xref: /openbmc/linux/drivers/tty/tty_buffer.c (revision ebee41c8)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Tty buffer allocation management
4  */
5 
6 #include <linux/types.h>
7 #include <linux/errno.h>
8 #include <linux/minmax.h>
9 #include <linux/tty.h>
10 #include <linux/tty_driver.h>
11 #include <linux/tty_flip.h>
12 #include <linux/timer.h>
13 #include <linux/string.h>
14 #include <linux/slab.h>
15 #include <linux/sched.h>
16 #include <linux/wait.h>
17 #include <linux/bitops.h>
18 #include <linux/delay.h>
19 #include <linux/module.h>
20 #include <linux/ratelimit.h>
21 #include "tty.h"
22 
23 #define MIN_TTYB_SIZE	256
24 #define TTYB_ALIGN_MASK	0xff
25 
26 /*
27  * Byte threshold to limit memory consumption for flip buffers.
28  * The actual memory limit is > 2x this amount.
29  */
30 #define TTYB_DEFAULT_MEM_LIMIT	(640 * 1024UL)
31 
32 /*
33  * We default to dicing tty buffer allocations to this many characters
34  * in order to avoid multiple page allocations. We know the size of
35  * tty_buffer itself but it must also be taken into account that the
36  * buffer is 256 byte aligned. See tty_buffer_find for the allocation
37  * logic this must match.
38  */
39 
40 #define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~TTYB_ALIGN_MASK)
41 
42 /**
43  * tty_buffer_lock_exclusive	-	gain exclusive access to buffer
44  * @port: tty port owning the flip buffer
45  *
46  * Guarantees safe use of the &tty_ldisc_ops.receive_buf() method by excluding
47  * the buffer work and any pending flush from using the flip buffer. Data can
48  * continue to be added concurrently to the flip buffer from the driver side.
49  *
50  * See also tty_buffer_unlock_exclusive().
51  */
tty_buffer_lock_exclusive(struct tty_port * port)52 void tty_buffer_lock_exclusive(struct tty_port *port)
53 {
54 	struct tty_bufhead *buf = &port->buf;
55 
56 	atomic_inc(&buf->priority);
57 	mutex_lock(&buf->lock);
58 }
59 EXPORT_SYMBOL_GPL(tty_buffer_lock_exclusive);
60 
61 /**
62  * tty_buffer_unlock_exclusive	-	release exclusive access
63  * @port: tty port owning the flip buffer
64  *
65  * The buffer work is restarted if there is data in the flip buffer.
66  *
67  * See also tty_buffer_lock_exclusive().
68  */
tty_buffer_unlock_exclusive(struct tty_port * port)69 void tty_buffer_unlock_exclusive(struct tty_port *port)
70 {
71 	struct tty_bufhead *buf = &port->buf;
72 	int restart;
73 
74 	restart = buf->head->commit != buf->head->read;
75 
76 	atomic_dec(&buf->priority);
77 	mutex_unlock(&buf->lock);
78 	if (restart)
79 		queue_work(system_unbound_wq, &buf->work);
80 }
81 EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);
82 
83 /**
84  * tty_buffer_space_avail	-	return unused buffer space
85  * @port: tty port owning the flip buffer
86  *
87  * Returns: the # of bytes which can be written by the driver without reaching
88  * the buffer limit.
89  *
90  * Note: this does not guarantee that memory is available to write the returned
91  * # of bytes (use tty_prepare_flip_string() to pre-allocate if memory
92  * guarantee is required).
