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1// SPDX-License-Identifier: GPL-2.0+
2/*
3 * comedi_buf.c
4 *
5 * COMEDI - Linux Control and Measurement Device Interface
6 * Copyright (C) 1997-2000 David A. Schleef <ds@schleef.org>
7 * Copyright (C) 2002 Frank Mori Hess <fmhess@users.sourceforge.net>
8 */
9
10#include <linux/vmalloc.h>
11#include <linux/slab.h>
12#include <linux/comedi/comedidev.h>
13#include "comedi_internal.h"
14
15#ifdef PAGE_KERNEL_NOCACHE
16#define COMEDI_PAGE_PROTECTION PAGE_KERNEL_NOCACHE
17#else
18#define COMEDI_PAGE_PROTECTION PAGE_KERNEL
19#endif
20
21static void comedi_buf_map_kref_release(struct kref *kref)
22{
23 struct comedi_buf_map *bm =
24 container_of(kref, struct comedi_buf_map, refcount);
25 struct comedi_buf_page *buf;
26 unsigned int i;
27
28 if (bm->page_list) {
29 if (bm->dma_dir != DMA_NONE) {
30 for (i = 0; i < bm->n_pages; i++) {
31 buf = &bm->page_list[i];
32 dma_free_coherent(dev: bm->dma_hw_dev, PAGE_SIZE,
33 cpu_addr: buf->virt_addr,
34 dma_handle: buf->dma_addr);
35 }
36 } else {
37 for (i = 0; i < bm->n_pages; i++) {
38 buf = &bm->page_list[i];
39 ClearPageReserved(virt_to_page(buf->virt_addr));
40 free_page((unsigned long)buf->virt_addr);
41 }
42 }
43 vfree(addr: bm->page_list);
44 }
45 if (bm->dma_dir != DMA_NONE)
46 put_device(dev: bm->dma_hw_dev);
47 kfree(objp: bm);
48}
49
50static void __comedi_buf_free(struct comedi_device *dev,
51 struct comedi_subdevice *s)
52{
53 struct comedi_async *async = s->async;
54 struct comedi_buf_map *bm;
55 unsigned long flags;
56
57 async->prealloc_bufsz = 0;
58 spin_lock_irqsave(&s->spin_lock, flags);
59 bm = async->buf_map;
60 async->buf_map = NULL;
61 spin_unlock_irqrestore(lock: &s->spin_lock, flags);
62 comedi_buf_map_put(bm);
63}
64
65static struct comedi_buf_map *
66comedi_buf_map_alloc(struct comedi_device *dev, enum dma_data_direction dma_dir,
67 unsigned int n_pages)
68{
69 struct comedi_buf_map *bm;
70 struct comedi_buf_page *buf;
71 unsigned int i;
72
73 bm = kzalloc(sizeof(*bm), GFP_KERNEL);
74 if (!bm)
75 return NULL;
76
77 kref_init(kref: &bm->refcount);
78 bm->dma_dir = dma_dir;
79 if (bm->dma_dir != DMA_NONE) {
80 /* Need ref to hardware device to free buffer later. */
81 bm->dma_hw_dev = get_device(dev: dev->hw_dev);
82 }
83
84 bm->page_list = vzalloc(sizeof(*buf) * n_pages);
85 if (!bm->page_list)
86 goto err;
87
88 if (bm->dma_dir != DMA_NONE) {
89 for (i = 0; i < n_pages; i++) {
90 buf = &bm->page_list[i];
91 buf->virt_addr =
92 dma_alloc_coherent(dev: bm->dma_hw_dev, PAGE_SIZE,
93 dma_handle: &buf->dma_addr, GFP_KERNEL);
94 if (!buf->virt_addr)
95 break;
96 }
97 } else {
98 for (i = 0; i < n_pages; i++) {
99 buf = &bm->page_list[i];
100 buf->virt_addr = (void *)get_zeroed_page(GFP_KERNEL);
101 if (!buf->virt_addr)
102 break;
103
104 SetPageReserved(virt_to_page(buf->virt_addr));
105 }
106 }
107 bm->n_pages = i;
108 if (i < n_pages)
109 goto err;
110
111 return bm;
112
113err:
114 comedi_buf_map_put(bm);
115 return NULL;
116}
117
118static void __comedi_buf_alloc(struct comedi_device *dev,
119 struct comedi_subdevice *s,
120 unsigned int n_pages)
121{
122 struct comedi_async *async = s->async;
123 struct comedi_buf_map *bm;
124 unsigned long flags;
125
126 if (!IS_ENABLED(CONFIG_HAS_DMA) && s->async_dma_dir != DMA_NONE) {
127 dev_err(dev->class_dev,
128 "dma buffer allocation not supported\n");
129 return;
130 }
131
132 bm = comedi_buf_map_alloc(dev, dma_dir: s->async_dma_dir, n_pages);
133 if (!bm)
134 return;
135
136 spin_lock_irqsave(&s->spin_lock, flags);
137 async->buf_map = bm;
138 spin_unlock_irqrestore(lock: &s->spin_lock, flags);
139 async->prealloc_bufsz = n_pages << PAGE_SHIFT;
140}
141
142void comedi_buf_map_get(struct comedi_buf_map *bm)
143{
144 if (bm)
145 kref_get(kref: &bm->refcount);
146}
147
148int comedi_buf_map_put(struct comedi_buf_map *bm)
