linux/drivers/devfreq/devfreq.c
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   1/*
   2 * devfreq: Generic Dynamic Voltage and Frequency Scaling (DVFS) Framework
   3 *          for Non-CPU Devices.
   4 *
   5 * Copyright (C) 2011 Samsung Electronics
   6 *      MyungJoo Ham <myungjoo.ham@samsung.com>
   7 *
   8 * This program is free software; you can redistribute it and/or modify
   9 * it under the terms of the GNU General Public License version 2 as
  10 * published by the Free Software Foundation.
  11 */
  12
  13#include <linux/kernel.h>
  14#include <linux/sched.h>
  15#include <linux/errno.h>
  16#include <linux/err.h>
  17#include <linux/init.h>
  18#include <linux/module.h>
  19#include <linux/slab.h>
  20#include <linux/stat.h>
  21#include <linux/opp.h>
  22#include <linux/devfreq.h>
  23#include <linux/workqueue.h>
  24#include <linux/platform_device.h>
  25#include <linux/list.h>
  26#include <linux/printk.h>
  27#include <linux/hrtimer.h>
  28#include "governor.h"
  29
  30struct class *devfreq_class;
  31
  32/*
  33 * devfreq_work periodically monitors every registered device.
  34 * The minimum polling interval is one jiffy. The polling interval is
  35 * determined by the minimum polling period among all polling devfreq
  36 * devices. The resolution of polling interval is one jiffy.
  37 */
  38static bool polling;
  39static struct workqueue_struct *devfreq_wq;
  40static struct delayed_work devfreq_work;
  41
  42/* wait removing if this is to be removed */
  43static struct devfreq *wait_remove_device;
  44
  45/* The list of all device-devfreq */
  46static LIST_HEAD(devfreq_list);
  47static DEFINE_MUTEX(devfreq_list_lock);
  48
  49/**
  50 * find_device_devfreq() - find devfreq struct using device pointer
  51 * @dev:        device pointer used to lookup device devfreq.
  52 *
  53 * Search the list of device devfreqs and return the matched device's
  54 * devfreq info. devfreq_list_lock should be held by the caller.
  55 */
  56static struct devfreq *find_device_devfreq(struct device *dev)
  57{
  58        struct devfreq *tmp_devfreq;
  59
  60        if (unlikely(IS_ERR_OR_NULL(dev))) {
  61                pr_err("DEVFREQ: %s: Invalid parameters\n", __func__);
  62                return ERR_PTR(-EINVAL);
  63        }
  64        WARN(!mutex_is_locked(&devfreq_list_lock),
  65             "devfreq_list_lock must be locked.");
  66
  67        list_for_each_entry(tmp_devfreq, &devfreq_list, node) {
  68                if (tmp_devfreq->dev.parent == dev)
  69                        return tmp_devfreq;
  70        }
  71
  72        return ERR_PTR(-ENODEV);
  73}
  74
  75/**
  76 * update_devfreq() - Reevaluate the device and configure frequency.
  77 * @devfreq:    the devfreq instance.
  78 *
  79 * Note: Lock devfreq->lock before calling update_devfreq
  80 *       This function is exported for governors.
  81 */
  82int update_devfreq(struct devfreq *devfreq)
  83{
  84        unsigned long freq;
  85        int err = 0;
  86        u32 flags = 0;
  87
  88        if (!mutex_is_locked(&devfreq->lock)) {
  89                WARN(true, "devfreq->lock must be locked by the caller.\n");
  90                return -EINVAL;
  91        }
  92
  93        /* Reevaluate the proper frequency */
  94        err = devfreq->governor->get_target_freq(devfreq, &freq);
  95        if (err)
  96                return err;
  97
  98        /*
  99         * Adjust the freuqency with user freq and QoS.
 100         *
 101         * List from the highest proiority
 102         * max_freq (probably called by thermal when it's too hot)
 103         * min_freq
 104         */
 105
 106        if (devfreq->min_freq && freq < devfreq->min_freq) {
 107                freq = devfreq->min_freq;
 108                flags &= ~DEVFREQ_FLAG_LEAST_UPPER_BOUND; /* Use GLB */
 109        }
 110        if (devfreq->max_freq && freq > devfreq->max_freq) {
 111                freq = devfreq->max_freq;
 112                flags |= DEVFREQ_FLAG_LEAST_UPPER_BOUND; /* Use LUB */
 113        }
 114
 115        err = devfreq->profile->target(devfreq->dev.parent, &freq, flags);
 116        if (err)
 117                return err;
 118
 119        devfreq->previous_freq = freq;
 120        return err;
 121}
 122
 123/**
 124 * devfreq_notifier_call() - Notify that the device frequency requirements
 125 *                         has been changed out of devfreq framework.