93  */
tty_buffer_space_avail(struct tty_port * port)94 unsigned int tty_buffer_space_avail(struct tty_port *port)
95 {
96 	int space = port->buf.mem_limit - atomic_read(&port->buf.mem_used);
97 
98 	return max(space, 0);
99 }
100 EXPORT_SYMBOL_GPL(tty_buffer_space_avail);
101 
tty_buffer_reset(struct tty_buffer * p,size_t size)102 static void tty_buffer_reset(struct tty_buffer *p, size_t size)
103 {
104 	p->used = 0;
105 	p->size = size;
106 	p->next = NULL;
107 	p->commit = 0;
108 	p->lookahead = 0;
109 	p->read = 0;
110 	p->flags = true;
111 }
112 
113 /**
114  * tty_buffer_free_all		-	free buffers used by a tty
115  * @port: tty port to free from
116  *
117  * Remove all the buffers pending on a tty whether queued with data or in the
118  * free ring. Must be called when the tty is no longer in use.
119  */
tty_buffer_free_all(struct tty_port * port)120 void tty_buffer_free_all(struct tty_port *port)
121 {
122 	struct tty_bufhead *buf = &port->buf;
123 	struct tty_buffer *p, *next;
124 	struct llist_node *llist;
125 	unsigned int freed = 0;
126 	int still_used;
127 
128 	while ((p = buf->head) != NULL) {
129 		buf->head = p->next;
130 		freed += p->size;
131 		if (p->size > 0)
132 			kfree(p);
133 	}
134 	llist = llist_del_all(&buf->free);
135 	llist_for_each_entry_safe(p, next, llist, free)
136 		kfree(p);
137 
138 	tty_buffer_reset(&buf->sentinel, 0);
139 	buf->head = &buf->sentinel;
140 	buf->tail = &buf->sentinel;
141 
142 	still_used = atomic_xchg(&buf->mem_used, 0);
143 	WARN(still_used != freed, "we still have not freed %d bytes!",
144 			still_used - freed);
145 }
146 
147 /**
148  * tty_buffer_alloc	-	allocate a tty buffer
149  * @port: tty port
150  * @size: desired size (characters)
151  *
152  * Allocate a new tty buffer to hold the desired number of characters. We
153  * round our buffers off in 256 character chunks to get better allocation
154  * behaviour.
155  *
156  * Returns: %NULL if out of memory or the allocation would exceed the per
157  * device queue.
158  */
tty_buffer_alloc(struct tty_port * port,size_t size)159 static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
160 {
161 	struct llist_node *free;
162 	struct tty_buffer *p;
163 
164 	/* Round the buffer size out */
165 	size = __ALIGN_MASK(size, TTYB_ALIGN_MASK);
166 
167 	if (size <= MIN_TTYB_SIZE) {
168 		free = llist_del_first(&port->buf.free);
169 		if (free) {
170 			p = llist_entry(free, struct tty_buffer, free);
171 			goto found;
172 		}
173 	}
174 
175 	/* Should possibly check if this fails for the largest buffer we
176 	 * have queued and recycle that ?
177 	 */
178 	if (atomic_read(&port->buf.mem_used) > port->buf.mem_limit)
179 		return NULL;
180 	p = kmalloc(struct_size(p, data, 2 * size), GFP_ATOMIC | __GFP_NOWARN);
181 	if (p == NULL)
182 		return NULL;
183 
184 found:
185 	tty_buffer_reset(p, size);
186 	atomic_add(size, &port->buf.mem_used);
187 	return p;
188 }
189 
190 /**
191  * tty_buffer_free		-	free a tty buffer
192  * @port: tty port owning the buffer
193  * @b: the buffer to free
194  *
195  * Free a tty buffer, or add it to the free list according to our internal
196  * strategy.
197  */
tty_buffer_free(struct tty_port * port,struct tty_buffer * b)198 static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
199 {
200 	struct tty_bufhead *buf = &port->buf;
201 
202 	/* Dumb strategy for now - should keep some stats */
203 	WARN_ON(atomic_sub_return(b->size, &buf->mem_used) < 0);
204 
205 	if (b->size > MIN_TTYB_SIZE)
206 		kfree(b);
207 	else if (b->size > 0)
208 		llist_add(&b->free, &buf->free);
209 }
210 
211 /**
212  * tty_buffer_flush		-	flush full tty buffers
213  * @tty: tty to flush
214  * @ld: optional ldisc ptr (must be referenced)
215  *
216  * Flush all the buffers containing receive data. If @ld != %NULL, flush the
217  * ldisc input buffer.
