1 files changed, 458 insertions, 379 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 9121c1f7aeef..1a5673acd9b1 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -1,548 +1,627 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Tty buffer allocation management
  */
 
 #include <linux/types.h>
 #include <linux/errno.h>
+#include <linux/minmax.h>
 #include <linux/tty.h>
+#include <linux/tty_buffer.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/init.h>
 #include <linux/wait.h>
 #include <linux/bitops.h>
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/ratelimit.h>
+#include "tty.h"
+
+#define MIN_TTYB_SIZE	256
+#define TTYB_ALIGN_MASK	0xff
+
+/*
+ * Byte threshold to limit memory consumption for flip buffers.
+ * The actual memory limit is > 2x this amount.
+ */
+#define TTYB_DEFAULT_MEM_LIMIT	(640 * 1024UL)
+
+/*
+ * 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
+ * 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) & ~TTYB_ALIGN_MASK)
 
 /**
- *	tty_buffer_free_all		-	free buffers used by a tty
- *	@tty: tty to free from
+ * tty_buffer_lock_exclusive	-	gain exclusive access to buffer
+ * @port: tty port owning the flip buffer
  *
- *	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
+ * Guarantees safe use of the &tty_ldisc_ops.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.
  *
- *	Locking: none
+ * See also tty_buffer_unlock_exclusive().
  */
-
-void tty_buffer_free_all(struct tty_port *port)
+void tty_buffer_lock_exclusive(struct tty_port *port)
 {
 	struct tty_bufhead *buf = &port->buf;
-	struct tty_buffer *thead;
 
-	while ((thead = buf->head) != NULL) {
-		buf->head = thead->next;
-		kfree(thead);
-	}
-	while ((thead = buf->free) != NULL) {
-		buf->free = thead->next;
-		kfree(thead);
-	}
-	buf->tail = NULL;
-	buf->memory_used = 0;
+	atomic_inc(&buf->priority);
+	mutex_lock(&buf->lock);
 }
+EXPORT_SYMBOL_GPL(tty_buffer_lock_exclusive);
 
 /**
- *	tty_buffer_alloc	-	allocate a tty buffer
- *	@tty: tty device
- *	@size: desired size (characters)
+ * tty_buffer_unlock_exclusive	-	release exclusive access
+ * @port: tty port owning the flip buffer
  *
- *	Allocate a new tty buffer to hold the desired number of characters.
- *	Return NULL if out of memory or the allocation would exceed the
- *	per device queue
+ * The buffer work is restarted if there is data in the flip buffer.
  *
- *	Locking: Caller must hold tty->buf.lock
+ * See also tty_buffer_lock_exclusive().
  */
+void tty_buffer_unlock_exclusive(struct tty_port *port)
+{
+	struct tty_bufhead *buf = &port->buf;
+	bool restart = buf->head->commit != buf->head->read;
 
-static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
+	atomic_dec(&buf->priority);
+	mutex_unlock(&buf->lock);
+
+	if (restart)
+		queue_work(system_dfl_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() to pre-allocate if memory
+ * guarantee is required).
+ */
+unsigned int tty_buffer_space_avail(struct tty_port *port)
 {
-	struct tty_buffer *p;
+	int space = port->buf.mem_limit - atomic_read(&port->buf.mem_used);
 
-	if (port->buf.memory_used + size > 65536)
-		return NULL;
-	p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
-	if (p == NULL)
-		return NULL;
+	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->lookahead = 0;
 	p->read = 0;
-	p->char_buf_ptr = (char *)(p->data);
-	p->flag_buf_ptr = (unsigned char *)p->char_buf_ptr + size;
-	port->buf.memory_used += size;
-	return p;
+	p->flags = true;
 }
 
 /**
- *	tty_buffer_free		-	free a tty buffer
- *	@tty: tty owning the buffer
- *	@b: the buffer to free
+ * tty_buffer_free_all		-	free buffers used by a tty
+ * @port: tty port to free from
  *
- *	Free a tty buffer, or add it to the free list according to our
- *	internal strategy
- *
- *	Locking: Caller must hold tty->buf.lock
+ * 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.
  */
-
-static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
+void tty_buffer_free_all(struct tty_port *port)
 {
 	struct tty_bufhead *buf = &port->buf;
+	struct tty_buffer *p, *next;
+	struct llist_node *llist;
+	unsigned int freed = 0;
+	int still_used;
+
+	while ((p = buf->head) != NULL) {
+		buf->head = p->next;
+		freed += p->size;
+		if (p->size > 0)
+			kfree(p);
+	}
+	llist = llist_del_all(&buf->free);
+	llist_for_each_entry_safe(p, next, llist, free)
+		kfree(p);
 
