[RFC,1/2] power: Userspace performance QoS

diff --git a/include/linux/perf_qos.h b/include/linux/perf_qos.h
new file mode 100644
index 000000000000..57529c40be4d
--- /dev/null
+++ b/include/linux/perf_qos.h
@@ -0,0 +1,45 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Performance QoS device abstraction
+ *
+ * Copyright (2024) Linaro Ltd
+ *
+ * Author: Daniel Lezcano <daniel.lezcano@linaro.org>
+ *
+ */
+#ifndef __PERF_QOS_H
+#define __PERF_QOS_H
+
+#include <uapi/linux/perf_qos_ioctl.h>
+
+struct perf_qos;
+
+/**
+ * struct perf_qos_value_descr - Performance constraint description
+ *
+ * @unit: the unit used for the constraint (normalized, throughput, ...)
+ * @limit_min: the minimal constraint limit to be set
+ * @limit_max: the maximal constraint limit to be set
+ */
+struct perf_qos_value_descr {
+	perf_qos_unit_t unit;
+	int limit_min;
+	int limit_max;
+};
+
+typedef int (*set_perf_limit_cb_t)(int);
+
+struct perf_qos_ops {
+	set_perf_limit_cb_t set_perf_limit_max;
+	set_perf_limit_cb_t set_perf_limit_min;
+};
+
+extern struct perf_qos *perf_qos_device_create(const char *name,
+					       struct perf_qos_ops *ops,
+					       struct perf_qos_value_descr *descr);
+
+extern int perf_qos_is_allowed(struct perf_qos *pq, int performance);
+
+extern void perf_qos_device_destroy(struct perf_qos *pq);
+
+#endif
diff --git a/include/uapi/linux/perf_qos_ioctl.h b/include/uapi/linux/perf_qos_ioctl.h
new file mode 100644
index 000000000000..a9fb8940c175
--- /dev/null
+++ b/include/uapi/linux/perf_qos_ioctl.h
@@ -0,0 +1,47 @@ 
+/* SPDX-License-Identifier: LGPL-2.0+ WITH Linux-syscall-note */
+/*
+ * Performance QoS device abstraction
+ *
+ * Copyright (2024) Linaro Ltd
+ *
+ * Author: Daniel Lezcano <daniel.lezcano@linaro.org>
+ *
+ */
+#ifndef __PERF_QOS_IOCTL_H
+#define __PERF_QOS_IOCTL_H
+
+#include <linux/types.h>
+
+enum {
+	PERF_QOS_IOC_SET_MIN_CMD,
+	PERF_QOS_IOC_GET_MIN_CMD,
+	PERF_QOS_IOC_SET_MAX_CMD,
+	PERF_QOS_IOC_GET_MAX_CMD,
+	PERF_QOS_IOC_GET_UNIT_CMD,
+	PERF_QOS_IOC_GET_LIMITS_CMD,
+	PERF_QOS_IOC_MAX_CMD,
+};
+
+typedef enum {
+	PERF_QOS_UNIT_NORMAL,
+	PERF_QOS_UNIT_KBPS,
+	PERF_QOS_UNIT_MAX
+} perf_qos_unit_t;
+
+struct perf_qos_ioctl_arg {
+	int value;
+	int limit_min;
+	int limit_max;
+	perf_qos_unit_t unit;
+};
+
+#define PERF_QOS_IOCTL_TYPE 'P'
+
+#define PERF_QOS_IOC_SET_MIN	_IOW(PERF_QOS_IOCTL_TYPE, PERF_QOS_IOC_SET_MIN_CMD,	struct perf_qos_ioctl_arg *)
