tty_buffer.c

/*
 * Tty buffer allocation management
 */

#include <linux/types.h>
#include <linux/errno.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/tty_flip.h>
#include <linux/timer.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/delay.h>
#include <linux/module.h>
#include <linux/ratelimit.h>


#define MIN_TTYB_SIZE	256
#define TTYB_ALIGN_MASK	255

/*
 * Byte threshold to limit memory consumption for flip buffers.
 * The actual memory limit is > 2x this amount.
 */
#define TTYB_DEFAULT_MEM_LIMIT	65536

/*
 * We default to dicing tty buffer allocations to this many characters
 * in order to avoid multiple page allocations. We know the size of
 * tty_buffer itself but it must also be taken into account that the
 * the buffer is 256 byte aligned. See tty_buffer_find for the allocation
 * logic this must match
 */

#define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)

/*
 * If all tty flip buffers have been processed by flush_to_ldisc() or
 * dropped by tty_buffer_flush(), check if the linked pty has been closed.
 * If so, wake the reader/poll to process
 */
static inline void check_other_closed(struct tty_struct *tty)
{
	unsigned long flags, old;

	/* transition from TTY_OTHER_CLOSED => TTY_OTHER_DONE must be atomic */
	for (flags = ACCESS_ONCE(tty->flags);
	     test_bit(TTY_OTHER_CLOSED, &flags);
	     ) {
		old = flags;
		__set_bit(TTY_OTHER_DONE, &flags);
		flags = cmpxchg(&tty->flags, old, flags);
		if (old == flags) {
			wake_up_interruptible(&tty->read_wait);
			break;
		}
	}
}

/**
 *	tty_buffer_lock_exclusive	-	gain exclusive access to buffer
 *	tty_buffer_unlock_exclusive	-	release exclusive access
 *
 *	@port - tty_port owning the flip buffer
 *
 *	Guarantees safe use of the line discipline's receive_buf() method by
 *	excluding the buffer work and any pending flush from using the flip
 *	buffer. Data can continue to be added concurrently to the flip buffer
 *	from the driver side.
 *
 *	On release, the buffer work is restarted if there is data in the
 *	flip buffer
 */

void tty_buffer_lock_exclusive(struct tty_port *port)
{
	struct tty_bufhead *buf = &port->buf;

	atomic_inc(&buf->priority);
	mutex_lock(&buf->lock);
}
EXPORT_SYMBOL_GPL(tty_buffer_lock_exclusive);

void tty_buffer_unlock_exclusive(struct tty_port *port)
{
	struct tty_bufhead *buf = &port->buf;
	int restart;

	restart = buf->head->commit != buf->head->read;

	atomic_dec(&buf->priority);
	mutex_unlock(&buf->lock);
	if (restart)
		queue_work(system_unbound_wq, &buf->work);
}
EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);

/**
 *	tty_buffer_space_avail	-	return unused buffer space
 *	@port - tty_port owning the flip buffer
 *
 *	Returns the # of bytes which can be written by the driver without
 *	reaching the buffer limit.
 *
 *	Note: this does not guarantee that memory is available to write
 *	the returned # of bytes (use tty_prepare_flip_string_xxx() to
 *	pre-allocate if memory guarantee is required).
 */

int tty_buffer_space_avail(struct tty_port *port)
{
	int space = port->buf.mem_limit - atomic_read(&port->buf.mem_used);
	return max(space, 0);
}
EXPORT_SYMBOL_GPL(tty_buffer_space_avail);

static void tty_buffer_reset(struct tty_buffer *p, size_t size)
{
	p->used = 0;
	p->size = size;
	p->next = NULL;
	p->commit = 0;
	p->read = 0;
	p->flags = 0;
}

/**
 *	tty_buffer_free_all		-	free buffers used by a tty
 *	@tty: tty to free from
 *
 *	Remove all the buffers pending on a tty whether queued with data
 *	or in the free ring. Must be called when the tty is no longer in use
 */

void tty_buffer_free_all(struct tty_port *port)
{
	struct tty_bufhead *buf = &port->buf;
	struct tty_buffer *p, *next;
	struct llist_node *llist;

	while ((p = buf->head) != NULL) {
		buf->head = p->next;
		if (p->size > 0)
			kfree(p);
	}
	llist = llist_del_all(&buf->free);
	llist_for_each_entry_safe(p, next, llist, free)
		kfree(p);

	tty_buffer_reset(&buf->sentinel, 0);
	buf->head = &buf->sentinel;
	buf->tail = &buf->sentinel;

	atomic_set(&buf->mem_used, 0);
}

/**
 *	tty_buffer_alloc	-	allocate a tty buffer
 *	@tty: tty device
 *	@size: desired size (characters)
 *
 *	Allocate a new tty buffer to hold the desired number of characters.
 *	We round our buffers off in 256 character chunks to get better
 *	allocation behaviour.
 *	Return NULL if out of memory or the allocation would exceed the
 *	per device queue
 */

static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
{
	struct llist_node *free;
	struct tty_buffer *p;