218  *
219  * Locking: takes buffer lock to ensure single-threaded flip buffer 'consumer'.
220  */
tty_buffer_flush(struct tty_struct * tty,struct tty_ldisc * ld)221 void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
222 {
223 	struct tty_port *port = tty->port;
224 	struct tty_bufhead *buf = &port->buf;
225 	struct tty_buffer *next;
226 
227 	atomic_inc(&buf->priority);
228 
229 	mutex_lock(&buf->lock);
230 	/* paired w/ release in __tty_buffer_request_room; ensures there are
231 	 * no pending memory accesses to the freed buffer
232 	 */
233 	while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
234 		tty_buffer_free(port, buf->head);
235 		buf->head = next;
236 	}
237 	buf->head->read = buf->head->commit;
238 	buf->head->lookahead = buf->head->read;
239 
240 	if (ld && ld->ops->flush_buffer)
241 		ld->ops->flush_buffer(tty);
242 
243 	atomic_dec(&buf->priority);
244 	mutex_unlock(&buf->lock);
245 }
246 
247 /**
248  * __tty_buffer_request_room	-	grow tty buffer if needed
249  * @port: tty port
250  * @size: size desired
251  * @flags: buffer has to store flags along character data
252  *
253  * Make at least @size bytes of linear space available for the tty buffer.
254  *
255  * Will change over to a new buffer if the current buffer is encoded as
256  * %TTY_NORMAL (so has no flags buffer) and the new buffer requires a flags
257  * buffer.
258  *
259  * Returns: the size we managed to find.
260  */
__tty_buffer_request_room(struct tty_port * port,size_t size,bool flags)261 static int __tty_buffer_request_room(struct tty_port *port, size_t size,
262 				     bool flags)
263 {
264 	struct tty_bufhead *buf = &port->buf;
265 	struct tty_buffer *n, *b = buf->tail;
266 	size_t left = (b->flags ? 1 : 2) * b->size - b->used;
267 	bool change = !b->flags && flags;
268 
269 	if (!change && left >= size)
270 		return size;
271 
272 	/* This is the slow path - looking for new buffers to use */
273 	n = tty_buffer_alloc(port, size);
274 	if (n == NULL)
275 		return change ? 0 : left;
276 
277 	n->flags = flags;
278 	buf->tail = n;
279 	/*
280 	 * Paired w/ acquire in flush_to_ldisc() and lookahead_bufs()
281 	 * ensures they see all buffer data.
282 	 */
283 	smp_store_release(&b->commit, b->used);
284 	/*
285 	 * Paired w/ acquire in flush_to_ldisc() and lookahead_bufs()
286 	 * ensures the latest commit value can be read before the head
287 	 * is advanced to the next buffer.
288 	 */
289 	smp_store_release(&b->next, n);
290 
291 	return size;
292 }
293 
tty_buffer_request_room(struct tty_port * port,size_t size)294 int tty_buffer_request_room(struct tty_port *port, size_t size)
295 {
296 	return __tty_buffer_request_room(port, size, true);
297 }
298 EXPORT_SYMBOL_GPL(tty_buffer_request_room);
299 
__tty_insert_flip_string_flags(struct tty_port * port,const u8 * chars,const u8 * flags,bool mutable_flags,size_t size)300 size_t __tty_insert_flip_string_flags(struct tty_port *port, const u8 *chars,
301 				      const u8 *flags, bool mutable_flags,
302 				      size_t size)
303 {
304 	bool need_flags = mutable_flags || flags[0] != TTY_NORMAL;
305 	size_t copied = 0;
306 
307 	do {
308 		size_t goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
309 		size_t space = __tty_buffer_request_room(port, goal, need_flags);
310 		struct tty_buffer *tb = port->buf.tail;
311 
312 		if (unlikely(space == 0))
313 			break;
314 
315 		memcpy(char_buf_ptr(tb, tb->used), chars, space);
316 
317 		if (mutable_flags) {
318 			memcpy(flag_buf_ptr(tb, tb->used), flags, space);
319 			flags += space;
320 		} else if (tb->flags) {
321 			memset(flag_buf_ptr(tb, tb->used), flags[0], space);
322 		} else {
323 			/* tb->flags should be available once requested */
324 			WARN_ON_ONCE(need_flags);
325 		}
326 
327 		tb->used += space;
328 		copied += space;
329 		chars += space;
330 
331 		/* There is a small chance that we need to split the data over
332 		 * several buffers. If this is the case we must loop.
