drivers/hwtracing/coresight/coresight-tmc-etf.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Copyright(C) 2016 Linaro Limited. All rights reserved.
   4  * Author: Mathieu Poirier <mathieu.poirier@linaro.org>
   5  */
   6
   7 #include <linux/atomic.h>
   8 #include <linux/circ_buf.h>
   9 #include <linux/coresight.h>
  10 #include <linux/perf_event.h>
  11 #include <linux/slab.h>
  12 #include "coresight-priv.h"
  13 #include "coresight-tmc.h"
  14 #include "coresight-etm-perf.h"
  15
  16 static int tmc_set_etf_buffer(struct coresight_device *csdev,
  17                               struct perf_output_handle *handle);
  18
  19 static void __tmc_etb_enable_hw(struct tmc_drvdata *drvdata)
  20 {
  21         CS_UNLOCK(drvdata->base);
  22
  23         /* Wait for TMCSReady bit to be set */
  24         tmc_wait_for_tmcready(drvdata);
  25
  26         writel_relaxed(TMC_MODE_CIRCULAR_BUFFER, drvdata->base + TMC_MODE);
  27         writel_relaxed(TMC_FFCR_EN_FMT | TMC_FFCR_EN_TI |
  28                        TMC_FFCR_FON_FLIN | TMC_FFCR_FON_TRIG_EVT |
  29                        TMC_FFCR_TRIGON_TRIGIN,
  30                        drvdata->base + TMC_FFCR);
  31
  32         writel_relaxed(drvdata->trigger_cntr, drvdata->base + TMC_TRG);
  33         tmc_enable_hw(drvdata);
  34
  35         CS_LOCK(drvdata->base);
  36 }
  37
  38 static int tmc_etb_enable_hw(struct tmc_drvdata *drvdata)
  39 {
  40         int rc = coresight_claim_device(drvdata->base);
  41
  42         if (rc)
  43                 return rc;
  44
  45         __tmc_etb_enable_hw(drvdata);
  46         return 0;
  47 }
  48
  49 static void tmc_etb_dump_hw(struct tmc_drvdata *drvdata)
  50 {
  51         char *bufp;
  52         u32 read_data, lost;
  53
  54         /* Check if the buffer wrapped around. */
  55         lost = readl_relaxed(drvdata->base + TMC_STS) & TMC_STS_FULL;
  56         bufp = drvdata->buf;
  57         drvdata->len = 0;
  58         while (1) {
  59                 read_data = readl_relaxed(drvdata->base + TMC_RRD);
  60                 if (read_data == 0xFFFFFFFF)
  61                         break;
  62                 memcpy(bufp, &read_data, 4);
  63                 bufp += 4;
  64                 drvdata->len += 4;
  65         }
  66
  67         if (lost)
  68                 coresight_insert_barrier_packet(drvdata->buf);
  69         return;
  70 }
  71
  72 static void __tmc_etb_disable_hw(struct tmc_drvdata *drvdata)
  73 {
  74         CS_UNLOCK(drvdata->base);
  75
  76         tmc_flush_and_stop(drvdata);
  77         /*
  78          * When operating in sysFS mode the content of the buffer needs to be
  79          * read before the TMC is disabled.