+#define PERF_QOS_IOC_GET_MIN	_IOR(PERF_QOS_IOCTL_TYPE, PERF_QOS_IOC_GET_MIN_CMD,	struct perf_qos_ioctl_arg *)
+#define PERF_QOS_IOC_SET_MAX	_IOW(PERF_QOS_IOCTL_TYPE, PERF_QOS_IOC_SET_MAX_CMD,	struct perf_qos_ioctl_arg *)
+#define PERF_QOS_IOC_GET_MAX	_IOR(PERF_QOS_IOCTL_TYPE, PERF_QOS_IOC_GET_MAX_CMD,	struct perf_qos_ioctl_arg *)
+#define PERF_QOS_IOC_GET_UNIT	_IOR(PERF_QOS_IOCTL_TYPE, PERF_QOS_IOC_GET_UNIT_CMD,	struct perf_qos_ioctl_arg *)
+#define PERF_QOS_IOC_GET_LIMITS	_IOR(PERF_QOS_IOCTL_TYPE, PERF_QOS_IOC_GET_LIMITS_CMD,	struct perf_qos_ioctl_arg *)
+
+#endif
diff --git a/kernel/power/Makefile b/kernel/power/Makefile
index 874ad834dc8d..e2e4d707ab6e 100644
--- a/kernel/power/Makefile
+++ b/kernel/power/Makefile
@@ -8,7 +8,7 @@  endif
 
 KASAN_SANITIZE_snapshot.o	:= n
 
-obj-y				+= qos.o
+obj-y				+= qos.o perf_qos.o
 obj-$(CONFIG_PM)		+= main.o
 obj-$(CONFIG_VT_CONSOLE_SLEEP)	+= console.o
 obj-$(CONFIG_FREEZER)		+= process.o
diff --git a/kernel/power/perf_qos.c b/kernel/power/perf_qos.c
new file mode 100644
index 000000000000..ca0619b07ae5
--- /dev/null
+++ b/kernel/power/perf_qos.c
@@ -0,0 +1,652 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Performance Quality of Service (Perf QoS) support base.
+ *
+ * Copyright (C) 2024 Linaro Ltd
+ *
+ * Author: Daniel Lezcano <daniel.lezcano@linaro.org>
+ *
+ */
+#include <linux/cdev.h>
+#include <linux/perf_qos.h>
+#include <linux/list_sort.h>
+
+#define DEVNAME "perf_qos"
+#define NUM_PERF_QOS_MINORS 128
+
+static DEFINE_IDR(perf_qos_minors);
+static struct class *perf_qos_class;
+static dev_t perf_qos_devt;
+
+typedef enum {
+	PERF_QOS_LIMIT_MAX,
+	PERF_QOS_LIMIT_MIN,
+} perf_qos_limit_t;
+
+/**
+ * struct perf_qos_constraint - structure holding a constraint information
+ *
+ * @soft_limit: an integer corresponding of the limit value set
+ * @hard_limit: an integer corresponding to the limit value allowed by the driver
+ * @kref: a refcount to the constraint responsible of its life cycle
+ * @set_perf_limit_cb: a callback to notify the backend driver about the limit change
+ * @node: the list node to attach this constraint with the list of constraints
+ * @head: the list of constraints the @node
+ *
+ * This structure has a couple of instanciation per perf QoS file
+ * opened by a process. The process can apply one or two constraints
+ * to the device.
+ *
+ * Other processes will allocate their own constraints which will be
+ * added in the list of constraints.