-	/* Dumb strategy for now - should keep some stats */
-	buf->memory_used -= b->size;
-	WARN_ON(buf->memory_used < 0);
+	tty_buffer_reset(&buf->sentinel, 0);
+	buf->head = &buf->sentinel;
+	buf->tail = &buf->sentinel;
 
-	if (b->size >= 512)
-		kfree(b);
-	else {
-		b->next = buf->free;
-		buf->free = b;
-	}
+	still_used = atomic_xchg(&buf->mem_used, 0);
+	WARN(still_used != freed, "we still have not freed %d bytes!",
+			still_used - freed);
 }
 
 /**
- *	__tty_buffer_flush		-	flush full tty buffers
- *	@tty: tty to flush
+ * tty_buffer_alloc	-	allocate a tty buffer
+ * @port: tty port
+ * @size: desired size (characters)
  *
- *	flush all the buffers containing receive data. Caller must
- *	hold the buffer lock and must have ensured no parallel flush to
- *	ldisc is running.
+ * 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.
  *
- *	Locking: Caller must hold tty->buf.lock
+ * Returns: %NULL if out of memory or the allocation would exceed the per
+ * device queue.
  */
-
-static void __tty_buffer_flush(struct tty_port *port)
+static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
 {
-	struct tty_bufhead *buf = &port->buf;
-	struct tty_buffer *thead;
+	struct llist_node *free;
+	struct tty_buffer *p;
 
-	if (unlikely(buf->head == NULL))
-		return;
-	while ((thead = buf->head->next) != NULL) {
-		tty_buffer_free(port, buf->head);
-		buf->head = thead;
+	/* 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;
+		}
 	}
-	WARN_ON(buf->head != buf->tail);
-	buf->head->read = buf->head->commit;
+
+	/* 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(struct_size(p, data, 2 * size), GFP_ATOMIC | __GFP_NOWARN);
+	if (p == NULL)
+		return NULL;
+
+found:
+	tty_buffer_reset(p, size);
+	atomic_add(size, &port->buf.mem_used);
+	return p;
 }
 
 /**
- *	tty_buffer_flush		-	flush full tty buffers
- *	@tty: tty to flush
- *
- *	flush all the buffers containing receive data. If the buffer is
- *	being processed by flush_to_ldisc then we defer the processing
- *	to that function
+ * tty_buffer_free		-	free a tty buffer
+ * @port: tty port owning the buffer
+ * @b: the buffer to free
  *
- *	Locking: none
+ * Free a tty buffer, or add it to the free list according to our internal
+ * strategy.
  */
-
-void tty_buffer_flush(struct tty_struct *tty)
+static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
 {
-	struct tty_port *port = tty->port;
 	struct tty_bufhead *buf = &port->buf;
-	unsigned long flags;
 