  80          */
  81         if (drvdata->mode == CS_MODE_SYSFS)
  82                 tmc_etb_dump_hw(drvdata);
  83         tmc_disable_hw(drvdata);
  84
  85         CS_LOCK(drvdata->base);
  86 }
  87
  88 static void tmc_etb_disable_hw(struct tmc_drvdata *drvdata)
  89 {
  90         __tmc_etb_disable_hw(drvdata);
  91         coresight_disclaim_device(drvdata->base);
  92 }
  93
  94 static void __tmc_etf_enable_hw(struct tmc_drvdata *drvdata)
  95 {
  96         CS_UNLOCK(drvdata->base);
  97
  98         /* Wait for TMCSReady bit to be set */
  99         tmc_wait_for_tmcready(drvdata);
 100
 101         writel_relaxed(TMC_MODE_HARDWARE_FIFO, drvdata->base + TMC_MODE);
 102         writel_relaxed(TMC_FFCR_EN_FMT | TMC_FFCR_EN_TI,
 103                        drvdata->base + TMC_FFCR);
 104         writel_relaxed(0x0, drvdata->base + TMC_BUFWM);
 105         tmc_enable_hw(drvdata);
 106
 107         CS_LOCK(drvdata->base);
 108 }
 109
 110 static int tmc_etf_enable_hw(struct tmc_drvdata *drvdata)
 111 {
 112         int rc = coresight_claim_device(drvdata->base);
 113
 114         if (rc)
 115                 return rc;
 116
 117         __tmc_etf_enable_hw(drvdata);
 118         return 0;
 119 }
 120
 121 static void tmc_etf_disable_hw(struct tmc_drvdata *drvdata)
 122 {
 123         CS_UNLOCK(drvdata->base);
 124
 125         tmc_flush_and_stop(drvdata);
 126         tmc_disable_hw(drvdata);
 127         coresight_disclaim_device_unlocked(drvdata->base);
 128         CS_LOCK(drvdata->base);
 129 }
 130
 131 /*
 132  * Return the available trace data in the buffer from @pos, with
 133  * a maximum limit of @len, updating the @bufpp on where to
 134  * find it.
 135  */
 136 ssize_t tmc_etb_get_sysfs_trace(struct tmc_drvdata *drvdata,
 137                                 loff_t pos, size_t len, char **bufpp)
 138 {
 139         ssize_t actual = len;
 140
 141         /* Adjust the len to available size @pos */
 142         if (pos + actual > drvdata->len)
 143                 actual = drvdata->len - pos;
 144         if (actual > 0)
 145                 *bufpp = drvdata->buf + pos;
 146         return actual;
 147 }
 148
 149 static int tmc_enable_etf_sink_sysfs(struct coresight_device *csdev)
 150 {
 151         int ret = 0;
 152         bool used = false;
 153         char *buf = NULL;
 154         unsigned long flags;
 155         struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 156
 157         /*
 158          * If we don't have a buffer release the lock and allocate memory.
 159          * Otherwise keep the lock and move along.
 160          */
 161         spin_lock_irqsave(&drvdata->spinlock, flags);
 162         if (!drvdata->buf) {
 163                 spin_unlock_irqrestore(&drvdata->spinlock, flags);
 164
 165                 /* Allocating the memory here while outside of the spinlock */
 166                 buf = kzalloc(drvdata->size, GFP_KERNEL);
 167                 if (!buf)
 168                         return -ENOMEM;
 169
 170                 /* Let's try again */
 171                 spin_lock_irqsave(&drvdata->spinlock, flags);
 172         }
 173
 174         if (drvdata->reading) {
 175                 ret = -EBUSY;
 176                 goto out;
 177         }
 178
 179         /*
 180          * In sysFS mode we can have multiple writers per sink.  Since this
 181          * sink is already enabled no memory is needed and the HW need not be
 182          * touched.
 183          */
 184         if (drvdata->mode == CS_MODE_SYSFS) {
 185                 atomic_inc(csdev->refcnt);
 186                 goto out;
 187         }
 188
 189         /*
 190          * If drvdata::buf isn't NULL, memory was allocated for a previous
 191          * trace run but wasn't read.  If so simply zero-out the memory.
 192          * Otherwise use the memory allocated above.
 193          *
 194          * The memory is freed when users read the buffer using the
 195          * /dev/xyz.{etf|etb} interface.  See tmc_read_unprepare_etf() for
 196          * details.