149{
150 if (bm)
151 return kref_put(kref: &bm->refcount, release: comedi_buf_map_kref_release);
152 return 1;
153}
154
155/* helper for "access" vm operation */
156int comedi_buf_map_access(struct comedi_buf_map *bm, unsigned long offset,
157 void *buf, int len, int write)
158{
159 unsigned int pgoff = offset_in_page(offset);
160 unsigned long pg = offset >> PAGE_SHIFT;
161 int done = 0;
162
163 while (done < len && pg < bm->n_pages) {
164 int l = min_t(int, len - done, PAGE_SIZE - pgoff);
165 void *b = bm->page_list[pg].virt_addr + pgoff;
166
167 if (write)
168 memcpy(b, buf, l);
169 else
170 memcpy(buf, b, l);
171 buf += l;
172 done += l;
173 pg++;
174 pgoff = 0;
175 }
176 return done;
177}
178
179/* returns s->async->buf_map and increments its kref refcount */
180struct comedi_buf_map *
181comedi_buf_map_from_subdev_get(struct comedi_subdevice *s)
182{
183 struct comedi_async *async = s->async;
184 struct comedi_buf_map *bm = NULL;
185 unsigned long flags;
186
187 if (!async)
188 return NULL;
189
190 spin_lock_irqsave(&s->spin_lock, flags);
191 bm = async->buf_map;
192 /* only want it if buffer pages allocated */
193 if (bm && bm->n_pages)
194 comedi_buf_map_get(bm);
195 else
196 bm = NULL;
197 spin_unlock_irqrestore(lock: &s->spin_lock, flags);
198
199 return bm;
200}
201
202bool comedi_buf_is_mmapped(struct comedi_subdevice *s)
203{
204 struct comedi_buf_map *bm = s->async->buf_map;
205
206 return bm && (kref_read(kref: &bm->refcount) > 1);
207}
208
209int comedi_buf_alloc(struct comedi_device *dev, struct comedi_subdevice *s,
210 unsigned long new_size)
211{
212 struct comedi_async *async = s->async;
213
214 lockdep_assert_held(&dev->mutex);
215
216 /* Round up new_size to multiple of PAGE_SIZE */
217 new_size = (new_size + PAGE_SIZE - 1) & PAGE_MASK;
218
219 /* if no change is required, do nothing */
220 if (async->prealloc_bufsz == new_size)
221 return 0;
222
223 /* deallocate old buffer */
224 __comedi_buf_free(dev, s);
225
226 /* allocate new buffer */
227 if (new_size) {
228 unsigned int n_pages = new_size >> PAGE_SHIFT;
229
230 __comedi_buf_alloc(dev, s, n_pages);
231 if (!async->prealloc_bufsz)
232 return -ENOMEM;
233 }
234
235 return 0;
236}
237
238void comedi_buf_reset(struct comedi_subdevice *s)
239{
240 struct comedi_async *async = s->async;
241
242 async->buf_write_alloc_count = 0;
243 async->buf_write_count = 0;
244 async->buf_read_alloc_count = 0;
245 async->buf_read_count = 0;
246
247 async->buf_write_ptr = 0;
248 async->buf_read_ptr = 0;
249
250 async->cur_chan = 0;
251 async->scans_done = 0;
252 async->scan_progress = 0;
253 async->munge_chan = 0;
254 async->munge_count = 0;
255 async->munge_ptr = 0;
256
257 async->events = 0;
258}
259
260static unsigned int comedi_buf_write_n_unalloc(struct comedi_subdevice *s)
261{
262 struct comedi_async *async = s->async;
263 unsigned int free_end = async->buf_read_count + async->prealloc_bufsz;
264
265 return free_end - async->buf_write_alloc_count;
266}
267
268unsigned int comedi_buf_write_n_available(struct comedi_subdevice *s)
269{
270 struct comedi_async *async = s->async;
271 unsigned int free_end = async->buf_read_count + async->prealloc_bufsz;
272
273 return free_end - async->buf_write_count;
274}
275
276unsigned int _comedi_buf_write_alloc(struct comedi_subdevice *s,
277 unsigned int nbytes)
278{
279 struct comedi_async *async = s->async;
280 unsigned int unalloc = comedi_buf_write_n_unalloc(s);
281
282 if (nbytes > unalloc)
283 nbytes = unalloc;
284
285 async->buf_write_alloc_count += nbytes;
286
287 /*
288 * ensure the async buffer 'counts' are read and updated
289 * before we write data to the write-alloc'ed buffer space
290 */
291 smp_mb();
292
293 return nbytes;
294}
295
296/**
297 * comedi_buf_write_alloc() - Reserve buffer space for writing
298 * @s: COMEDI subdevice.