333 		 */
334 	} while (unlikely(size > copied));
335 
336 	return copied;
337 }
338 EXPORT_SYMBOL(__tty_insert_flip_string_flags);
339 
340 /**
341  * tty_prepare_flip_string	-	make room for characters
342  * @port: tty port
343  * @chars: return pointer for character write area
344  * @size: desired size
345  *
346  * Prepare a block of space in the buffer for data.
347  *
348  * This is used for drivers that need their own block copy routines into the
349  * buffer. There is no guarantee the buffer is a DMA target!
350  *
351  * Returns: the length available and buffer pointer (@chars) to the space which
352  * is now allocated and accounted for as ready for normal characters.
353  */
tty_prepare_flip_string(struct tty_port * port,u8 ** chars,size_t size)354 size_t tty_prepare_flip_string(struct tty_port *port, u8 **chars, size_t size)
355 {
356 	size_t space = __tty_buffer_request_room(port, size, false);
357 
358 	if (likely(space)) {
359 		struct tty_buffer *tb = port->buf.tail;
360 
361 		*chars = char_buf_ptr(tb, tb->used);
362 		if (tb->flags)
363 			memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
364 		tb->used += space;
365 	}
366 
367 	return space;
368 }
369 EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
370 
371 /**
372  * tty_ldisc_receive_buf	-	forward data to line discipline
373  * @ld: line discipline to process input
374  * @p: char buffer
375  * @f: %TTY_NORMAL, %TTY_BREAK, etc. flags buffer
376  * @count: number of bytes to process
377  *
378  * Callers other than flush_to_ldisc() need to exclude the kworker from
379  * concurrent use of the line discipline, see paste_selection().
380  *
381  * Returns: the number of bytes processed.
382  */
tty_ldisc_receive_buf(struct tty_ldisc * ld,const u8 * p,const u8 * f,size_t count)383 size_t tty_ldisc_receive_buf(struct tty_ldisc *ld, const u8 *p, const u8 *f,
384 			     size_t count)
385 {
386 	if (ld->ops->receive_buf2)
387 		count = ld->ops->receive_buf2(ld->tty, p, f, count);
388 	else {
389 		count = min_t(size_t, count, ld->tty->receive_room);
390 		if (count && ld->ops->receive_buf)
391 			ld->ops->receive_buf(ld->tty, p, f, count);
392 	}
393 	return count;
394 }
395 EXPORT_SYMBOL_GPL(tty_ldisc_receive_buf);
396 
lookahead_bufs(struct tty_port * port,struct tty_buffer * head)397 static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
398 {
399 	head->lookahead = max(head->lookahead, head->read);
400 
401 	while (head) {
402 		struct tty_buffer *next;
403 		unsigned int count;
404 
405 		/*
406 		 * Paired w/ release in __tty_buffer_request_room();
407 		 * ensures commit value read is not stale if the head
408 		 * is advancing to the next buffer.
409 		 */
410 		next = smp_load_acquire(&head->next);
411 		/*
412 		 * Paired w/ release in __tty_buffer_request_room() or in
413 		 * tty_buffer_flush(); ensures we see the committed buffer data.