 197          */
 198         if (drvdata->buf) {
 199                 memset(drvdata->buf, 0, drvdata->size);
 200         } else {
 201                 used = true;
 202                 drvdata->buf = buf;
 203         }
 204
 205         ret = tmc_etb_enable_hw(drvdata);
 206         if (!ret) {
 207                 drvdata->mode = CS_MODE_SYSFS;
 208                 atomic_inc(csdev->refcnt);
 209         } else {
 210                 /* Free up the buffer if we failed to enable */
 211                 used = false;
 212         }
 213 out:
 214         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 215
 216         /* Free memory outside the spinlock if need be */
 217         if (!used)
 218                 kfree(buf);
 219
 220         return ret;
 221 }
 222
 223 static int tmc_enable_etf_sink_perf(struct coresight_device *csdev, void *data)
 224 {
 225         int ret = 0;
 226         pid_t pid;
 227         unsigned long flags;
 228         struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 229         struct perf_output_handle *handle = data;
 230         struct cs_buffers *buf = etm_perf_sink_config(handle);
 231
 232         spin_lock_irqsave(&drvdata->spinlock, flags);
 233         do {
 234                 ret = -EINVAL;
 235                 if (drvdata->reading)
 236                         break;
 237                 /*
 238                  * No need to continue if the ETB/ETF is already operated
 239                  * from sysFS.
 240                  */
 241                 if (drvdata->mode == CS_MODE_SYSFS) {
 242                         ret = -EBUSY;
 243                         break;
 244                 }
 245
 246                 /* Get a handle on the pid of the process to monitor */
 247                 pid = buf->pid;
 248
 249                 if (drvdata->pid != -1 && drvdata->pid != pid) {
 250                         ret = -EBUSY;
 251                         break;
 252                 }
 253
 254                 ret = tmc_set_etf_buffer(csdev, handle);
 255                 if (ret)
 256                         break;
 257
 258                 /*
 259                  * No HW configuration is needed if the sink is already in
 260                  * use for this session.
 261                  */
 262                 if (drvdata->pid == pid) {
 263                         atomic_inc(csdev->refcnt);
 264                         break;
 265                 }
 266
 267                 ret  = tmc_etb_enable_hw(drvdata);
 268                 if (!ret) {
 269                         /* Associate with monitored process. */
 270                         drvdata->pid = pid;
 271                         drvdata->mode = CS_MODE_PERF;
 272                         atomic_inc(csdev->refcnt);
 273                 }
 274         } while (0);
 275         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 276
 277         return ret;
 278 }
 279
 280 static int tmc_enable_etf_sink(struct coresight_device *csdev,
 281                                u32 mode, void *data)
 282 {
 283         int ret;
 284
 285         switch (mode) {
 286         case CS_MODE_SYSFS:
 287                 ret = tmc_enable_etf_sink_sysfs(csdev);
 288                 break;
 289         case CS_MODE_PERF:
 290                 ret = tmc_enable_etf_sink_perf(csdev, data);
 291                 break;
 292         /* We shouldn't be here */
 293         default:
 294                 ret = -EINVAL;
 295                 break;
 296         }
 297
 298         if (ret)
 299                 return ret;
 300
 301         dev_dbg(&csdev->dev, "TMC-ETB/ETF enabled\n");
 302         return 0;
 303 }
 304
 305 static int tmc_disable_etf_sink(struct coresight_device *csdev)
 306 {
 307         unsigned long flags;
 308         struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 309
 310         spin_lock_irqsave(&drvdata->spinlock, flags);
 311
 312         if (drvdata->reading) {
 313                 spin_unlock_irqrestore(&drvdata->spinlock, flags);
 314                 return -EBUSY;
 315         }
 316
 317         if (atomic_dec_return(csdev->refcnt)) {
 318                 spin_unlock_irqrestore(&drvdata->spinlock, flags);
 319                 return -EBUSY;
 320         }
 321
 322         /* Complain if we (somehow) got out of sync */
 323         WARN_ON_ONCE(drvdata->mode == CS_MODE_DISABLED);
 324         tmc_etb_disable_hw(drvdata);
 325         /* Dissociate from monitored process. */
 326         drvdata->pid = -1;
 327         drvdata->mode = CS_MODE_DISABLED;
 328
 329         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 330
 331         dev_dbg(&csdev->dev, "TMC-ETB/ETF disabled\n");
 332         return 0;
 333 }
 334
 335 static int tmc_enable_etf_link(struct coresight_device *csdev,
 336                                int inport, int outport)
 337 {
 338         int ret = 0;