	/* Round the buffer size out */
	size = __ALIGN_MASK(size, TTYB_ALIGN_MASK);

	if (size <= MIN_TTYB_SIZE) {
		free = llist_del_first(&port->buf.free);
		if (free) {
			p = llist_entry(free, struct tty_buffer, free);
			goto found;
		}
	}

	/* Should possibly check if this fails for the largest buffer we
	   have queued and recycle that ? */
	if (atomic_read(&port->buf.mem_used) > port->buf.mem_limit)
		return NULL;
	p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
	if (p == NULL)
		return NULL;

found:
	tty_buffer_reset(p, size);
	atomic_add(size, &port->buf.mem_used);
	return p;
}

/**
 *	tty_buffer_free		-	free a tty buffer
 *	@tty: tty owning the buffer
 *	@b: the buffer to free
 *
 *	Free a tty buffer, or add it to the free list according to our
 *	internal strategy
 */

static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
{
	struct tty_bufhead *buf = &port->buf;

	/* Dumb strategy for now - should keep some stats */
	WARN_ON(atomic_sub_return(b->size, &buf->mem_used) < 0);

	if (b->size > MIN_TTYB_SIZE)
		kfree(b);
	else if (b->size > 0)
		llist_add(&b->free, &buf->free);
}

/**
 *	tty_buffer_flush		-	flush full tty buffers
 *	@tty: tty to flush
 *	@ld:  optional ldisc ptr (must be referenced)
 *
 *	flush all the buffers containing receive data. If ld != NULL,
 *	flush the ldisc input buffer.
 *
 *	Locking: takes buffer lock to ensure single-threaded flip buffer
 *		 'consumer'
 */

void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
{
	struct tty_port *port = tty->port;
	struct tty_bufhead *buf = &port->buf;
	struct tty_buffer *next;

	atomic_inc(&buf->priority);

	mutex_lock(&buf->lock);
	/* paired w/ release in __tty_buffer_request_room; ensures there are
	 * no pending memory accesses to the freed buffer
	 */
	while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
		tty_buffer_free(port, buf->head);
		buf->head = next;
	}
	buf->head->read = buf->head->commit;

	if (ld && ld->ops->flush_buffer)
		ld->ops->flush_buffer(tty);

	check_other_closed(tty);

	atomic_dec(&buf->priority);
	mutex_unlock(&buf->lock);
}

/**
 *	tty_buffer_request_room		-	grow tty buffer if needed
 *	@tty: tty structure
 *	@size: size desired
 *	@flags: buffer flags if new buffer allocated (default = 0)
 *
 *	Make at least size bytes of linear space available for the tty
 *	buffer. If we fail return the size we managed to find.
 *
 *	Will change over to a new buffer if the current buffer is encoded as
 *	TTY_NORMAL (so has no flags buffer) and the new buffer requires
 *	a flags buffer.
 */
static int __tty_buffer_request_room(struct tty_port *port, size_t size,
				     int flags)
{
	struct tty_bufhead *buf = &port->buf;
	struct tty_buffer *b, *n;
	int left, change;

	b = buf->tail;
	if (b->flags & TTYB_NORMAL)
		left = 2 * b->size - b->used;
	else
		left = b->size - b->used;

	change = (b->flags & TTYB_NORMAL) && (~flags & TTYB_NORMAL);
	if (change || left < size) {
		/* This is the slow path - looking for new buffers to use */
		n = tty_buffer_alloc(port, size);
		if (n != NULL) {
			n->flags = flags;
			buf->tail = n;
			/* paired w/ acquire in flush_to_ldisc(); ensures
			 * flush_to_ldisc() sees buffer data.
			 */
			smp_store_release(&b->commit, b->used);
			/* paired w/ acquire in flush_to_ldisc(); ensures the
			 * latest commit value can be read before the head is
			 * advanced to the next buffer
			 */
			smp_store_release(&b->next, n);
		} else if (change)
			size = 0;
		else
			size = left;
	}
	return size;
}

int tty_buffer_request_room(struct tty_port *port, size_t size)
{
	return __tty_buffer_request_room(port, size, 0);
}
EXPORT_SYMBOL_GPL(tty_buffer_request_room);