 126 * @nb          the notifier_block (supposed to be devfreq->nb)
 127 * @type        not used
 128 * @devp        not used
 129 *
 130 * Called by a notifier that uses devfreq->nb.
 131 */
 132static int devfreq_notifier_call(struct notifier_block *nb, unsigned long type,
 133                                 void *devp)
 134{
 135        struct devfreq *devfreq = container_of(nb, struct devfreq, nb);
 136        int ret;
 137
 138        mutex_lock(&devfreq->lock);
 139        ret = update_devfreq(devfreq);
 140        mutex_unlock(&devfreq->lock);
 141
 142        return ret;
 143}
 144
 145/**
 146 * _remove_devfreq() - Remove devfreq from the device.
 147 * @devfreq:    the devfreq struct
 148 * @skip:       skip calling device_unregister().
 149 *
 150 * Note that the caller should lock devfreq->lock before calling
 151 * this. _remove_devfreq() will unlock it and free devfreq
 152 * internally. devfreq_list_lock should be locked by the caller
 153 * as well (not relased at return)
 154 *
 155 * Lock usage:
 156 * devfreq->lock: locked before call.
 157 *                unlocked at return (and freed)
 158 * devfreq_list_lock: locked before call.
 159 *                    kept locked at return.
 160 *                    if devfreq is centrally polled.
 161 *
 162 * Freed memory:
 163 * devfreq
 164 */
 165static void _remove_devfreq(struct devfreq *devfreq, bool skip)
 166{
 167        if (!mutex_is_locked(&devfreq->lock)) {
 168                WARN(true, "devfreq->lock must be locked by the caller.\n");
 169                return;
 170        }
 171        if (!devfreq->governor->no_central_polling &&
 172            !mutex_is_locked(&devfreq_list_lock)) {
 173                WARN(true, "devfreq_list_lock must be locked by the caller.\n");
 174                return;
 175        }
 176
 177        if (devfreq->being_removed)
 178                return;
 179
 180        devfreq->being_removed = true;
 181
 182        if (devfreq->profile->exit)
 183                devfreq->profile->exit(devfreq->dev.parent);
 184
 185        if (devfreq->governor->exit)
 186                devfreq->governor->exit(devfreq);
 187
 188        if (!skip && get_device(&devfreq->dev)) {
 189                device_unregister(&devfreq->dev);
 190                put_device(&devfreq->dev);
 191        }
 192
 193        if (!devfreq->governor->no_central_polling)
 194                list_del(&devfreq->node);
 195
 196        mutex_unlock(&devfreq->lock);
 197        mutex_destroy(&devfreq->lock);
 198
 199        kfree(devfreq);
 200}
 201
 202/**
 203 * devfreq_dev_release() - Callback for struct device to release the device.
 204 * @dev:        the devfreq device
 205 *
 206 * This calls _remove_devfreq() if _remove_devfreq() is not called.
 207 * Note that devfreq_dev_release() could be called by _remove_devfreq() as
 208 * well as by others unregistering the device.
 209 */
 210static void devfreq_dev_release(struct device *dev)
 211{
 212        struct devfreq *devfreq = to_devfreq(dev);
 213        bool central_polling = !devfreq->governor->no_central_polling;
 214
 215        /*
 216         * If devfreq_dev_release() was called by device_unregister() of
 217         * _remove_devfreq(), we cannot mutex_lock(&devfreq->lock) and
 218         * being_removed is already set. This also partially checks the case
 219         * where devfreq_dev_release() is called from a thread other than
 220         * the one called _remove_devfreq(); however, this case is
 221         * dealt completely with another following being_removed check.
 222         *
 223         * Because being_removed is never being
 224         * unset, we do not need to worry about race conditions on
 225         * being_removed.
 226         */
 227        if (devfreq->being_removed)
 228                return;
 229
 230        if (central_polling)
 231                mutex_lock(&devfreq_list_lock);
 232
 233        mutex_lock(&devfreq->lock);
 234
 235        /*
 236         * Check being_removed flag again for the case where
 237         * devfreq_dev_release() was called in a thread other than the one
 238         * possibly called _remove_devfreq().