414 		 */
415 		count = smp_load_acquire(&head->commit) - head->lookahead;
416 		if (!count) {
417 			head = next;
418 			continue;
419 		}
420 
421 		if (port->client_ops->lookahead_buf) {
422 			u8 *p, *f = NULL;
423 
424 			p = char_buf_ptr(head, head->lookahead);
425 			if (head->flags)
426 				f = flag_buf_ptr(head, head->lookahead);
427 
428 			port->client_ops->lookahead_buf(port, p, f, count);
429 		}
430 
431 		head->lookahead += count;
432 	}
433 }
434 
435 static size_t
receive_buf(struct tty_port * port,struct tty_buffer * head,size_t count)436 receive_buf(struct tty_port *port, struct tty_buffer *head, size_t count)
437 {
438 	u8 *p = char_buf_ptr(head, head->read);
439 	const u8 *f = NULL;
440 	size_t n;
441 
442 	if (head->flags)
443 		f = flag_buf_ptr(head, head->read);
444 
445 	n = port->client_ops->receive_buf(port, p, f, count);
446 	if (n > 0)
447 		memset(p, 0, n);
448 	return n;
449 }
450 
451 /**
452  * flush_to_ldisc		-	flush data from buffer to ldisc
453  * @work: tty structure passed from work queue.
454  *
455  * This routine is called out of the software interrupt to flush data from the
456  * buffer chain to the line discipline.
457  *
458  * The receive_buf() method is single threaded for each tty instance.
459  *
460  * Locking: takes buffer lock to ensure single-threaded flip buffer 'consumer'.
461  */
flush_to_ldisc(struct work_struct * work)462 static void flush_to_ldisc(struct work_struct *work)
463 {
464 	struct tty_port *port = container_of(work, struct tty_port, buf.work);
465 	struct tty_bufhead *buf = &port->buf;
466 
467 	mutex_lock(&buf->lock);
468 
469 	while (1) {
470 		struct tty_buffer *head = buf->head;
471 		struct tty_buffer *next;
472 		size_t count, rcvd;
473 
474 		/* Ldisc or user is trying to gain exclusive access */
475 		if (atomic_read(&buf->priority))
476 			break;
477 
478 		/* paired w/ release in __tty_buffer_request_room();
479 		 * ensures commit value read is not stale if the head
480 		 * is advancing to the next buffer
481 		 */
482 		next = smp_load_acquire(&head->next);
483 		/* paired w/ release in __tty_buffer_request_room() or in
484 		 * tty_buffer_flush(); ensures we see the committed buffer data
485 		 */
486 		count = smp_load_acquire(&head->commit) - head->read;
487 		if (!count) {
488 			if (next == NULL)
489 				break;
490 			buf->head = next;
491 			tty_buffer_free(port, head);
492 			continue;
493 		}
494 
495 		rcvd = receive_buf(port, head, count);
496 		head->read += rcvd;
497 		if (rcvd < count)
498 			lookahead_bufs(port, head);
499 		if (!rcvd)
500 			break;
501 
502 		if (need_resched())
503 			cond_resched();
504 	}
505 
506 	mutex_unlock(&buf->lock);
507 
508 }
509 
tty_flip_buffer_commit(struct tty_buffer * tail)510 static inline void tty_flip_buffer_commit(struct tty_buffer *tail)
511 {
512 	/*
513 	 * Paired w/ acquire in flush_to_ldisc(); ensures flush_to_ldisc() sees
514 	 * buffer data.
515 	 */
516 	smp_store_release(&tail->commit, tail->used);
517 }
518 
519 /**
520  * tty_flip_buffer_push		-	push terminal buffers
521  * @port: tty port to push
522  *
523  * Queue a push of the terminal flip buffers to the line discipline. Can be
524  * called from IRQ/atomic context.
525  *
526  * In the event of the queue being busy for flipping the work will be held off
527  * and retried later.
