoS core
	* code and if needed call the per-device and the global notification
	* callbacks
	*/
	static int apply_constraint(struct dev_pm_qos_request *req,
	enum pm_qos_req_action action, int value)
	{
	int ret, curr_value;

	ret = pm_qos_update_target(req->dev->power.constraints,
	&req->node, action, value);

	if (ret) {
	/* Call the global callbacks if needed */
	curr_value = pm_qos_read_value(req->dev->power.constraints);
	blocking_notifier_call_chain(&dev_pm_notifiers,
	(unsigned long)curr_value,
	req);
	}

	return ret;
	}

	/*
	* dev_pm_qos_constraints_allocate
	* @dev: device to allocate data for
	*
	* Called at the first call to add_request, for constraint data allocation
	* Must be called with the dev_pm_qos_mtx mutex held
	*/
	static int dev_pm_qos_constraints_allocate(struct device *dev)
	{
	struct pm_qos_constraints *c;
	struct blocking_notifier_head *n;

	c = kzalloc(sizeof(*c), GFP_KERNEL);
	if (!c)
	return -ENOMEM;

	n = kzalloc(sizeof(*n), GFP_KERNEL);
	if (!n) {
	kfree(c);
	return -ENOMEM;
	}
	BLOCKING_INIT_NOTIFIER_HEAD(n);

	plist_head_init(&c->list);
	c->target_value = PM_QOS_DEV_LAT_DEFAULT_VALUE;
	c->default_value = PM_QOS_DEV_LAT_DEFAULT_VALUE;
	c->type = PM_QOS_MIN;
	c->notifiers = n;

	spin_lock_irq(&dev->power.lock);
	dev->power.constraints = c;
	spin_unlock_irq(&dev->power.lock);

	return 0;
	}

	/**
	* dev_pm_qos_constraints_init - Initalize device's PM QoS constraints pointer.
	* @dev: target device
	*
	* Called from the device PM subsystem during device insertion under
	* device_pm_lock().
	*/
	void dev_pm_qos_constraints_init(struct device *dev)
	{
	mutex_lock(&dev_pm_qos_mtx);
	dev->power.constraints = NULL;
	dev->power.power_state = PMSG_ON;
	mutex_unlock(&dev_pm_qos_mtx);
	}

	/**
	* dev_pm_qos_constraints_destroy
	* @dev: target device
	*
	* Called from the device PM subsystem on device removal under device_pm_lock().
	*/
	void dev_pm_qos_constraints_destroy(struct device *dev)
	{
	struct dev_pm_qos_request req, tmp;
	struct pm_qos_constraints *c;

	/*
	* If the device's PM QoS resume latency limit has been exposed to user
	* space, it has to be hidden at this point.
	*/
	dev_pm_qos_hide_latency_limit(dev);

	mutex_lock(&dev_pm_qos_mtx);

	dev->power.power_state = PMSG_INVALID;
	c = dev->power.constraints;
	if (!c)
	goto out;

	/* Flush the constraints list for the device */
	plist_for_each_entry_safe(req, tmp, &c->list, node) {
	/*
	* Update constraints list and call the notification
	* callbacks if needed
	*/
	apply_constraint(req, PM_QOS_REMOVE_REQ, PM_QOS_DEFAULT_VALUE);
	memset(req, 0, sizeof(*req));
	}

	spin_lock_irq(&dev->power.lock);
	dev->power.constraints = NULL;
	spin_unlock_irq(&dev->power.lock);

	kfree(c->notifiers);
	kfree(c);

	out:
	mutex_unlock(&dev_pm_qos_mtx);
	}

	/**
	* dev_pm_qos_add_request - inserts new qos request into the list
	* @dev: target device for the constraint
	* @req: pointer to a preallocated handle
	* @value: defines the qos request
	*
	* This function inserts a new entry in the device constraints list of
	* requested qos performance characteristics. It recomputes the aggregate
	* QoS expectations of parameters and initializes the dev_pm_qos_request
	* handle. Caller needs to save this handle for later use in updates and
	* removal.
	*
	* Returns 1 if the aggregated constraint value has changed,
	* 0 if the aggregated constraint value has not changed,
	* -EINVAL in case of wrong parameters, -ENOMEM if there's not enough memory
	* to allocate for data structures, -ENODEV if the device has just been removed
	* from the system.
	*/
	int dev_pm_qos_add_request(struct device dev, struct dev_pm_qos_request req,
	s32 value)
	{
	int ret = 0;