+ */
+struct perf_qos_constraint {
+	int soft_limit;
+	int hard_limit;
+	set_perf_limit_cb_t set_perf_limit_cb;
+	struct kref kref;
+	struct list_head node;
+	struct list_head *head;
+};
+
+/**
+ * struct perf_qos - structure owning the constraint information for
+ * 			the device
+ *
+ * @lock: lock to protect the actions on the list of constraints
+ * @perf_qos_cdev: a struct cdev used for the device destruction
+ * @ops: the ops given by the backend driver to notify the change of constraint
+ * @descr: a constraint descriptor giving the units and the boundaries
+ * @perf_min: the list of the minimal performance constraints
+ * @perf_max: the list of the maximal performance constraints
+ */
+struct perf_qos {
+	spinlock_t lock;
+	struct cdev perf_qos_cdev;
+	struct perf_qos_ops *ops;
+	struct perf_qos_value_descr *descr;
+	struct list_head perf_min;
+	struct list_head perf_max;
+};
+
+/**
+ * struct perf_qos_data - structure with the requested constraints
+ *
+ * @pqc_min: the requested performance constraint giving the minimal value
+ * @pqc_max: the requested performance constraint giving the maximal value
+ */
+struct perf_qos_data {
+	struct perf_qos_constraint *pqc_min;
+	struct perf_qos_constraint *pqc_max;
+};
+
+static struct perf_qos_constraint *perf_qos_constraint_find(struct list_head *list, int value)
+{
+	struct perf_qos_constraint *pcq;
+
+	list_for_each_entry(pcq, list, node) {
+		if (pcq->soft_limit == value)
+			return pcq;
+	}
+
+	return NULL;
+}
+
+static int perf_qos_constraint_cmp(void *data,
+				   const struct list_head *l1,
+				   const struct list_head *l2)
+{
+	struct perf_qos_constraint *pqc1 = container_of(l1, struct perf_qos_constraint, node);
+	struct perf_qos_constraint *pqc2 = container_of(l2, struct perf_qos_constraint, node);
+
+	/*
+	 * The comparison will depend if we apply a max or min
+	 * performance constraint. If the soft limit is lesser than
+	 * the hard limit, that means it is a maximum limitation.
+	 */
+	if (pqc1->soft_limit < pqc1->hard_limit)
+		return pqc1->soft_limit - pqc2->soft_limit;
+
+	return pqc2->soft_limit - pqc1->soft_limit;
+}
+
+static int perf_qos_del(struct perf_qos_constraint *pcq)
+{
+	const struct perf_qos_constraint *first;
+	int new_limit;
+
+	first = list_first_entry(pcq->head, struct perf_qos_constraint, node);
+
+	list_del(&pcq->node);
+
+	/*
+	 * The active constraint is not the one we removed, so there
+	 * is nothing more to do
+	 */
+	if (first != pcq)
+		return 0;
+
+	/*
+	 * As we remove the first entry, then get the new first entry
+	 * to apply the next constraint. If there is no more
+	 * constraint set, reset to the original limit. Otherwise, use
+	 * the new constraint value.
+	 */
+	if (list_empty(pcq->head))
+		new_limit = pcq->hard_limit;
+	else {
+		first = list_first_entry(pcq->head, struct perf_qos_constraint, node);
+		new_limit = first->soft_limit;
+	};
+
+	/*
+	 * Notify the backend driver to update its performance level
+	 * if needed. If the performance level is currently inside the
+	 * new limits, nothing will happen. Otherwise it must be
+	 * adjust the current performance level to be inside the
+	 * authorized limits
+	 */
+	pcq->set_perf_limit_cb(new_limit);
+
+	return 1;
+}
+
+static int perf_qos_add(struct perf_qos_constraint *pcq)
+{
+	const struct perf_qos_constraint *first;
+
+	list_add(&pcq->node, pcq->head);
+
+	list_sort(NULL, pcq->head, perf_qos_constraint_cmp);
+
+	/*