 339         unsigned long flags;
 340         struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 341         bool first_enable = false;
 342
 343         spin_lock_irqsave(&drvdata->spinlock, flags);
 344         if (drvdata->reading) {
 345                 spin_unlock_irqrestore(&drvdata->spinlock, flags);
 346                 return -EBUSY;
 347         }
 348
 349         if (atomic_read(&csdev->refcnt[0]) == 0) {
 350                 ret = tmc_etf_enable_hw(drvdata);
 351                 if (!ret) {
 352                         drvdata->mode = CS_MODE_SYSFS;
 353                         first_enable = true;
 354                 }
 355         }
 356         if (!ret)
 357                 atomic_inc(&csdev->refcnt[0]);
 358         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 359
 360         if (first_enable)
 361                 dev_dbg(&csdev->dev, "TMC-ETF enabled\n");
 362         return ret;
 363 }
 364
 365 static void tmc_disable_etf_link(struct coresight_device *csdev,
 366                                  int inport, int outport)
 367 {
 368         unsigned long flags;
 369         struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 370         bool last_disable = false;
 371
 372         spin_lock_irqsave(&drvdata->spinlock, flags);
 373         if (drvdata->reading) {
 374                 spin_unlock_irqrestore(&drvdata->spinlock, flags);
 375                 return;
 376         }
 377
 378         if (atomic_dec_return(&csdev->refcnt[0]) == 0) {
 379                 tmc_etf_disable_hw(drvdata);
 380                 drvdata->mode = CS_MODE_DISABLED;
 381                 last_disable = true;
 382         }
 383         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 384
 385         if (last_disable)
 386                 dev_dbg(&csdev->dev, "TMC-ETF disabled\n");
 387 }
 388
 389 static void *tmc_alloc_etf_buffer(struct coresight_device *csdev,
 390                                   struct perf_event *event, void **pages,
 391                                   int nr_pages, bool overwrite)
 392 {
 393         int node;
 394         struct cs_buffers *buf;
 395
 396         node = (event->cpu == -1) ? NUMA_NO_NODE : cpu_to_node(event->cpu);
 397
 398         /* Allocate memory structure for interaction with Perf */
 399         buf = kzalloc_node(sizeof(struct cs_buffers), GFP_KERNEL, node);
 400         if (!buf)
 401                 return NULL;
 402
 403         buf->pid = task_pid_nr(event->owner);
 404         buf->snapshot = overwrite;
 405         buf->nr_pages = nr_pages;
 406         buf->data_pages = pages;
 407
 408         return buf;
 409 }
 410
 411 static void tmc_free_etf_buffer(void *config)
 412 {
 413         struct cs_buffers *buf = config;
 414
 415         kfree(buf);
 416 }
 417
 418 static int tmc_set_etf_buffer(struct coresight_device *csdev,
 419                               struct perf_output_handle *handle)
 420 {
 421         int ret = 0;
 422         unsigned long head;
 423         struct cs_buffers *buf = etm_perf_sink_config(handle);
 424
 425         if (!buf)
 426                 return -EINVAL;
 427
 428         /* wrap head around to the amount of space we have */
 429         head = handle->head & ((buf->nr_pages << PAGE_SHIFT) - 1);
 430
 431         /* find the page to write to */
 432         buf->cur = head / PAGE_SIZE;
 433
 434         /* and offset within that page */
 435         buf->offset = head % PAGE_SIZE;
 436
 437         local_set(&buf->data_size, 0);
 438
 439         return ret;
 440 }
 441
 442 static unsigned long tmc_update_etf_buffer(struct coresight_device *csdev,
 443                                   struct perf_output_handle *handle,
 444                                   void *sink_config)
 445 {
 446         bool lost = false;
 447         int i, cur;
 448         const u32 *barrier;
 449         u32 *buf_ptr;
 450         u64 read_ptr, write_ptr;
 451         u32 status;
 452         unsigned long offset, to_read = 0, flags;
 453         struct cs_buffers *buf = sink_config;
 454         struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 455
 456         if (!buf)
 457                 return 0;
 458
 459         /* This shouldn't happen */
 460         if (WARN_ON_ONCE(drvdata->mode != CS_MODE_PERF))
 461                 return 0;
 462
 463         spin_lock_irqsave(&drvdata->spinlock, flags);
 464
 465         /* Don't do anything if another tracer is using this sink */
 466         if (atomic_read(csdev->refcnt) != 1)
 467                 goto out;
 468
 469         CS_UNLOCK(drvdata->base);
 470
 471         tmc_flush_and_stop(drvdata);
 472
 473         read_ptr = tmc_read_rrp(drvdata);
 474         write_ptr = tmc_read_rwp(drvdata);
 475
 476         /*
 477          * Get a hold of the status register and see if a wrap around
 478          * has occurred.  If so adjust things accordingly.