 239         */
 240        if (devfreq->being_removed) {
 241                mutex_unlock(&devfreq->lock);
 242                goto out;
 243        }
 244
 245        /* devfreq->lock is unlocked and removed in _removed_devfreq() */
 246        _remove_devfreq(devfreq, true);
 247
 248out:
 249        if (central_polling)
 250                mutex_unlock(&devfreq_list_lock);
 251}
 252
 253/**
 254 * devfreq_monitor() - Periodically poll devfreq objects.
 255 * @work: the work struct used to run devfreq_monitor periodically.
 256 *
 257 */
 258static void devfreq_monitor(struct work_struct *work)
 259{
 260        static unsigned long last_polled_at;
 261        struct devfreq *devfreq, *tmp;
 262        int error;
 263        unsigned long jiffies_passed;
 264        unsigned long next_jiffies = ULONG_MAX, now = jiffies;
 265        struct device *dev;
 266
 267        /* Initially last_polled_at = 0, polling every device at bootup */
 268        jiffies_passed = now - last_polled_at;
 269        last_polled_at = now;
 270        if (jiffies_passed == 0)
 271                jiffies_passed = 1;
 272
 273        mutex_lock(&devfreq_list_lock);
 274        list_for_each_entry_safe(devfreq, tmp, &devfreq_list, node) {
 275                mutex_lock(&devfreq->lock);
 276                dev = devfreq->dev.parent;
 277
 278                /* Do not remove tmp for a while */
 279                wait_remove_device = tmp;
 280
 281                if (devfreq->governor->no_central_polling ||
 282                    devfreq->next_polling == 0) {
 283                        mutex_unlock(&devfreq->lock);
 284                        continue;
 285                }
 286                mutex_unlock(&devfreq_list_lock);
 287
 288                /*
 289                 * Reduce more next_polling if devfreq_wq took an extra
 290                 * delay. (i.e., CPU has been idled.)
 291                 */
 292                if (devfreq->next_polling <= jiffies_passed) {
 293                        error = update_devfreq(devfreq);
 294
 295                        /* Remove a devfreq with an error. */
 296                        if (error && error != -EAGAIN) {
 297
 298                                dev_err(dev, "Due to update_devfreq error(%d), devfreq(%s) is removed from the device\n",
 299                                        error, devfreq->governor->name);
 300
 301                                /*
 302                                 * Unlock devfreq before locking the list
 303                                 * in order to avoid deadlock with
 304                                 * find_device_devfreq or others
 305                                 */
 306                                mutex_unlock(&devfreq->lock);
 307                                mutex_lock(&devfreq_list_lock);
 308                                /* Check if devfreq is already removed */
 309                                if (IS_ERR(find_device_devfreq(dev)))
 310                                        continue;
 311                                mutex_lock(&devfreq->lock);
 312                                /* This unlocks devfreq->lock and free it */
 313                                _remove_devfreq(devfreq, false);
 314                                continue;
 315                        }
 316                        devfreq->next_polling = devfreq->polling_jiffies;
 317                } else {
 318                        devfreq->next_polling -= jiffies_passed;
 319                }
 320
 321                if (devfreq->next_polling)
 322                        next_jiffies = (next_jiffies > devfreq->next_polling) ?
 323                                        devfreq->next_polling : next_jiffies;
 324
 325                mutex_unlock(&devfreq->lock);
 326                mutex_lock(&devfreq_list_lock);
 327        }
 328        wait_remove_device = NULL;
 329        mutex_unlock(&devfreq_list_lock);
 330
 331        if (next_jiffies > 0 && next_jiffies < ULONG_MAX) {
 332                polling = true;
 333                queue_delayed_work(devfreq_wq, &devfreq_work, next_jiffies);
 334        } else {
 335                polling = false;
 336        }
 337}
 338
 339/**
 340 * devfreq_add_device() - Add devfreq feature to the device
 341 * @dev:        the device to add devfreq feature.
 342 * @profile:    device-specific profile to run devfreq.
 343 * @governor:   the policy to choose frequency.
 344 * @data:       private data for the governor. The devfreq framework does not
 345 *              touch this value.