/**
 *	tty_insert_flip_string_fixed_flag - Add characters to the tty buffer
 *	@port: tty port
 *	@chars: characters
 *	@flag: flag value for each character
 *	@size: size
 *
 *	Queue a series of bytes to the tty buffering. All the characters
 *	passed are marked with the supplied flag. Returns the number added.
 */

int tty_insert_flip_string_fixed_flag(struct tty_port *port,
		const unsigned char *chars, char flag, size_t size)
{
	int copied = 0;
	do {
		int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
		int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;
		int space = __tty_buffer_request_room(port, goal, flags);
		struct tty_buffer *tb = port->buf.tail;
		if (unlikely(space == 0))
			break;
		memcpy(char_buf_ptr(tb, tb->used), chars, space);
		if (~tb->flags & TTYB_NORMAL)
			memset(flag_buf_ptr(tb, tb->used), flag, space);
		tb->used += space;
		copied += space;
		chars += space;
		/* There is a small chance that we need to split the data over
		   several buffers. If this is the case we must loop */
	} while (unlikely(size > copied));
	return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_fixed_flag);

/**
 *	tty_insert_flip_string_flags	-	Add characters to the tty buffer
 *	@port: tty port
 *	@chars: characters
 *	@flags: flag bytes
 *	@size: size
 *
 *	Queue a series of bytes to the tty buffering. For each character
 *	the flags array indicates the status of the character. Returns the
 *	number added.
 */

int tty_insert_flip_string_flags(struct tty_port *port,
		const unsigned char *chars, const char *flags, size_t size)
{
	int copied = 0;
	do {
		int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
		int space = tty_buffer_request_room(port, goal);
		struct tty_buffer *tb = port->buf.tail;
		if (unlikely(space == 0))
			break;
		memcpy(char_buf_ptr(tb, tb->used), chars, space);
		memcpy(flag_buf_ptr(tb, tb->used), flags, space);
		tb->used += space;
		copied += space;
		chars += space;
		flags += space;
		/* There is a small chance that we need to split the data over
		   several buffers. If this is the case we must loop */
	} while (unlikely(size > copied));
	return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_flags);

/**
 *	tty_schedule_flip	-	push characters to ldisc
 *	@port: tty port to push from
 *
 *	Takes any pending buffers and transfers their ownership to the
 *	ldisc side of the queue. It then schedules those characters for
 *	processing by the line discipline.
 */

void tty_schedule_flip(struct tty_port *port)
{
	struct tty_bufhead *buf = &port->buf;

	/* paired w/ acquire in flush_to_ldisc(); ensures
	 * flush_to_ldisc() sees buffer data.
	 */
	smp_store_release(&buf->tail->commit, buf->tail->used);
	queue_work(system_unbound_wq, &buf->work);
}
EXPORT_SYMBOL(tty_schedule_flip);

/**
 *	tty_prepare_flip_string		-	make room for characters
 *	@port: tty port
 *	@chars: return pointer for character write area
 *	@size: desired size
 *
 *	Prepare a block of space in the buffer for data. Returns the length
 *	available and buffer pointer to the space which is now allocated and
 *	accounted for as ready for normal characters. This is used for drivers
 *	that need their own block copy routines into the buffer. There is no
 *	guarantee the buffer is a DMA target!
 */

int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars,
		size_t size)
{
	int space = __tty_buffer_request_room(port, size, TTYB_NORMAL);
	if (likely(space)) {
		struct tty_buffer *tb = port->buf.tail;
		*chars = char_buf_ptr(tb, tb->used);
		if (~tb->flags & TTYB_NORMAL)
			memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
		tb->used += space;
	}
	return space;
}
EXPORT_SYMBOL_GPL(tty_prepare_flip_string);


static int
receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
{
	struct tty_ldisc *disc = tty->ldisc;
	unsigned char *p = char_buf_ptr(head, head->read);
	char	      *f = NULL;

	if (~head->flags & TTYB_NORMAL)
		f = flag_buf_ptr(head, head->read);

	if (disc->ops->receive_buf2)
		count = disc->ops->receive_buf2(tty, p, f, count);
	else {
		count = min_t(int, count, tty->receive_room);
		if (count && disc->ops->receive_buf)
			disc->ops->receive_buf(tty, p, f, count);
	}
	return count;
}