299 * @nbytes: Maximum space to reserve in bytes.
300 *
301 * Reserve up to @nbytes bytes of space to be written in the COMEDI acquisition
302 * data buffer associated with the subdevice. The amount reserved is limited
303 * by the space available.
304 *
305 * Return: The amount of space reserved in bytes.
306 */
307unsigned int comedi_buf_write_alloc(struct comedi_subdevice *s,
308 unsigned int nbytes)
309{
310 if (comedi_get_is_subdevice_running(s)) {
311 nbytes = _comedi_buf_write_alloc(s, nbytes);
312 comedi_put_is_subdevice_running(s);
313 } else {
314 nbytes = 0;
315 }
316 return nbytes;
317}
318EXPORT_SYMBOL_GPL(comedi_buf_write_alloc);
319
320/*
321 * munging is applied to data by core as it passes between user
322 * and kernel space
323 */
324static unsigned int comedi_buf_munge(struct comedi_subdevice *s,
325 unsigned int num_bytes)
326{
327 struct comedi_async *async = s->async;
328 struct comedi_buf_page *buf_page_list = async->buf_map->page_list;
329 unsigned int count = 0;
330 const unsigned int num_sample_bytes = comedi_bytes_per_sample(s);
331
332 if (!s->munge || (async->cmd.flags & CMDF_RAWDATA) || async->cmd.chanlist_len == 0) {
333 async->munge_count += num_bytes;
334 return num_bytes;
335 }
336
337 /* don't munge partial samples */
338 num_bytes -= num_bytes % num_sample_bytes;
339 while (count < num_bytes) {
340 /*
341 * Do not munge beyond page boundary.
342 * Note: prealloc_bufsz is a multiple of PAGE_SIZE.
343 */
344 unsigned int page = async->munge_ptr >> PAGE_SHIFT;
345 unsigned int offset = offset_in_page(async->munge_ptr);
346 unsigned int block_size =
347 min(num_bytes - count, PAGE_SIZE - offset);
348
349 s->munge(s->device, s, buf_page_list[page].virt_addr + offset,
350 block_size, async->munge_chan);
351
352 /*
353 * ensure data is munged in buffer before the
354 * async buffer munge_count is incremented
355 */
356 smp_wmb();
357
358 async->munge_chan += block_size / num_sample_bytes;
359 async->munge_chan %= async->cmd.chanlist_len;
360 async->munge_count += block_size;
361 async->munge_ptr += block_size;
362 if (async->munge_ptr == async->prealloc_bufsz)
363 async->munge_ptr = 0;
364 count += block_size;
365 }
366
367 return count;
368}
369
370unsigned int comedi_buf_write_n_allocated(struct comedi_subdevice *s)
371{
372 struct comedi_async *async = s->async;
373
374 return async->buf_write_alloc_count - async->buf_write_count;
375}
376
377unsigned int _comedi_buf_write_free(struct comedi_subdevice *s,
378 unsigned int nbytes)
379{
380 struct comedi_async *async = s->async;
381 unsigned int allocated = comedi_buf_write_n_allocated(s);
382
383 if (nbytes > allocated)
384 nbytes = allocated;
385
386 async->buf_write_count += nbytes;
387 async->buf_write_ptr += nbytes;
388 comedi_buf_munge(s, num_bytes: async->buf_write_count - async->munge_count);
389 if (async->buf_write_ptr >= async->prealloc_bufsz)
390 async->buf_write_ptr %= async->prealloc_bufsz;
391
392 return nbytes;
393}
394
395/**
396 * comedi_buf_write_free() - Free buffer space after it is written
397 * @s: COMEDI subdevice.