+	 * A sort happened resulting in a different constraint at the head
+	 */
+	first = list_first_entry(pcq->head, struct perf_qos_constraint, node);
+
+	/*
+	 * The inserted constraint did not become the active one, so
+	 * we can bail out
+	 */
+	if (pcq != first)
+		return 0;
+
+	/*
+	 * Notify the backend driver to update its performance level
+	 * if needed. If the performance level is currently inside the
+	 * new limits, nothing will happen. Otherwise it must be
+	 * adjust the current performance level to be inside the
+	 * authorized limits
+	 */
+	pcq->set_perf_limit_cb(first->soft_limit);
+
+	return 1;
+}
+
+static void perf_qos_constraint_release(struct kref *kref)
+{
+	struct perf_qos_constraint *pcq;
+
+	pcq = container_of(kref, struct perf_qos_constraint, kref);
+
+	/*
+	 * The removal of the constraint results in the change of the
+	 * first entry of the list which means it was the active
+	 * one. We need to apply the next constraint of the list
+	 */
+	if (perf_qos_del(pcq)) {
+		/* Something to do */
+	}
+
+	kfree(pcq);
+}
+
+static void perf_qos_constraint_put(struct perf_qos_constraint *pcq)
+{
+	kref_put(&pcq->kref, perf_qos_constraint_release);
+}
+
+static void perf_qos_constraint_get(struct perf_qos_constraint *pcq)
+{
+	kref_get(&pcq->kref);
+}
+
+static struct perf_qos_constraint *perf_qos_constraint_alloc(struct perf_qos *pq, int soft_limit,
+							     struct list_head *perf, perf_qos_limit_t limit)
+{
+	struct perf_qos_constraint *pqc;
+
+	pqc = kzalloc(sizeof(*pqc), GFP_KERNEL);
+	if (!pqc)
+		return NULL;
+
+	kref_init(&pqc->kref);
+	INIT_LIST_HEAD(&pqc->node);
+
+	if (limit == PERF_QOS_LIMIT_MAX) {
+		pqc->set_perf_limit_cb = pq->ops->set_perf_limit_max;
+		pqc->hard_limit = pq->descr->limit_max;
+	} else {
+		pqc->set_perf_limit_cb = pq->ops->set_perf_limit_min;
+		pqc->hard_limit = pq->descr->limit_min;
+	}
+
+	pqc->head = perf;
+	pqc->soft_limit = soft_limit;
+
+	return pqc;
+}
+
+static int perf_qos_open(struct inode *inode, struct file *file)
+{
+	struct perf_qos_data *pqd;
+	struct perf_qos *pq;
+
+	pq = idr_find(&perf_qos_minors, iminor(inode));
+	if (!pq)
+		return -ENODEV;
+
+	inode->i_private = pq;
+
+	pqd = kzalloc(sizeof(*pqd), GFP_KERNEL);
+	if (!pqd)
+		return -ENOMEM;
+
+	file->private_data = pqd;
+
+	return 0;
+}
+
+static int perf_qos_release(struct inode *inode, struct file *file)
+{
+	struct perf_qos *pq = inode->i_private;
+	struct perf_qos_data *pqd = file->private_data;
+
+	spin_lock(&pq->lock);
+
+	if (pqd->pqc_min)
+		perf_qos_constraint_put(pqd->pqc_min);
+
+	if (pqd->pqc_max)
+		perf_qos_constraint_put(pqd->pqc_max);
+
+	spin_unlock(&pq->lock);
+
+	kfree(pqd);
+
+	return 0;
+}
+
+static int perf_qos_unset(struct perf_qos_constraint **cur_pqc,
+			  perf_qos_limit_t limit, struct list_head *perf, int value)
+{
+	/*
+	 * Removing a constraint:
+	 *
+	 * - if it exists then *current_pqc is set. We decrement the
+         *   refcount and update the current constraint by setting it
+         *   to NULL
+	 *
+	 * - if the current constraint does not exist then, it is an
+         *   error and we should exit with an error
+	 */
+	if (!(*cur_pqc))
+		return -EINVAL;
+
+	perf_qos_constraint_put(*cur_pqc);
+	*cur_pqc = NULL;
+
+	return 0;
+}
+
+static int perf_qos_set(struct perf_qos *pq, struct perf_qos_constraint **cur_pqc,
+			perf_qos_limit_t limit, struct list_head *perf, int value)
+{
+	struct perf_qos_constraint *pqc;
+	int ret = 0;
+
+	/*
+	 * We are trying to set the same constraint.