 346 */
 347struct devfreq *devfreq_add_device(struct device *dev,
 348                                   struct devfreq_dev_profile *profile,
 349                                   const struct devfreq_governor *governor,
 350                                   void *data)
 351{
 352        struct devfreq *devfreq;
 353        int err = 0;
 354
 355        if (!dev || !profile || !governor) {
 356                dev_err(dev, "%s: Invalid parameters.\n", __func__);
 357                return ERR_PTR(-EINVAL);
 358        }
 359
 360
 361        if (!governor->no_central_polling) {
 362                mutex_lock(&devfreq_list_lock);
 363                devfreq = find_device_devfreq(dev);
 364                mutex_unlock(&devfreq_list_lock);
 365                if (!IS_ERR(devfreq)) {
 366                        dev_err(dev, "%s: Unable to create devfreq for the device. It already has one.\n", __func__);
 367                        err = -EINVAL;
 368                        goto err_out;
 369                }
 370        }
 371
 372        devfreq = kzalloc(sizeof(struct devfreq), GFP_KERNEL);
 373        if (!devfreq) {
 374                dev_err(dev, "%s: Unable to create devfreq for the device\n",
 375                        __func__);
 376                err = -ENOMEM;
 377                goto err_out;
 378        }
 379
 380        mutex_init(&devfreq->lock);
 381        mutex_lock(&devfreq->lock);
 382        devfreq->dev.parent = dev;
 383        devfreq->dev.class = devfreq_class;
 384        devfreq->dev.release = devfreq_dev_release;
 385        devfreq->profile = profile;
 386        devfreq->governor = governor;
 387        devfreq->previous_freq = profile->initial_freq;
 388        devfreq->data = data;
 389        devfreq->next_polling = devfreq->polling_jiffies
 390                              = msecs_to_jiffies(devfreq->profile->polling_ms);
 391        devfreq->nb.notifier_call = devfreq_notifier_call;
 392
 393        dev_set_name(&devfreq->dev, dev_name(dev));
 394        err = device_register(&devfreq->dev);
 395        if (err) {
 396                put_device(&devfreq->dev);
 397                goto err_dev;
 398        }
 399
 400        if (governor->init)
 401                err = governor->init(devfreq);
 402        if (err)
 403                goto err_init;
 404
 405        mutex_unlock(&devfreq->lock);
 406
 407        if (governor->no_central_polling)
 408                goto out;
 409
 410        mutex_lock(&devfreq_list_lock);
 411
 412        list_add(&devfreq->node, &devfreq_list);
 413
 414        if (devfreq_wq && devfreq->next_polling && !polling) {
 415                polling = true;
 416                queue_delayed_work(devfreq_wq, &devfreq_work,
 417                                   devfreq->next_polling);
 418        }
 419        mutex_unlock(&devfreq_list_lock);
 420out:
 421        return devfreq;
 422
 423err_init:
 424        device_unregister(&devfreq->dev);
 425err_dev:
 426        mutex_unlock(&devfreq->lock);
 427        kfree(devfreq);
 428err_out:
 429        return ERR_PTR(err);
 430}
 431
 432/**
 433 * devfreq_remove_device() - Remove devfreq feature from a device.