 479          */
 480         status = readl_relaxed(drvdata->base + TMC_STS);
 481         if (status & TMC_STS_FULL) {
 482                 lost = true;
 483                 to_read = drvdata->size;
 484         } else {
 485                 to_read = CIRC_CNT(write_ptr, read_ptr, drvdata->size);
 486         }
 487
 488         /*
 489          * The TMC RAM buffer may be bigger than the space available in the
 490          * perf ring buffer (handle->size).  If so advance the RRP so that we
 491          * get the latest trace data.  In snapshot mode none of that matters
 492          * since we are expected to clobber stale data in favour of the latest
 493          * traces.
 494          */
 495         if (!buf->snapshot && to_read > handle->size) {
 496                 u32 mask = tmc_get_memwidth_mask(drvdata);
 497
 498                 /*
 499                  * Make sure the new size is aligned in accordance with the
 500                  * requirement explained in function tmc_get_memwidth_mask().
 501                  */
 502                 to_read = handle->size & mask;
 503                 /* Move the RAM read pointer up */
 504                 read_ptr = (write_ptr + drvdata->size) - to_read;
 505                 /* Make sure we are still within our limits */
 506                 if (read_ptr > (drvdata->size - 1))
 507                         read_ptr -= drvdata->size;
 508                 /* Tell the HW */
 509                 tmc_write_rrp(drvdata, read_ptr);
 510                 lost = true;
 511         }
 512
 513         /*
 514          * Don't set the TRUNCATED flag in snapshot mode because 1) the
 515          * captured buffer is expected to be truncated and 2) a full buffer
 516          * prevents the event from being re-enabled by the perf core,
 517          * resulting in stale data being send to user space.
 518          */
 519         if (!buf->snapshot && lost)
 520                 perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
 521
 522         cur = buf->cur;
 523         offset = buf->offset;
 524         barrier = coresight_barrier_pkt;
 525
 526         /* for every byte to read */
 527         for (i = 0; i < to_read; i += 4) {
 528                 buf_ptr = buf->data_pages[cur] + offset;
 529                 *buf_ptr = readl_relaxed(drvdata->base + TMC_RRD);
 530
 531                 if (lost && *barrier) {
 532                         *buf_ptr = *barrier;
 533                         barrier++;
 534                 }
 535
 536                 offset += 4;
 537                 if (offset >= PAGE_SIZE) {
 538                         offset = 0;
 539                         cur++;
 540                         /* wrap around at the end of the buffer */
 541                         cur &= buf->nr_pages - 1;
 542                 }
 543         }
 544
 545         /*
 546          * In snapshot mode we simply increment the head by the number of byte
 547          * that were written.  User space function  cs_etm_find_snapshot() will
 548          * figure out how many bytes to get from the AUX buffer based on the
 549          * position of the head.