+	 */
+	if (*cur_pqc && ((*cur_pqc)->soft_limit == value)) {
+		ret = -EALREADY;
+		goto out;
+	}
+
+	/*
+	 * Case 2 : Adding a constraint:
+	 *
+	 * - it already exists because it was created by another
+	 *   process, we increment the refcount
+	 *
+	 * - it already exists because we created it before, we
+	 *   return an error
+	 *
+	 * - it does not exist but there is a previous different
+         *   constraint we set before. It is a constraint change. We
+         *   must release the previous constraint and create a new
+         *   one. However, we apply the new constraint and then we
+         *   remove the old one in order to not have the backend
+         *   driver with a window where there is no constraint at all
+	 *
+	 * - it does not exist and there is no previous constraint. It
+         *   is a new constraint. We allocate the constraint, apply it
+         *   and set it as the current constraint
+	 */
+	pqc = perf_qos_constraint_find(perf, value);
+	if (pqc) {
+		perf_qos_constraint_get(pqc);
+	} else {
+		pqc = perf_qos_constraint_alloc(pq, value, perf, limit);
+		if (!pqc) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		/*
+		 * The new constraint has to be applied because it
+		 * results in a change of the first entry of the list
+		 * of constraints
+		 */
+		if (perf_qos_add(pqc)) {
+			/* Something to do */
+		}
+	}
+
+	/*
+	 * We previously set a constraint, let's release the refcount
+	 * as we change it. The constraint can be freed if we are the
+	 * last one having a reference to it or if we are the creator
+	 * and no other process held a refcount on it.
+	 */
+	if ((*cur_pqc))
+		perf_qos_constraint_put(*cur_pqc);
+
+	*cur_pqc = pqc;
+out:
+	return ret;
+}
+
+static int ioctl_perf_qos_set_max(struct perf_qos *pq,
+				  struct perf_qos_data *pqd,
+				  struct perf_qos_ioctl_arg *pqia)
+{
+	if (pqia->value > pq->descr->limit_max)
+		return -EINVAL;
+
+	if (pqia->value == pq->descr->limit_max)
+		return perf_qos_unset(&pqd->pqc_max, PERF_QOS_LIMIT_MAX,
+				    &pq->perf_max, pqia->value);
+	else
+		return perf_qos_set(pq, &pqd->pqc_max, PERF_QOS_LIMIT_MAX,
+				    &pq->perf_max, pqia->value);
+}
+
+static int ioctl_perf_qos_set_min(struct perf_qos *pq,
+				  struct perf_qos_data *pqd,
+				  struct perf_qos_ioctl_arg *pqia)
+{
+	if (pqia->value < pq->descr->limit_min)
+		return -EINVAL;
+
+	if (pqia->value == pq->descr->limit_min)
+		return perf_qos_unset(&pqd->pqc_min, PERF_QOS_LIMIT_MIN,
+				    &pq->perf_min, pqia->value);
+	else
+		return perf_qos_set(pq, &pqd->pqc_min, PERF_QOS_LIMIT_MIN,
+				    &pq->perf_min, pqia->value);
+}
+
+static int perf_qos_get(struct list_head *perf, int *value)
+{
+	struct perf_qos_constraint *pqc;
+
+	/*
+	 * We may not have set any performance constraint yet but
+	 * another process may have set one, so we get the head of
+	 * performance constraint list
+	 */