 434 * @devfreq     the devfreq instance to be removed
 435 */
 436int devfreq_remove_device(struct devfreq *devfreq)
 437{
 438        bool central_polling;
 439
 440        if (!devfreq)
 441                return -EINVAL;
 442
 443        central_polling = !devfreq->governor->no_central_polling;
 444
 445        if (central_polling) {
 446                mutex_lock(&devfreq_list_lock);
 447                while (wait_remove_device == devfreq) {
 448                        mutex_unlock(&devfreq_list_lock);
 449                        schedule();
 450                        mutex_lock(&devfreq_list_lock);
 451                }
 452        }
 453
 454        mutex_lock(&devfreq->lock);
 455        _remove_devfreq(devfreq, false); /* it unlocks devfreq->lock */
 456
 457        if (central_polling)
 458                mutex_unlock(&devfreq_list_lock);
 459
 460        return 0;
 461}
 462
 463static ssize_t show_governor(struct device *dev,
 464                             struct device_attribute *attr, char *buf)
 465{
 466        return sprintf(buf, "%s\n", to_devfreq(dev)->governor->name);
 467}
 468
 469static ssize_t show_freq(struct device *dev,
 470                         struct device_attribute *attr, char *buf)
 471{
 472        return sprintf(buf, "%lu\n", to_devfreq(dev)->previous_freq);
 473}
 474
 475static ssize_t show_polling_interval(struct device *dev,
 476                                     struct device_attribute *attr, char *buf)
 477{
 478        return sprintf(buf, "%d\n", to_devfreq(dev)->profile->polling_ms);
 479}
 480
 481static ssize_t store_polling_interval(struct device *dev,
 482                                      struct device_attribute *attr,
 483                                      const char *buf, size_t count)
 484{
 485        struct devfreq *df = to_devfreq(dev);
 486        unsigned int value;
 487        int ret;
 488
 489        ret = sscanf(buf, "%u", &value);
 490        if (ret != 1)
 491                goto out;
 492
 493        mutex_lock(&df->lock);
 494        df->profile->polling_ms = value;
 495        df->next_polling = df->polling_jiffies
 496                         = msecs_to_jiffies(value);
 497        mutex_unlock(&df->lock);
 498
 499        ret = count;
 500
 501        if (df->governor->no_central_polling)
 502                goto out;
 503
 504        mutex_lock(&devfreq_list_lock);
 505        if (df->next_polling > 0 && !polling) {
 506                polling = true;
 507                queue_delayed_work(devfreq_wq, &devfreq_work,
 508                                   df->next_polling);
 509        }
 510        mutex_unlock(&devfreq_list_lock);
 511out:
 512        return ret;
 513}
 514
 515static ssize_t show_central_polling(struct device *dev,
 516                                    struct device_attribute *attr, char *buf)
 517{
 518        return sprintf(buf, "%d\n",
 519                       !to_devfreq(dev)->governor->no_central_polling);
 520}
 521
 522static ssize_t store_min_freq(struct device *dev, struct device_attribute *attr,
 523                              const char *buf, size_t count)
 524{
 525        struct devfreq *df = to_devfreq(dev);
 526        unsigned long value;
 527        int ret;
 528        unsigned long max;
 529
 530        ret = sscanf(buf, "%lu", &value);
 531        if (ret != 1)
 532                goto out;
 533
 534        mutex_lock(&df->lock);
 535        max = df->max_freq;
 536        if (value && max && value > max) {
 537                ret = -EINVAL;
 538                goto unlock;
 539        }
 540
 541        df->min_freq = value;
 542        update_devfreq(df);
 543        ret = count;
 544unlock:
 545        mutex_unlock(&df->lock);
 546out:
 547        return ret;
 548}
 549
 550static ssize_t show_min_freq(struct device *dev, struct device_attribute *attr,
 551                             char *buf)
 552{
 553        return sprintf(buf, "%lu\n", to_devfreq(dev)->min_freq);
 554}
 555
 556static ssize_t store_max_freq(struct device *dev, struct device_attribute *attr,
 557                              const char *buf, size_t count)
 558{
 559        struct devfreq *df = to_devfreq(dev);
 560        unsigned long value;
 561        int ret;
 562        unsigned long min;
 563
 564        ret = sscanf(buf, "%lu", &value);
 565        if (ret != 1)
 566                goto out;
 567
 568        mutex_lock(&df->lock);
 569        min = df->min_freq;
 570        if (value && min && value < min) {
 571                ret = -EINVAL;
 572                goto unlock;
 573        }
 574
 575        df->max_freq = value;
 576        update_devfreq(df);
 577        ret = count;
 578unlock:
 579        mutex_unlock(&df->lock);
 580out:
 581        return ret;
 582}
 583
 584static ssize_t show_max_freq(struct device *dev, struct device_attribute *attr,
 585                             char *buf)
 586{
 587        return sprintf(buf, "%lu\n", to_devfreq(dev)->max_freq);
 588}
 589
 590static struct device_attribute devfreq_attrs[] = {
 591        __ATTR(governor, S_IRUGO, show_governor, NULL),
 592        __ATTR(cur_freq, S_IRUGO, show_freq, NULL),
 593        __ATTR(central_polling, S_IRUGO, show_central_polling, NULL),
 594        __ATTR(polling_interval, S_IRUGO | S_IWUSR, show_polling_interval,
 595               store_polling_interval),
 596        __ATTR(min_freq, S_IRUGO | S_IWUSR, show_min_freq, store_min_freq),
 597        __ATTR(max_freq, S_IRUGO | S_IWUSR, show_max_freq, store_max_freq),
 598        { },
 599};
 600
 601/**
 602 * devfreq_start_polling() - Initialize data structure for devfreq framework and
 603 *                         start polling registered devfreq devices.