398 * @nbytes: Maximum space to free in bytes.
399 *
400 * Free up to @nbytes bytes of space previously reserved for writing in the
401 * COMEDI acquisition data buffer associated with the subdevice. The amount of
402 * space freed is limited to the amount that was reserved. The freed space is
403 * assumed to have been filled with sample data by the writer.
404 *
405 * If the samples in the freed space need to be "munged", do so here. The
406 * freed space becomes available for allocation by the reader.
407 *
408 * Return: The amount of space freed in bytes.
409 */
410unsigned int comedi_buf_write_free(struct comedi_subdevice *s,
411 unsigned int nbytes)
412{
413 if (comedi_get_is_subdevice_running(s)) {
414 nbytes = _comedi_buf_write_free(s, nbytes);
415 comedi_put_is_subdevice_running(s);
416 } else {
417 nbytes = 0;
418 }
419 return nbytes;
420}
421EXPORT_SYMBOL_GPL(comedi_buf_write_free);
422
423unsigned int _comedi_buf_read_n_available(struct comedi_subdevice *s)
424{
425 struct comedi_async *async = s->async;
426 unsigned int num_bytes;
427
428 if (!async)
429 return 0;
430
431 num_bytes = async->munge_count - async->buf_read_count;
432
433 /*
434 * ensure the async buffer 'counts' are read before we
435 * attempt to read data from the buffer
436 */
437 smp_rmb();
438
439 return num_bytes;
440}
441
442/**
443 * comedi_buf_read_n_available() - Determine amount of readable buffer space
444 * @s: COMEDI subdevice.
445 *
446 * Determine the amount of readable buffer space in the COMEDI acquisition data
447 * buffer associated with the subdevice. The readable buffer space is that
448 * which has been freed by the writer and "munged" to the sample data format
449 * expected by COMEDI if necessary.
450 *
451 * Return: The amount of readable buffer space.
452 */
453unsigned int comedi_buf_read_n_available(struct comedi_subdevice *s)
454{
455 unsigned int num_bytes;
456
457 if (comedi_get_is_subdevice_running(s)) {
458 num_bytes = _comedi_buf_read_n_available(s);
459 comedi_put_is_subdevice_running(s);
460 } else {
461 num_bytes = 0;
462 }
463 return num_bytes;
464}
465EXPORT_SYMBOL_GPL(comedi_buf_read_n_available);
466
467unsigned int _comedi_buf_read_alloc(struct comedi_subdevice *s,
468 unsigned int nbytes)
469{
470 struct comedi_async *async = s->async;
471 unsigned int available;
472
473 available = async->munge_count - async->buf_read_alloc_count;
474 if (nbytes > available)
475 nbytes = available;
476
477 async->buf_read_alloc_count += nbytes;
478
479 /*
480 * ensure the async buffer 'counts' are read before we
481 * attempt to read data from the read-alloc'ed buffer space
482 */
483 smp_rmb();
484
485 return nbytes;
486}
487
488/**
489 * comedi_buf_read_alloc() - Reserve buffer space for reading
490 * @s: COMEDI subdevice.
491 * @nbytes: Maximum space to reserve in bytes.
492 *
493 * Reserve up to @nbytes bytes of previously written and "munged" buffer space
494 * for reading in the COMEDI acquisition data buffer associated with the
495 * subdevice. The amount reserved is limited to the space available. The
496 * reader can read from the reserved space and then free it. A reader is also
497 * allowed to read from the space before reserving it as long as it determines
498 * the amount of readable data available, but the space needs to be marked as
499 * reserved before it can be freed.
500 *
501 * Return: The amount of space reserved in bytes.