+	if (list_empty(perf))
+		return -ENODATA;
+
+	pqc = list_first_entry(perf, struct perf_qos_constraint, node);
+
+	*value = pqc->soft_limit;
+
+	return 0;
+}
+
+static int ioctl_perf_qos_get_min(struct perf_qos *pq,
+				  struct perf_qos_data *pqd,
+				  struct perf_qos_ioctl_arg *pqia)
+{
+	return perf_qos_get(&pq->perf_min, &pqia->value);
+}
+
+static int ioctl_perf_qos_get_max(struct perf_qos *pq,
+				  struct perf_qos_data *pqd,
+				  struct perf_qos_ioctl_arg *pqia)
+{
+	return perf_qos_get(&pq->perf_max, &pqia->value);
+}
+
+static int ioctl_perf_qos_get_unit(struct perf_qos *pq,
+				   struct perf_qos_data *pqd,
+				   struct perf_qos_ioctl_arg *pqia)
+{
+	pqia->unit = pq->descr->unit;
+
+	return 0;
+}
+
+static int ioctl_perf_qos_get_limits(struct perf_qos *pq,
+				     struct perf_qos_data *pqd,
+				     struct perf_qos_ioctl_arg *pqia)
+{
+	pqia->limit_min = pq->descr->limit_min;
+	pqia->limit_max = pq->descr->limit_max;
+
+	return 0;
+}
+
+typedef int (*perf_qos_ioctl_ops_t)(struct perf_qos *pq,
+				    struct perf_qos_data *pqd,
+				    struct perf_qos_ioctl_arg *pqia);
+
+static long perf_qos_ioctl(struct file *file, unsigned int ucmd,
+			   unsigned long arg)
+{
+	struct perf_qos_data *pqd = file->private_data;
+	struct perf_qos *pq = file->f_inode->i_private;
+	struct perf_qos_ioctl_arg pqia;
+	int cmd = _IOC_NR(ucmd);
+	int dir = _IOC_DIR(ucmd);
+	int type = _IOC_TYPE(ucmd);
+	int ret;
+
+	perf_qos_ioctl_ops_t perf_qos_ioctl_ops[] = {
+		[PERF_QOS_IOC_SET_MAX_CMD]  	= ioctl_perf_qos_set_max,
+		[PERF_QOS_IOC_SET_MIN_CMD]  	= ioctl_perf_qos_set_min,
+		[PERF_QOS_IOC_GET_MAX_CMD]  	= ioctl_perf_qos_get_max,
+		[PERF_QOS_IOC_GET_MIN_CMD]  	= ioctl_perf_qos_get_min,
+		[PERF_QOS_IOC_GET_UNIT_CMD] 	= ioctl_perf_qos_get_unit,
+		[PERF_QOS_IOC_GET_LIMITS_CMD] 	= ioctl_perf_qos_get_limits,
+	};
+
+	if (type != PERF_QOS_IOCTL_TYPE)
+		return -EINVAL;
+
+	if (cmd < 0 || cmd >= PERF_QOS_IOC_MAX_CMD)
+		return -EINVAL;
+
+	if (dir & _IOC_WRITE) {
+		if (copy_from_user(&pqia, (typeof(pqia) *)arg, sizeof(pqia)))
+			return -EACCES;
+	}
+
+	spin_lock(&pq->lock);
+	ret = perf_qos_ioctl_ops[cmd](pq, pqd, &pqia);
+	spin_unlock(&pq->lock);
+
+	if (ret)
+		goto out;
+
+	if (dir & _IOC_READ) {
+		if (copy_to_user((typeof(pqia) *)arg, &pqia, sizeof(pqia)))
+			return -EACCES;
+	}
+out:
+	return ret;
+}
+
+static const struct file_operations perf_qos_fops = {
+	.owner          = THIS_MODULE,
+	.open		= perf_qos_open,
+	.release	= perf_qos_release,
+	.unlocked_ioctl = perf_qos_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl	= perf_qos_ioctl,
+#endif
+};
+
+void perf_qos_device_destroy(struct perf_qos *pq)
+{
+	idr_remove(&perf_qos_minors, MINOR(pq->perf_qos_cdev.dev));
+	device_destroy(perf_qos_class, pq->perf_qos_cdev.dev);
+	cdev_del(&pq->perf_qos_cdev);
+	kfree(pq->descr);
+	kfree(pq->ops);
+	kfree(pq);
+}
+EXPORT_SYMBOL_GPL(perf_qos_device_destroy);