	if (!dev \|\| !req) /guard against callers passing in null /
	return -EINVAL;

	if (WARN(dev_pm_qos_request_active(req),
	"%s() called for already added request\n", __func__))
	return -EINVAL;

	req->dev = dev;

	mutex_lock(&dev_pm_qos_mtx);

	if (!dev->power.constraints) {
	if (dev->power.power_state.event == PM_EVENT_INVALID) {
	/* The device has been removed from the system. */
	req->dev = NULL;
	ret = -ENODEV;
	goto out;
	} else {
	/*
	* Allocate the constraints data on the first call to
	* add_request, i.e. only if the data is not already
	* allocated and if the device has not been removed.
	*/
	ret = dev_pm_qos_constraints_allocate(dev);
	}
	}

	if (!ret)
	ret = apply_constraint(req, PM_QOS_ADD_REQ, value);

	out:
	mutex_unlock(&dev_pm_qos_mtx);

	return ret;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_add_request);

	/**
	* dev_pm_qos_update_request - modifies an existing qos request
	* @req : handle to list element holding a dev_pm_qos request to use
	* @new_value: defines the qos request
	*
	* Updates an existing dev PM qos request along with updating the
	* target value.
	*
	* Attempts are made to make this code callable on hot code paths.
	*
	* Returns 1 if the aggregated constraint value has changed,
	* 0 if the aggregated constraint value has not changed,
	* -EINVAL in case of wrong parameters, -ENODEV if the device has been
	* removed from the system
	*/
	int dev_pm_qos_update_request(struct dev_pm_qos_request *req,
	s32 new_value)
	{
	int ret = 0;

	if (!req) /guard against callers passing in null /
	return -EINVAL;

	if (WARN(!dev_pm_qos_request_active(req),
	"%s() called for unknown object\n", __func__))
	return -EINVAL;

	mutex_lock(&dev_pm_qos_mtx);

	if (req->dev->power.constraints) {
	if (new_value != req->node.prio)
	ret = apply_constraint(req, PM_QOS_UPDATE_REQ,
	new_value);
	} else {
	/* Return if the device has been removed */
	ret = -ENODEV;
	}

	mutex_unlock(&dev_pm_qos_mtx);
	return ret;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_update_request);

	/**
	* dev_pm_qos_remove_request - modifies an existing qos request
	* @req: handle to request list element
	*
	* Will remove pm qos request from the list of constraints and
	* recompute the current target value. Call this on slow code paths.
	*
	* Returns 1 if the aggregated constraint value has changed,
	* 0 if the aggregated constraint value has not changed,
	* -EINVAL in case of wrong parameters, -ENODEV if the device has been
	* removed from the system
	*/
	int dev_pm_qos_remove_request(struct dev_pm_qos_request *req)
	{
	int ret = 0;

	if (!req) /guard against callers passing in null /
	return -EINVAL;

	if (WARN(!dev_pm_qos_request_active(req),
	"%s() called for unknown object\n", __func__))
	return -EINVAL;

	mutex_lock(&dev_pm_qos_mtx);

	if (req->dev->power.constraints) {
	ret = apply_constraint(req, PM_QOS_REMOVE_REQ,
	PM_QOS_DEFAULT_VALUE);
	memset(req, 0, sizeof(*req));
	} else {
	/* Return if the device has been removed */
	ret = -ENODEV;
	}

	mutex_unlock(&dev_pm_qos_mtx);
	return ret;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_remove_request);

	/**
	* dev_pm_qos_add_notifier - sets notification entry for changes to target value
	* of per-device PM QoS constraints
	*
	* @dev: target device for the constraint
	* @notifier: notifier block managed by caller.
	*
	* Will register the notifier into a notification chain that gets called
	* upon changes to the target value for the device.
	*
	* If the device's constraints object doesn't exist when this routine is called,
	* it will be created (or error code will be returned if that fails).
	*/
	int dev_pm_qos_add_notifier(struct device dev, struct notifier_block notifier)
	{
	int ret = 0;

	mutex_lock(&dev_pm_qos_mtx);

	if (!dev->power.constraints)
	ret = dev->power.power_state.event != PM_EVENT_INVALID ?
	dev_pm_qos_constraints_allocate(dev) : -ENODEV;

	if (!ret)
	ret = blocking_notifier_chain_register(
	dev->power.constraints->notifiers, notifier);

	mutex_unlock(&dev_pm_qos_mtx);
	return ret;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_add_notifier);

