tty: move tty_port workqueue to be a kthread

This makes each tty_port have their own kthread, hopefully allowing them
a bit more freedom in scheduling when they reveive data.

Based on a patch from Philip Cuadra <philipcuadra@google.com>
Based on a patch from Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 36106419
Test: run Bluetooth audio, ensure separate tty thread is created.
Signed-off-by: Philip Cuadra <philipcuadra@google.com>
Change-Id: I9fd25235b26a66acb37a40304c356209b74ad46c

drivers/tty/tty_buffer.c

@@ -71,7 +71,7 @@ void tty_buffer_unlock_exclusive(struct tty_port *port)
 	atomic_dec(&buf->priority);
 	mutex_unlock(&buf->lock);
 	if (restart)
-		queue_work(system_unbound_wq, &buf->work);
+		queue_kthread_work(&port->worker, &buf->work);
 }
 EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);
 
@@ -132,6 +132,7 @@ void tty_buffer_free_all(struct tty_port *port)
 	buf->tail = &buf->sentinel;
 
 	atomic_set(&buf->mem_used, 0);
+	kthread_stop(port->worker_thread);
 }
 
 /**
@@ -404,7 +405,7 @@ void tty_schedule_flip(struct tty_port *port)
 	 * flush_to_ldisc() sees buffer data.
 	 */
 	smp_store_release(&buf->tail->commit, buf->tail->used);
-	queue_work(system_unbound_wq, &buf->work);
+	queue_kthread_work(&port->worker, &buf->work);
 }
 EXPORT_SYMBOL(tty_schedule_flip);
 
@@ -472,7 +473,7 @@ receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
  *		 'consumer'
  */
 
-static void flush_to_ldisc(struct work_struct *work)
+static void flush_to_ldisc(struct kthread_work *work)
 {
 	struct tty_port *port = container_of(work, struct tty_port, buf.work);
 	struct tty_bufhead *buf = &port->buf;
@@ -562,8 +563,20 @@ void tty_buffer_init(struct tty_port *port)
 	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;
+	init_kthread_work(&buf->work, flush_to_ldisc);
+	init_kthread_worker(&port->worker);
+	port->worker_thread = kthread_run(kthread_worker_fn, &port->worker,
+					  "tty_worker_thread");
+	if (IS_ERR(port->worker_thread)) {
+		/*
+		 * Not good, we can't unwind, this tty is going to be really
+		 * sad...
+		 */
+		pr_err("Unable to start tty_worker_thread\n");
+	}
+
+
 }
 
 /**
@@ -591,15 +604,15 @@ void tty_buffer_set_lock_subclass(struct tty_port *port)
 
 bool tty_buffer_restart_work(struct tty_port *port)
 {
-	return queue_work(system_unbound_wq, &port->buf.work);
+	return queue_kthread_work(&port->worker, &port->buf.work);
 }
 
 bool tty_buffer_cancel_work(struct tty_port *port)
 {
-	return cancel_work_sync(&port->buf.work);
+	return kthread_cancel_work_sync(&port->buf.work);
 }
 
 void tty_buffer_flush_work(struct tty_port *port)
 {
-	flush_work(&port->buf.work);
+	flush_kthread_work(&port->buf.work);
 }

include/linux/tty.h

@@ -12,7 +12,7 @@
 #include <uapi/linux/tty.h>
 #include <linux/rwsem.h>
 #include <linux/llist.h>
-
+#include <linux/kthread.h>
 
 /*
  * Lock subclasses for tty locks
@@ -82,7 +82,7 @@ static inline char *flag_buf_ptr(struct tty_buffer *b, int ofs)
 
 struct tty_bufhead {
 	struct tty_buffer *head;	/* Queue head */
-	struct work_struct work;
+	struct kthread_work work;
 	struct mutex	   lock;
 	atomic_t	   priority;
 	struct tty_buffer sentinel;
@@ -240,6 +240,8 @@ struct tty_port {
 						   based drain is needed else
 						   set to size of fifo */
 	struct kref		kref;		/* Ref counter */
+	struct kthread_worker   worker;         /* worker thread */
+	struct task_struct      *worker_thread; /* worker thread */
 };
 
 /*