 550          */
 551         if (buf->snapshot)
 552                 handle->head += to_read;
 553
 554         CS_LOCK(drvdata->base);
 555 out:
 556         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 557
 558         return to_read;
 559 }
 560
 561 static const struct coresight_ops_sink tmc_etf_sink_ops = {
 562         .enable         = tmc_enable_etf_sink,
 563         .disable        = tmc_disable_etf_sink,
 564         .alloc_buffer   = tmc_alloc_etf_buffer,
 565         .free_buffer    = tmc_free_etf_buffer,
 566         .update_buffer  = tmc_update_etf_buffer,
 567 };
 568
 569 static const struct coresight_ops_link tmc_etf_link_ops = {
 570         .enable         = tmc_enable_etf_link,
 571         .disable        = tmc_disable_etf_link,
 572 };
 573
 574 const struct coresight_ops tmc_etb_cs_ops = {
 575         .sink_ops       = &tmc_etf_sink_ops,
 576 };
 577
 578 const struct coresight_ops tmc_etf_cs_ops = {
 579         .sink_ops       = &tmc_etf_sink_ops,
 580         .link_ops       = &tmc_etf_link_ops,
 581 };
 582
 583 int tmc_read_prepare_etb(struct tmc_drvdata *drvdata)
 584 {
 585         enum tmc_mode mode;
 586         int ret = 0;
 587         unsigned long flags;
 588
 589         /* config types are set a boot time and never change */
 590         if (WARN_ON_ONCE(drvdata->config_type != TMC_CONFIG_TYPE_ETB &&
 591                          drvdata->config_type != TMC_CONFIG_TYPE_ETF))
 592                 return -EINVAL;
 593
 594         spin_lock_irqsave(&drvdata->spinlock, flags);
 595
 596         if (drvdata->reading) {
 597                 ret = -EBUSY;
 598                 goto out;
 599         }
 600
 601         /* Don't interfere if operated from Perf */
 602         if (drvdata->mode == CS_MODE_PERF) {
 603                 ret = -EINVAL;
 604                 goto out;
 605         }
 606
 607         /* If drvdata::buf is NULL the trace data has been read already */
 608         if (drvdata->buf == NULL) {
 609                 ret = -EINVAL;
 610                 goto out;
 611         }
 612
 613         /* Disable the TMC if need be */
 614         if (drvdata->mode == CS_MODE_SYSFS) {
 615                 /* There is no point in reading a TMC in HW FIFO mode */
 616                 mode = readl_relaxed(drvdata->base + TMC_MODE);
 617                 if (mode != TMC_MODE_CIRCULAR_BUFFER) {
 618                         ret = -EINVAL;
 619                         goto out;
 620                 }
 621                 __tmc_etb_disable_hw(drvdata);
 622         }
 623
 624         drvdata->reading = true;
 625 out:
 626         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 627
 628         return ret;
 629 }
 630
 631 int tmc_read_unprepare_etb(struct tmc_drvdata *drvdata)
 632 {
 633         char *buf = NULL;
 634         enum tmc_mode mode;
 635         unsigned long flags;
 636
 637         /* config types are set a boot time and never change */
 638         if (WARN_ON_ONCE(drvdata->config_type != TMC_CONFIG_TYPE_ETB &&
 639                          drvdata->config_type != TMC_CONFIG_TYPE_ETF))
 640                 return -EINVAL;
 641
 642         spin_lock_irqsave(&drvdata->spinlock, flags);
 643
 644         /* Re-enable the TMC if need be */
 645         if (drvdata->mode == CS_MODE_SYSFS) {
 646                 /* There is no point in reading a TMC in HW FIFO mode */
 647                 mode = readl_relaxed(drvdata->base + TMC_MODE);
 648                 if (mode != TMC_MODE_CIRCULAR_BUFFER) {
 649                         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 650                         return -EINVAL;
 651                 }
 652                 /*
 653                  * The trace run will continue with the same allocated trace
 654                  * buffer. As such zero-out the buffer so that we don't end
 655                  * up with stale data.
 656                  *
 657                  * Since the tracer is still enabled drvdata::buf
 658                  * can't be NULL.
 659                  */
 660                 memset(drvdata->buf, 0, drvdata->size);
 661                 __tmc_etb_enable_hw(drvdata);
 662         } else {
 663                 /*
 664                  * The ETB/ETF is not tracing and the buffer was just read.
 665                  * As such prepare to free the trace buffer.
 666                  */
 667                 buf = drvdata->buf;
 668                 drvdata->buf = NULL;
 669         }
 670
 671         drvdata->reading = false;
 672         spin_unlock_irqrestore(&drvdata->spinlock, flags);
 673
 674         /*
 675          * Free allocated memory outside of the spinlock.  There is no need
 676          * to assert the validity of 'buf' since calling kfree(NULL) is safe.
 677          */
 678         kfree(buf);
 679
 680         return 0;
 681 }