	/**
	* dev_pm_qos_remove_notifier - deletes notification for changes to target value
	* of per-device PM QoS constraints
	*
	* @dev: target device for the constraint
	* @notifier: notifier block to be removed.
	*
	* Will remove the notifier from the notification chain that gets called
	* upon changes to the target value.
	*/
	int dev_pm_qos_remove_notifier(struct device *dev,
	struct notifier_block *notifier)
	{
	int retval = 0;

	mutex_lock(&dev_pm_qos_mtx);

	/* Silently return if the constraints object is not present. */
	if (dev->power.constraints)
	retval = blocking_notifier_chain_unregister(
	dev->power.constraints->notifiers,
	notifier);

	mutex_unlock(&dev_pm_qos_mtx);
	return retval;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_remove_notifier);

	/**
	* dev_pm_qos_add_global_notifier - sets notification entry for changes to
	* target value of the PM QoS constraints for any device
	*
	* @notifier: notifier block managed by caller.
	*
	* Will register the notifier into a notification chain that gets called
	* upon changes to the target value for any device.
	*/
	int dev_pm_qos_add_global_notifier(struct notifier_block *notifier)
	{
	return blocking_notifier_chain_register(&dev_pm_notifiers, notifier);
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_add_global_notifier);

	/**
	* dev_pm_qos_remove_global_notifier - deletes notification for changes to
	* target value of PM QoS constraints for any device
	*
	* @notifier: notifier block to be removed.
	*
	* Will remove the notifier from the notification chain that gets called
	* upon changes to the target value for any device.
	*/
	int dev_pm_qos_remove_global_notifier(struct notifier_block *notifier)
	{
	return blocking_notifier_chain_unregister(&dev_pm_notifiers, notifier);
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_remove_global_notifier);

	/**
	* dev_pm_qos_add_ancestor_request - Add PM QoS request for device's ancestor.
	* @dev: Device whose ancestor to add the request for.
	* @req: Pointer to the preallocated handle.
	* @value: Constraint latency value.
	*/
	int dev_pm_qos_add_ancestor_request(struct device *dev,
	struct dev_pm_qos_request *req, s32 value)
	{
	struct device *ancestor = dev->parent;
	int error = -ENODEV;

	while (ancestor && !ancestor->power.ignore_children)
	ancestor = ancestor->parent;

	if (ancestor)
	error = dev_pm_qos_add_request(ancestor, req, value);

	if (error)
	req->dev = NULL;

	return error;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_add_ancestor_request);

	#ifdef CONFIG_PM_RUNTIME
	static void __dev_pm_qos_drop_user_request(struct device *dev)
	{
	dev_pm_qos_remove_request(dev->power.pq_req);
	dev->power.pq_req = NULL;
	}

	/**
	* dev_pm_qos_expose_latency_limit - Expose PM QoS latency limit to user space.
	* @dev: Device whose PM QoS latency limit is to be exposed to user space.
	* @value: Initial value of the latency limit.
	*/
	int dev_pm_qos_expose_latency_limit(struct device *dev, s32 value)
	{
	struct dev_pm_qos_request *req;
	int ret;

	if (!device_is_registered(dev) \|\| value < 0)
	return -EINVAL;

	if (dev->power.pq_req)
	return -EEXIST;

	req = kzalloc(sizeof(*req), GFP_KERNEL);
	if (!req)
	return -ENOMEM;

	ret = dev_pm_qos_add_request(dev, req, value);
	if (ret < 0)
	return ret;

	dev->power.pq_req = req;
	ret = pm_qos_sysfs_add(dev);
	if (ret)
	__dev_pm_qos_drop_user_request(dev);

	return ret;
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_expose_latency_limit);

	/**
	* dev_pm_qos_hide_latency_limit - Hide PM QoS latency limit from user space.
	* @dev: Device whose PM QoS latency limit is to be hidden from user space.
	*/
	void dev_pm_qos_hide_latency_limit(struct device *dev)
	{
	if (dev->power.pq_req) {
	pm_qos_sysfs_remove(dev);
	__dev_pm_qos_drop_user_request(dev);
	}
	}
	EXPORT_SYMBOL_GPL(dev_pm_qos_hide_latency_limit);
	#endif /* CONFIG_PM_RUNTIME */