-	spin_lock_irqsave(&buf->lock, flags);
-
-	/* If the data is being pushed to the tty layer then we can't
-	   process it here. Instead set a flag and the flush_to_ldisc
-	   path will process the flush request before it exits */
-	if (test_bit(TTYP_FLUSHING, &port->iflags)) {
-		set_bit(TTYP_FLUSHPENDING, &port->iflags);
-		spin_unlock_irqrestore(&buf->lock, flags);
-		wait_event(tty->read_wait,
-				test_bit(TTYP_FLUSHPENDING, &port->iflags) == 0);
-		return;
-	} else
-		__tty_buffer_flush(port);
-	spin_unlock_irqrestore(&buf->lock, flags);
+	/* 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_find		-	find a free tty buffer
- *	@tty: tty owning the buffer
- *	@size: characters wanted
+ * tty_buffer_flush		-	flush full tty buffers
+ * @tty: tty to flush
+ * @ld: optional ldisc ptr (must be referenced)
  *
- *	Locate an existing suitable tty buffer or if we are lacking one then
- *	allocate a new one. We round our buffers off in 256 character chunks
- *	to get better allocation behaviour.
+ * Flush all the buffers containing receive data. If @ld != %NULL, flush the
+ * ldisc input buffer.
  *
- *	Locking: Caller must hold tty->buf.lock
+ * Locking: takes buffer lock to ensure single-threaded flip buffer 'consumer'.
  */
-
-static struct tty_buffer *tty_buffer_find(struct tty_port *port, size_t size)
+void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
 {
-	struct tty_buffer **tbh = &port->buf.free;
-	while ((*tbh) != NULL) {
-		struct tty_buffer *t = *tbh;
-		if (t->size >= size) {
-			*tbh = t->next;
-			t->next = NULL;
-			t->used = 0;
-			t->commit = 0;
-			t->read = 0;
-			port->buf.memory_used += t->size;
-			return t;
-		}
-		tbh = &((*tbh)->next);
+	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;
 	}
-	/* Round the buffer size out */
-	size = (size + 0xFF) & ~0xFF;
-	return tty_buffer_alloc(port, size);
-	/* Should possibly check if this fails for the largest buffer we
-	   have queued and recycle that ? */
+	buf->head->read = buf->head->commit;
+	buf->head->lookahead = buf->head->read;
+
+	if (ld && ld->ops->flush_buffer)
+		ld->ops->flush_buffer(tty);
+
+	atomic_dec(&buf->priority);
+	mutex_unlock(&buf->lock);
 }
+
 /**
- *	tty_buffer_request_room		-	grow tty buffer if needed
- *	@tty: tty structure
- *	@size: size desired
+ * __tty_buffer_request_room	-	grow tty buffer if needed
+ * @port: tty port
+ * @size: size desired
+ * @flags: buffer has to store flags along character data
  *
- *	Make at least size bytes of linear space available for the tty
- *	buffer. If we fail return the size we managed to find.
+ * Make at least @size bytes of linear space available for the tty buffer.
  *
- *	Locking: Takes port->buf.lock
+ * 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.
+ *
+ * Returns: the size we managed to find.
  */
-int tty_buffer_request_room(struct tty_port *port, size_t size)
+static int __tty_buffer_request_room(struct tty_port *port, size_t size,
+				     bool flags)
 {
 	struct tty_bufhead *buf = &port->buf;
-	struct tty_buffer *b, *n;
-	int left;
-	unsigned long flags;
-	spin_lock_irqsave(&buf->lock, flags);
-	/* OPTIMISATION: We could keep a per tty "zero" sized buffer to
-	   remove this conditional if its worth it. This would be invisible
-	   to the callers */
-	b = buf->tail;
-	if (b != NULL)
-		left = b->size - b->used;
-	else
-		left = 0;
-
-	if (left < size) {
-		/* This is the slow path - looking for new buffers to use */
-		if ((n = tty_buffer_find(port, size)) != NULL) {
-			if (b != NULL) {
-				b->next = n;
-				b->commit = b->used;
-			} else
-				buf->head = n;
-			buf->tail = n;
-		} else
-			size = left;
-	}
-	spin_unlock_irqrestore(&buf->lock, flags);
+	struct tty_buffer *n, *b = buf->tail;
+	size_t left = (b->flags ? 1 : 2) * b->size - b->used;
+	bool change = !b->flags && flags;
+
+	if (!change && left >= size)
+		return size;
+
+	/* This is the slow path - looking for new buffers to use */
+	n = tty_buffer_alloc(port, size);
+	if (n == NULL)
+		return change ? 0 : left;
+
+	n->flags = flags;
+	buf->tail = n;
+	/*
+	 * Paired w/ acquire in flush_to_ldisc() and lookahead_bufs()
+	 * ensures they see all buffer data.
+	 */
+	smp_store_release(&b->commit, b->used);
+	/*
+	 * Paired w/ acquire in flush_to_ldisc() and lookahead_bufs()
+	 * ensures the latest commit value can be read before the head
+	 * is advanced to the next buffer.
+	 */
+	smp_store_release(&b->next, n);
+
 	return size;
 }
-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.
- *
- *	Locking: Called functions may take port->buf.lock
- */
-
-int tty_insert_flip_string_fixed_flag(struct tty_port *port,
-		const unsigned char *chars, char flag, size_t size)
+int tty_buffer_request_room(struct tty_port *port, 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 there is no space then tb may be NULL */
-		if (unlikely(space == 0)) {
-			break;
-		}
-		memcpy(tb->char_buf_ptr + tb->used, chars, space);
-		memset(tb->flag_buf_ptr + 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;
+	return __tty_buffer_request_room(port, size, true);
 }
-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.
- *
- *	Locking: Called functions may take port->buf.lock
- */
+EXPORT_SYMBOL_GPL(tty_buffer_request_room);
 