+
+int perf_qos_is_allowed(struct perf_qos *pq, int performance)
+{
+	const struct perf_qos_constraint *first;
+	int allowed = 1;
+
+	spin_lock(&pq->lock);
+
+	first = list_first_entry(&pq->perf_min, struct perf_qos_constraint, node);
+	if (performance < first->soft_limit)
+		allowed = 0;
+
+	first = list_first_entry(&pq->perf_max, struct perf_qos_constraint, node);
+	if (performance > first->soft_limit)
+		allowed = 0;
+
+	spin_unlock(&pq->lock);
+
+	return allowed;
+}
+EXPORT_SYMBOL_GPL(perf_qos_is_allowed);
+
+struct perf_qos *perf_qos_device_create(const char *name,
+					struct perf_qos_ops *ops,
+					struct perf_qos_value_descr *descr)
+{
+	struct device *dev;
+	struct perf_qos *pq;
+	dev_t devt;
+	int minor;
+	int ret;
+
+	if (!ops->set_perf_limit_max || !ops->set_perf_limit_min)
+		return ERR_PTR(-EINVAL);
+
+	if (descr->unit < 0 || descr->unit >= PERF_QOS_UNIT_MAX)
+		return ERR_PTR(-EINVAL);
+
+	if (descr->limit_min > descr->limit_max)
+		return ERR_PTR(-EINVAL);
+
+	if (descr->unit == PERF_QOS_UNIT_NORMAL) {
+		if (descr->limit_min < 0 || descr->limit_max > 1024)
+			return ERR_PTR(-EINVAL);
+	}
+
+	pq = kzalloc(sizeof(*pq), GFP_KERNEL);
+	if (!pq)
+		return ERR_PTR(-ENOMEM);
+
+	INIT_LIST_HEAD(&pq->perf_min);
+	INIT_LIST_HEAD(&pq->perf_max);
+	spin_lock_init(&pq->lock);
+
+	pq->ops = kmemdup(ops, sizeof(*ops), GFP_KERNEL);
+	if (!pq->ops) {
+		ret = -ENOMEM;
+		goto out_kfree_pq;
+	}
+
+	pq->descr = kmemdup(descr, sizeof(*descr), GFP_KERNEL);
+	if (!pq->descr) {
+		ret = -ENOMEM;
+		goto out_kfree_pq_ops;
+	}
+
+	minor = idr_alloc(&perf_qos_minors, pq, 0,
+			  NUM_PERF_QOS_MINORS, GFP_KERNEL);
+	if (minor < 0)
+		goto out_kfree_pq_descr;
+
+	devt = MKDEV(MAJOR(perf_qos_devt), minor);
+
+	cdev_init(&pq->perf_qos_cdev, &perf_qos_fops);
+
+	ret = cdev_add(&pq->perf_qos_cdev, devt, 1);
+	if (ret < 0)
+		goto out_idr_remove;
+
+	dev = device_create(perf_qos_class, NULL, devt, NULL, name);
+	if (IS_ERR(dev)) {
+		ret = PTR_ERR(dev);
+		goto out_cdev_del;
+	}
+
+	return pq;
+
+out_cdev_del:
+	cdev_del(&pq->perf_qos_cdev);
+
+out_idr_remove:
+	idr_remove(&perf_qos_minors, minor);
+
+out_kfree_pq_descr:
+	kfree(pq->descr);
+
+out_kfree_pq_ops:
+	kfree(pq->ops);
+
+out_kfree_pq:
+	kfree(pq);
+
+	return ERR_PTR(ret);
+}
+EXPORT_SYMBOL_GPL(perf_qos_device_create);
+
+static char *perf_qos_devnode(const struct device *dev, umode_t *mode)
+{
+	return kasprintf(GFP_KERNEL, "%s/%s", DEVNAME, dev_name(dev));
+}
+
+static int perf_qos_init(void)
+{
+	int ret;
+
+	ret = alloc_chrdev_region(&perf_qos_devt, 0,
+				  NUM_PERF_QOS_MINORS, DEVNAME);
+	if (ret)
+		return ret;
+
+	perf_qos_class = class_create(DEVNAME);
+	if (IS_ERR(perf_qos_class)) {
+		unregister_chrdev_region(perf_qos_devt, NUM_PERF_QOS_MINORS);
+		return PTR_ERR(perf_qos_class);
+	}
+	perf_qos_class->devnode = perf_qos_devnode;
+
+	return 0;
+}
+
+subsys_initcall(perf_qos_init);