/**
 *	flush_to_ldisc
 *	@work: tty structure passed from work queue.
 *
 *	This routine is called out of the software interrupt to flush data
 *	from the buffer chain to the line discipline.
 *
 *	The receive_buf method is single threaded for each tty instance.
 *
 *	Locking: takes buffer lock to ensure single-threaded flip buffer
 *		 'consumer'
 */

static void flush_to_ldisc(struct work_struct *work)
{
	struct tty_port *port = container_of(work, struct tty_port, buf.work);
	struct tty_bufhead *buf = &port->buf;
	struct tty_struct *tty;
	struct tty_ldisc *disc;

	tty = READ_ONCE(port->itty);
	if (tty == NULL)
		return;

	disc = tty_ldisc_ref(tty);
	if (disc == NULL)
		return;

	mutex_lock(&buf->lock);

	while (1) {
		struct tty_buffer *head = buf->head;
		struct tty_buffer *next;
		int count;

		/* Ldisc or user is trying to gain exclusive access */
		if (atomic_read(&buf->priority))
			break;

		/* paired w/ release in __tty_buffer_request_room();
		 * ensures commit value read is not stale if the head
		 * is advancing to the next buffer
		 */
		next = smp_load_acquire(&head->next);
		/* paired w/ release in __tty_buffer_request_room() or in
		 * tty_buffer_flush(); ensures we see the committed buffer data
		 */
		count = smp_load_acquire(&head->commit) - head->read;
		if (!count) {
			if (next == NULL) {
				check_other_closed(tty);
				break;
			}
			buf->head = next;
			tty_buffer_free(port, head);
			continue;
		}

		count = receive_buf(tty, head, count);
		if (!count)
			break;
		head->read += count;
	}

	mutex_unlock(&buf->lock);

	tty_ldisc_deref(disc);
}

/**
 *	tty_flip_buffer_push	-	terminal
 *	@port: tty port to push
 *
 *	Queue a push of the terminal flip buffers to the line discipline.
 *	Can be called from IRQ/atomic context.
 *
 *	In the event of the queue being busy for flipping the work will be
 *	held off and retried later.
 */

void tty_flip_buffer_push(struct tty_port *port)
{
	tty_schedule_flip(port);
}
EXPORT_SYMBOL(tty_flip_buffer_push);

/**
 *	tty_buffer_init		-	prepare a tty buffer structure
 *	@tty: tty to initialise
 *
 *	Set up the initial state of the buffer management for a tty device.
 *	Must be called before the other tty buffer functions are used.
 */

void tty_buffer_init(struct tty_port *port)
{
	struct tty_bufhead *buf = &port->buf;

	mutex_init(&buf->lock);
	tty_buffer_reset(&buf->sentinel, 0);
	buf->head = &buf->sentinel;
	buf->tail = &buf->sentinel;
	init_llist_head(&buf->free);
	atomic_set(&buf->mem_used, 0);
	atomic_set(&buf->priority, 0);
	INIT_WORK(&buf->work, flush_to_ldisc);
	buf->mem_limit = TTYB_DEFAULT_MEM_LIMIT;
}

/**
 *	tty_buffer_set_limit	-	change the tty buffer memory limit
 *	@port: tty port to change
 *
 *	Change the tty buffer memory limit.
 *	Must be called before the other tty buffer functions are used.
 */

int tty_buffer_set_limit(struct tty_port *port, int limit)
{
	if (limit < MIN_TTYB_SIZE)
		return -EINVAL;
	port->buf.mem_limit = limit;
	return 0;
}
EXPORT_SYMBOL_GPL(tty_buffer_set_limit);

/* slave ptys can claim nested buffer lock when handling BRK and INTR */
void tty_buffer_set_lock_subclass(struct tty_port *port)
{
	lockdep_set_subclass(&port->buf.lock, TTY_LOCK_SLAVE);
}

bool tty_buffer_restart_work(struct tty_port *port)
{
	return queue_work(system_unbound_wq, &port->buf.work);
}

bool tty_buffer_cancel_work(struct tty_port *port)
{
	return cancel_work_sync(&port->buf.work);
}