 604 */
 605static int __init devfreq_start_polling(void)
 606{
 607        mutex_lock(&devfreq_list_lock);
 608        polling = false;
 609        devfreq_wq = create_freezable_workqueue("devfreq_wq");
 610        INIT_DEFERRABLE_WORK(&devfreq_work, devfreq_monitor);
 611        mutex_unlock(&devfreq_list_lock);
 612
 613        devfreq_monitor(&devfreq_work.work);
 614        return 0;
 615}
 616late_initcall(devfreq_start_polling);
 617
 618static int __init devfreq_init(void)
 619{
 620        devfreq_class = class_create(THIS_MODULE, "devfreq");
 621        if (IS_ERR(devfreq_class)) {
 622                pr_err("%s: couldn't create class\n", __FILE__);
 623                return PTR_ERR(devfreq_class);
 624        }
 625        devfreq_class->dev_attrs = devfreq_attrs;
 626        return 0;
 627}
 628subsys_initcall(devfreq_init);
 629
 630static void __exit devfreq_exit(void)
 631{
 632        class_destroy(devfreq_class);
 633}
 634module_exit(devfreq_exit);
 635
 636/*
 637 * The followings are helper functions for devfreq user device drivers with
 638 * OPP framework.
 639 */
 640
 641/**
 642 * devfreq_recommended_opp() - Helper function to get proper OPP for the
 643 *                           freq value given to target callback.
 644 * @dev         The devfreq user device. (parent of devfreq)
 645 * @freq        The frequency given to target function
 646 * @flags       Flags handed from devfreq framework.
 647 *
 648 */
 649struct opp *devfreq_recommended_opp(struct device *dev, unsigned long *freq,
 650                                    u32 flags)
 651{
 652        struct opp *opp;
 653
 654        if (flags & DEVFREQ_FLAG_LEAST_UPPER_BOUND) {
 655                /* The freq is an upper bound. opp should be lower */
 656                opp = opp_find_freq_floor(dev, freq);
 657
 658                /* If not available, use the closest opp */
 659                if (opp == ERR_PTR(-ENODEV))
 660                        opp = opp_find_freq_ceil(dev, freq);
 661        } else {
 662                /* The freq is an lower bound. opp should be higher */
 663                opp = opp_find_freq_ceil(dev, freq);
 664
 665                /* If not available, use the closest opp */
 666                if (opp == ERR_PTR(-ENODEV))
 667                        opp = opp_find_freq_floor(dev, freq);
 668        }
 669
 670        return opp;
 671}
 672
 673/**
 674 * devfreq_register_opp_notifier() - Helper function to get devfreq notified
 675 *                                 for any changes in the OPP availability
 676 *                                 changes
 677 * @dev         The devfreq user device. (parent of devfreq)
 678 * @devfreq     The devfreq object.
 679 */
 680int devfreq_register_opp_notifier(struct device *dev, struct devfreq *devfreq)
 681{
 682        struct srcu_notifier_head *nh = opp_get_notifier(dev);
 683
 684        if (IS_ERR(nh))
 685                return PTR_ERR(nh);
 686        return srcu_notifier_chain_register(nh, &devfreq->nb);
 687}
 688
 689/**
 690 * devfreq_unregister_opp_notifier() - Helper function to stop getting devfreq
 691 *                                   notified for any changes in the OPP
 692 *                                   availability changes anymore.
 693 * @dev         The devfreq user device. (parent of devfreq)
 694 * @devfreq     The devfreq object.
 695 *
 696 * At exit() callback of devfreq_dev_profile, this must be included if
 697 * devfreq_recommended_opp is used.
 698 */
 699int devfreq_unregister_opp_notifier(struct device *dev, struct devfreq *devfreq)
 700{
 701        struct srcu_notifier_head *nh = opp_get_notifier(dev);
 702
 703        if (IS_ERR(nh))
 704                return PTR_ERR(nh);
 705        return srcu_notifier_chain_unregister(nh, &devfreq->nb);
 706}
 707
 708MODULE_AUTHOR("MyungJoo Ham <myungjoo.ham@samsung.com>");
 709MODULE_DESCRIPTION("devfreq class support");
 710MODULE_LICENSE("GPL");
 711
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