-int tty_insert_flip_string_flags(struct tty_port *port,
-		const unsigned char *chars, const char *flags, size_t size)
+size_t __tty_insert_flip_string_flags(struct tty_port *port, const u8 *chars,
+				      const u8 *flags, bool mutable_flags,
+				      size_t size)
 {
-	int copied = 0;
+	bool need_flags = mutable_flags || flags[0] != TTY_NORMAL;
+	size_t copied = 0;
+
 	do {
-		int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
-		int space = tty_buffer_request_room(port, goal);
+		size_t goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
+		size_t space = __tty_buffer_request_room(port, goal, need_flags);
 		struct tty_buffer *tb = port->buf.tail;
-		/* If there is no space then tb may be NULL */
-		if (unlikely(space == 0)) {
+
+		if (unlikely(space == 0))
 			break;
+
+		memcpy(char_buf_ptr(tb, tb->used), chars, space);
+
+		if (mutable_flags) {
+			memcpy(flag_buf_ptr(tb, tb->used), flags, space);
+			flags += space;
+		} else if (tb->flags) {
+			memset(flag_buf_ptr(tb, tb->used), flags[0], space);
+		} else {
+			/* tb->flags should be available once requested */
+			WARN_ON_ONCE(need_flags);
 		}
-		memcpy(tb->char_buf_ptr + tb->used, chars, space);
-		memcpy(tb->flag_buf_ptr + 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 */
+		 * several buffers. If this is the case we must loop.
+		 */
 	} while (unlikely(size > copied));
+
 	return copied;
 }
-EXPORT_SYMBOL(tty_insert_flip_string_flags);
+EXPORT_SYMBOL(__tty_insert_flip_string_flags);
 
 /**
- *	tty_schedule_flip	-	push characters to ldisc
- *	@port: tty port to push from
+ * tty_prepare_flip_string	-	make room for characters
+ * @port: tty port
+ * @chars: return pointer for character write area
+ * @size: desired size
  *
- *	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.
- *	Note that this function can only be used when the low_latency flag
- *	is unset. Otherwise the workqueue won't be flushed.
- *
- *	Locking: Takes port->buf.lock
- */
-
-void tty_schedule_flip(struct tty_port *port)
-{
-	struct tty_bufhead *buf = &port->buf;
-	unsigned long flags;
-	WARN_ON(port->low_latency);
-
-	spin_lock_irqsave(&buf->lock, flags);
-	if (buf->tail != NULL)
-		buf->tail->commit = buf->tail->used;
-	spin_unlock_irqrestore(&buf->lock, flags);
-	schedule_work(&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.
  *
- *	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!
+ * 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!
  *
- *	Locking: May call functions taking port->buf.lock
+ * Returns: the length available and buffer pointer (@chars) to the space which
+ * is now allocated and accounted for as ready for normal characters.
  */
-
-int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars,
-		size_t size)
+size_t tty_prepare_flip_string(struct tty_port *port, u8 **chars, size_t size)
 {
-	int space = tty_buffer_request_room(port, size);
+	size_t space = __tty_buffer_request_room(port, size, false);
+
 	if (likely(space)) {
 		struct tty_buffer *tb = port->buf.tail;
-		*chars = tb->char_buf_ptr + tb->used;
-		memset(tb->flag_buf_ptr + tb->used, TTY_NORMAL, space);
+
+		*chars = char_buf_ptr(tb, tb->used);
+		if (tb->flags)
+			memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
 		tb->used += space;
 	}
+
 	return space;
 }
 EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
 
 /**
- *	tty_prepare_flip_string_flags	-	make room for characters
- *	@port: tty port
- *	@chars: return pointer for character write area
- *	@flags: return pointer for status flag write area
- *	@size: desired size
+ * tty_ldisc_receive_buf	-	forward data to line discipline
+ * @ld: line discipline to process input
+ * @p: char buffer
+ * @f: %TTY_NORMAL, %TTY_BREAK, etc. flags buffer
+ * @count: number of bytes to process
  *
- *	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 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!
+ * Callers other than flush_to_ldisc() need to exclude the kworker from
+ * concurrent use of the line discipline, see paste_selection().
  *
- *	Locking: May call functions taking port->buf.lock
+ * Returns: the number of bytes processed.
  */
+size_t tty_ldisc_receive_buf(struct tty_ldisc *ld, const u8 *p, const u8 *f,
+			     size_t count)
+{
+	if (ld->ops->receive_buf2)
+		count = ld->ops->receive_buf2(ld->tty, p, f, count);
+	else {
+		count = min_t(size_t, count, ld->tty->receive_room);
+		if (count && ld->ops->receive_buf)
+			ld->ops->receive_buf(ld->tty, p, f, count);
+	}
+	return count;
+}
+EXPORT_SYMBOL_GPL(tty_ldisc_receive_buf);
 