502 */
503unsigned int comedi_buf_read_alloc(struct comedi_subdevice *s,
504 unsigned int nbytes)
505{
506 if (comedi_get_is_subdevice_running(s)) {
507 nbytes = _comedi_buf_read_alloc(s, nbytes);
508 comedi_put_is_subdevice_running(s);
509 } else {
510 nbytes = 0;
511 }
512 return nbytes;
513}
514EXPORT_SYMBOL_GPL(comedi_buf_read_alloc);
515
516static unsigned int comedi_buf_read_n_allocated(struct comedi_async *async)
517{
518 return async->buf_read_alloc_count - async->buf_read_count;
519}
520
521unsigned int _comedi_buf_read_free(struct comedi_subdevice *s,
522 unsigned int nbytes)
523{
524 struct comedi_async *async = s->async;
525 unsigned int allocated;
526
527 /*
528 * ensure data has been read out of buffer before
529 * the async read count is incremented
530 */
531 smp_mb();
532
533 allocated = comedi_buf_read_n_allocated(async);
534 if (nbytes > allocated)
535 nbytes = allocated;
536
537 async->buf_read_count += nbytes;
538 async->buf_read_ptr += nbytes;
539 async->buf_read_ptr %= async->prealloc_bufsz;
540 return nbytes;
541}
542
543/**
544 * comedi_buf_read_free() - Free buffer space after it has been read
545 * @s: COMEDI subdevice.
546 * @nbytes: Maximum space to free in bytes.
547 *
548 * Free up to @nbytes bytes of buffer space previously reserved for reading in
549 * the COMEDI acquisition data buffer associated with the subdevice. The
550 * amount of space freed is limited to the amount that was reserved.
551 *
552 * The freed space becomes available for allocation by the writer.
553 *
554 * Return: The amount of space freed in bytes.
555 */
556unsigned int comedi_buf_read_free(struct comedi_subdevice *s,
557 unsigned int nbytes)
558{
559 if (comedi_get_is_subdevice_running(s)) {
560 nbytes = _comedi_buf_read_free(s, nbytes);
561 comedi_put_is_subdevice_running(s);
562 } else {
563 nbytes = 0;
564 }
565 return nbytes;
566}
567EXPORT_SYMBOL_GPL(comedi_buf_read_free);
568
569static void comedi_buf_memcpy_to(struct comedi_subdevice *s,
570 const void *data, unsigned int num_bytes)
571{
572 struct comedi_async *async = s->async;
573 struct comedi_buf_page *buf_page_list = async->buf_map->page_list;
574 unsigned int write_ptr = async->buf_write_ptr;
575
576 while (num_bytes) {
577 /*
578 * Do not copy beyond page boundary.
579 * Note: prealloc_bufsz is a multiple of PAGE_SIZE.
580 */
581 unsigned int page = write_ptr >> PAGE_SHIFT;
582 unsigned int offset = offset_in_page(write_ptr);
583 unsigned int block_size = min(num_bytes, PAGE_SIZE - offset);
584
585 memcpy(buf_page_list[page].virt_addr + offset,
586 data, block_size);
587
588 data += block_size;
589 num_bytes -= block_size;
590 write_ptr += block_size;
591 if (write_ptr == async->prealloc_bufsz)
592 write_ptr = 0;
593 }
594}
595
596static void comedi_buf_memcpy_from(struct comedi_subdevice *s,
597 void *dest, unsigned int nbytes)
598{
599 struct comedi_async *async = s->async;
600 struct comedi_buf_page *buf_page_list = async->buf_map->page_list;
601 unsigned int read_ptr = async->buf_read_ptr;
602
603 while (nbytes) {
604 /*
605 * Do not copy beyond page boundary.
606 * Note: prealloc_bufsz is a multiple of PAGE_SIZE.
607 */
608 unsigned int page = read_ptr >> PAGE_SHIFT;
609 unsigned int offset = offset_in_page(read_ptr);
610 unsigned int block_size = min(nbytes, PAGE_SIZE - offset);
611
612 memcpy(dest, buf_page_list[page].virt_addr + offset,
613 block_size);
614 nbytes -= block_size;
615 dest += block_size;
616 read_ptr += block_size;
617 if (read_ptr == async->prealloc_bufsz)
618 read_ptr = 0;
619 }
620}
621
622static unsigned int _comedi_buf_write_samples(struct comedi_subdevice *s,
623 const void *data,
624 unsigned int nsamples)
625{
626 unsigned int max_samples;
627 unsigned int nbytes;
628
629 /*
630 * Make sure there is enough room in the buffer for all the samples.
631 * If not, clamp the nsamples to the number that will fit, flag the
632 * buffer overrun and add the samples that fit.