528  */
tty_flip_buffer_push(struct tty_port * port)529 void tty_flip_buffer_push(struct tty_port *port)
530 {
531 	struct tty_bufhead *buf = &port->buf;
532 
533 	tty_flip_buffer_commit(buf->tail);
534 	queue_work(system_unbound_wq, &buf->work);
535 }
536 EXPORT_SYMBOL(tty_flip_buffer_push);
537 
538 /**
539  * tty_insert_flip_string_and_push_buffer - add characters to the tty buffer and
540  *	push
541  * @port: tty port
542  * @chars: characters
543  * @size: size
544  *
545  * The function combines tty_insert_flip_string() and tty_flip_buffer_push()
546  * with the exception of properly holding the @port->lock.
547  *
548  * To be used only internally (by pty currently).
549  *
550  * Returns: the number added.
551  */
tty_insert_flip_string_and_push_buffer(struct tty_port * port,const u8 * chars,size_t size)552 int tty_insert_flip_string_and_push_buffer(struct tty_port *port,
553 					   const u8 *chars, size_t size)
554 {
555 	struct tty_bufhead *buf = &port->buf;
556 	unsigned long flags;
557 
558 	spin_lock_irqsave(&port->lock, flags);
559 	size = tty_insert_flip_string(port, chars, size);
560 	if (size)
561 		tty_flip_buffer_commit(buf->tail);
562 	spin_unlock_irqrestore(&port->lock, flags);
563 
564 	queue_work(system_unbound_wq, &buf->work);
565 
566 	return size;
567 }
568 
569 /**
570  * tty_buffer_init		-	prepare a tty buffer structure
571  * @port: tty port to initialise
572  *
573  * Set up the initial state of the buffer management for a tty device. Must be
574  * called before the other tty buffer functions are used.
575  */
tty_buffer_init(struct tty_port * port)576 void tty_buffer_init(struct tty_port *port)
577 {
578 	struct tty_bufhead *buf = &port->buf;
579 
580 	mutex_init(&buf->lock);
581 	tty_buffer_reset(&buf->sentinel, 0);
582 	buf->head = &buf->sentinel;
583 	buf->tail = &buf->sentinel;
584 	init_llist_head(&buf->free);
585 	atomic_set(&buf->mem_used, 0);
586 	atomic_set(&buf->priority, 0);
587 	INIT_WORK(&buf->work, flush_to_ldisc);
588 	buf->mem_limit = TTYB_DEFAULT_MEM_LIMIT;
589 }
590 
591 /**
592  * tty_buffer_set_limit		-	change the tty buffer memory limit
593  * @port: tty port to change
594  * @limit: memory limit to set
595  *
596  * Change the tty buffer memory limit.
597  *
598  * Must be called before the other tty buffer functions are used.
599  */
tty_buffer_set_limit(struct tty_port * port,int limit)600 int tty_buffer_set_limit(struct tty_port *port, int limit)
601 {
602 	if (limit < MIN_TTYB_SIZE)
603 		return -EINVAL;
604 	port->buf.mem_limit = limit;
605 	return 0;
606 }
607 EXPORT_SYMBOL_GPL(tty_buffer_set_limit);
608 
609 /* slave ptys can claim nested buffer lock when handling BRK and INTR */
tty_buffer_set_lock_subclass(struct tty_port * port)610 void tty_buffer_set_lock_subclass(struct tty_port *port)
611 {
612 	lockdep_set_subclass(&port->buf.lock, TTY_LOCK_SLAVE);
613 }
614 
tty_buffer_restart_work(struct tty_port * port)615 bool tty_buffer_restart_work(struct tty_port *port)
616 {
617 	return queue_work(system_unbound_wq, &port->buf.work);
618 }
619 
tty_buffer_cancel_work(struct tty_port * port)620 bool tty_buffer_cancel_work(struct tty_port *port)
621 {
622 	return cancel_work_sync(&port->buf.work);
623 }
624 
tty_buffer_flush_work(struct tty_port * port)625 void tty_buffer_flush_work(struct tty_port *port)
626 {
627 	flush_work(&port->buf.work);
628 }
629