-int tty_prepare_flip_string_flags(struct tty_port *port,
-			unsigned char **chars, char **flags, size_t size)
+static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
 {
-	int space = tty_buffer_request_room(port, size);
-	if (likely(space)) {
-		struct tty_buffer *tb = port->buf.tail;
-		*chars = tb->char_buf_ptr + tb->used;
-		*flags = tb->flag_buf_ptr + tb->used;
-		tb->used += space;
+	head->lookahead = max(head->lookahead, head->read);
+
+	while (head) {
+		struct tty_buffer *next;
+		unsigned int count;
+
+		/*
+		 * 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->lookahead;
+		if (!count) {
+			head = next;
+			continue;
+		}
+
+		if (port->client_ops->lookahead_buf) {
+			u8 *p, *f = NULL;
+
+			p = char_buf_ptr(head, head->lookahead);
+			if (head->flags)
+				f = flag_buf_ptr(head, head->lookahead);
+
+			port->client_ops->lookahead_buf(port, p, f, count);
+		}
+
+		head->lookahead += count;
 	}
-	return space;
 }
-EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);
 
+static size_t
+receive_buf(struct tty_port *port, struct tty_buffer *head, size_t count)
+{
+	u8 *p = char_buf_ptr(head, head->read);
+	const u8 *f = NULL;
+	size_t n;
+
+	if (head->flags)
+		f = flag_buf_ptr(head, head->read);
 
+	n = port->client_ops->receive_buf(port, p, f, count);
+	if (n > 0)
+		memset(p, 0, n);
+	return n;
+}
 
 /**
- *	flush_to_ldisc
- *	@work: tty structure passed from work queue.
+ * flush_to_ldisc		-	flush data from buffer 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.
+ * This routine is called out of the software interrupt to flush data from the
+ * buffer chain to the line discipline.
  *
- *	Locking: holds tty->buf.lock to guard buffer list. Drops the lock
- *	while invoking the line discipline receive_buf method. The
- *	receive_buf method is single threaded for each tty instance.
+ * 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;
-	unsigned long 	flags;
-	struct tty_ldisc *disc;
-
-	tty = port->itty;
-	if (tty == NULL)
-		return;
-
-	disc = tty_ldisc_ref(tty);
-	if (disc == NULL)	/*  !TTY_LDISC */
-		return;
-
-	spin_lock_irqsave(&buf->lock, flags);
-
-	if (!test_and_set_bit(TTYP_FLUSHING, &port->iflags)) {
-		struct tty_buffer *head;
-		while ((head = buf->head) != NULL) {
-			int count;
-			char *char_buf;
-			unsigned char *flag_buf;
-
-			count = head->commit - head->read;
-			if (!count) {
-				if (head->next == NULL)
-					break;
-				buf->head = head->next;
-				tty_buffer_free(port, head);
-				continue;
-			}
-			if (!tty->receive_room)
-				break;
-			if (count > tty->receive_room)
-				count = tty->receive_room;
-			char_buf = head->char_buf_ptr + head->read;
-			flag_buf = head->flag_buf_ptr + head->read;
-			head->read += count;
-			spin_unlock_irqrestore(&buf->lock, flags);
-			disc->ops->receive_buf(tty, char_buf,
-							flag_buf, count);
-			spin_lock_irqsave(&buf->lock, flags);
-			/* Ldisc or user is trying to flush the buffers.
-			   We may have a deferred request to flush the
-			   input buffer, if so pull the chain under the lock
-			   and empty the queue */
-			if (test_bit(TTYP_FLUSHPENDING, &port->iflags)) {
-				__tty_buffer_flush(port);
-				clear_bit(TTYP_FLUSHPENDING, &port->iflags);
-				wake_up(&tty->read_wait);
+
+	mutex_lock(&buf->lock);
+
+	while (1) {
+		struct tty_buffer *head = buf->head;
+		struct tty_buffer *next;
+		size_t count, rcvd;
+
+		/* 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)
 				break;
-			}
+			buf->head = next;
+			tty_buffer_free(port, head);
+			continue;
 		}
-		clear_bit(TTYP_FLUSHING, &port->iflags);
+
+		rcvd = receive_buf(port, head, count);
+		head->read += rcvd;
+		if (rcvd < count)
+			lookahead_bufs(port, head);
+		if (!rcvd)
+			break;
+
+		cond_resched();
 	}
 