633 */
634 max_samples = comedi_bytes_to_samples(s, nbytes: comedi_buf_write_n_unalloc(s));
635 if (nsamples > max_samples) {
636 dev_warn(s->device->class_dev, "buffer overrun\n");
637 s->async->events |= COMEDI_CB_OVERFLOW;
638 nsamples = max_samples;
639 }
640
641 if (nsamples == 0)
642 return 0;
643
644 nbytes = comedi_samples_to_bytes(s, nsamples);
645 nbytes = _comedi_buf_write_alloc(s, nbytes);
646 comedi_buf_memcpy_to(s, data, num_bytes: nbytes);
647 _comedi_buf_write_free(s, nbytes);
648 _comedi_inc_scan_progress(s, num_bytes: nbytes);
649 s->async->events |= COMEDI_CB_BLOCK;
650
651 return nbytes;
652}
653
654/**
655 * comedi_buf_write_samples() - Write sample data to COMEDI buffer
656 * @s: COMEDI subdevice.
657 * @data: Pointer to source samples.
658 * @nsamples: Number of samples to write.
659 *
660 * Write up to @nsamples samples to the COMEDI acquisition data buffer
661 * associated with the subdevice, mark it as written and update the
662 * acquisition scan progress. If there is not enough room for the specified
663 * number of samples, the number of samples written is limited to the number
664 * that will fit and the %COMEDI_CB_OVERFLOW event flag is set to cause the
665 * acquisition to terminate with an overrun error. Set the %COMEDI_CB_BLOCK
666 * event flag if any samples are written to cause waiting tasks to be woken
667 * when the event flags are processed.
668 *
669 * Return: The amount of data written in bytes.
670 */
671unsigned int comedi_buf_write_samples(struct comedi_subdevice *s,
672 const void *data, unsigned int nsamples)
673{
674 unsigned int nbytes;
675
676 if (comedi_get_is_subdevice_running(s)) {
677 nbytes = _comedi_buf_write_samples(s, data, nsamples);
678 comedi_put_is_subdevice_running(s);
679 } else {
680 nbytes = 0;
681 }
682 return nbytes;
683}
684EXPORT_SYMBOL_GPL(comedi_buf_write_samples);
685
686static unsigned int _comedi_buf_read_samples(struct comedi_subdevice *s,
687 void *data, unsigned int nsamples)
688{
689 unsigned int max_samples;
690 unsigned int nbytes;
691
692 /* clamp nsamples to the number of full samples available */
693 max_samples = comedi_bytes_to_samples(s,
694 nbytes: _comedi_buf_read_n_available(s));
695 if (nsamples > max_samples)
696 nsamples = max_samples;
697
698 if (nsamples == 0)
699 return 0;
700
701 nbytes = _comedi_buf_read_alloc(s,
702 nbytes: comedi_samples_to_bytes(s, nsamples));
703 comedi_buf_memcpy_from(s, dest: data, nbytes);
704 _comedi_buf_read_free(s, nbytes);
705 _comedi_inc_scan_progress(s, num_bytes: nbytes);
706 s->async->events |= COMEDI_CB_BLOCK;
707
708 return nbytes;
709}
710
711/**
712 * comedi_buf_read_samples() - Read sample data from COMEDI buffer
713 * @s: COMEDI subdevice.
714 * @data: Pointer to destination.
715 * @nsamples: Maximum number of samples to read.
716 *
717 * Read up to @nsamples samples from the COMEDI acquisition data buffer
718 * associated with the subdevice, mark it as read and update the acquisition
719 * scan progress. Limit the number of samples read to the number available.
720 * Set the %COMEDI_CB_BLOCK event flag if any samples are read to cause waiting
721 * tasks to be woken when the event flags are processed.
722 *
723 * Return: The amount of data read in bytes.
724 */
725unsigned int comedi_buf_read_samples(struct comedi_subdevice *s,
726 void *data, unsigned int nsamples)
727{
728 unsigned int nbytes;
729
730 if (comedi_get_is_subdevice_running(s)) {
731 nbytes = _comedi_buf_read_samples(s, data, nsamples);
732 comedi_put_is_subdevice_running(s);
733 } else {
734 nbytes = 0;
735 }
736 return nbytes;
737}
738EXPORT_SYMBOL_GPL(comedi_buf_read_samples);
739

source code of linux/drivers/comedi/comedi_buf.c