-	spin_unlock_irqrestore(&buf->lock, flags);
+	mutex_unlock(&buf->lock);
+
+}
 
-	tty_ldisc_deref(disc);
+static inline void tty_flip_buffer_commit(struct tty_buffer *tail)
+{
+	/*
+	 * Paired w/ acquire in flush_to_ldisc(); ensures flush_to_ldisc() sees
+	 * buffer data.
+	 */
+	smp_store_release(&tail->commit, tail->used);
 }
 
 /**
- *	tty_flush_to_ldisc
- *	@tty: tty to push
+ * tty_flip_buffer_push		-	push terminal buffers
+ * @port: tty port to push
  *
- *	Push the terminal flip buffers to the line discipline.
+ * Queue a push of the terminal flip buffers to the line discipline. Can be
+ * called from IRQ/atomic context.
  *
- *	Must not be called from IRQ context.
+ * In the event of the queue being busy for flipping the work will be held off
+ * and retried later.
  */
-void tty_flush_to_ldisc(struct tty_struct *tty)
+void tty_flip_buffer_push(struct tty_port *port)
 {
-	if (!tty->port->low_latency)
-		flush_work(&tty->port->buf.work);
+	struct tty_bufhead *buf = &port->buf;
+
+	tty_flip_buffer_commit(buf->tail);
+	queue_work(system_dfl_wq, &buf->work);
 }
+EXPORT_SYMBOL(tty_flip_buffer_push);
 
 /**
- *	tty_flip_buffer_push	-	terminal
- *	@port: tty port to push
+ * tty_insert_flip_string_and_push_buffer - add characters to the tty buffer and
+ *	push
+ * @port: tty port
+ * @chars: characters
+ * @size: size
  *
- *	Queue a push of the terminal flip buffers to the line discipline. This
- *	function must not be called from IRQ context if port->low_latency is
- *	set.
+ * The function combines tty_insert_flip_string() and tty_flip_buffer_push()
+ * with the exception of properly holding the @port->lock.
  *
- *	In the event of the queue being busy for flipping the work will be
- *	held off and retried later.
+ * To be used only internally (by pty currently).
  *
- *	Locking: tty buffer lock. Driver locks in low latency mode.
+ * Returns: the number added.
  */
-
-void tty_flip_buffer_push(struct tty_port *port)
+int tty_insert_flip_string_and_push_buffer(struct tty_port *port,
+					   const u8 *chars, size_t size)
 {
 	struct tty_bufhead *buf = &port->buf;
 	unsigned long flags;
 
-	spin_lock_irqsave(&buf->lock, flags);
-	if (buf->tail != NULL)
-		buf->tail->commit = buf->tail->used;
-	spin_unlock_irqrestore(&buf->lock, flags);
+	spin_lock_irqsave(&port->lock, flags);
+	size = tty_insert_flip_string(port, chars, size);
+	if (size)
+		tty_flip_buffer_commit(buf->tail);
+	spin_unlock_irqrestore(&port->lock, flags);
 
-	if (port->low_latency)
-		flush_to_ldisc(&buf->work);
-	else
-		schedule_work(&buf->work);
+	queue_work(system_dfl_wq, &buf->work);
+
+	return size;
 }
-EXPORT_SYMBOL(tty_flip_buffer_push);
 
 /**
- *	tty_buffer_init		-	prepare a tty buffer structure
- *	@tty: tty to initialise
+ * tty_buffer_init		-	prepare a tty buffer structure
+ * @port: tty port 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.
- *
- *	Locking: none
+ * 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;
 
-	spin_lock_init(&buf->lock);
-	buf->head = NULL;
-	buf->tail = NULL;
-	buf->free = NULL;
-	buf->memory_used = 0;
+	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
+ * @limit: memory limit to set
+ *
+ * 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_dfl_wq, &port->buf.work);
+}
+
+bool tty_buffer_cancel_work(struct tty_port *port)
+{
+	return cancel_work_sync(&port->buf.work);
+}
+
+void tty_buffer_flush_work(struct tty_port *port)
+{
+	flush_work(&port->buf.work);
+}