greybus: loopback: convert loopback wake/sleep to a waitqueue

Current code will incrementally poll for gb->type == 0 and sleep.
This type of polling strategy wastes cycles.

This patch changes the sleep strategy by introducing a wait-queue which
waits for gb->type != 0 or kthread_should_stop() to wake-up and work or
to wake-up and terminate.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/loopback.c

@@ -31,6 +31,7 @@ struct gb_loopback {
 	u8 version_minor;
 
 	struct task_struct *task;
+	wait_queue_head_t wq;
 
 	int type;
 	u32 size;
@@ -113,22 +114,24 @@ static DEVICE_ATTR_RW(field)
 static void gb_loopback_reset_stats(struct gb_loopback *gb);
 static void gb_loopback_check_attr(struct gb_loopback *gb)
 {
+	if (gb->ms_wait > GB_LOOPBACK_MS_WAIT_MAX)
+		gb->ms_wait = GB_LOOPBACK_MS_WAIT_MAX;
+	if (gb->size > gb->size_max)
+		gb->size = gb->size_max;
+	gb->error = 0;
+	gb->iteration_count = 0;
+	gb_loopback_reset_stats(gb);
+
 	switch (gb->type) {
 	case GB_LOOPBACK_TYPE_PING:
 	case GB_LOOPBACK_TYPE_TRANSFER:
 	case GB_LOOPBACK_TYPE_SINK:
+		wake_up(&gb->wq);
 		break;
 	default:
 		gb->type = 0;
 		break;
 	}
-	if (gb->ms_wait > GB_LOOPBACK_MS_WAIT_MAX)
-		gb->ms_wait = GB_LOOPBACK_MS_WAIT_MAX;
-	if (gb->size > gb->size_max)
-		gb->size = gb->size_max;
-	gb->error = 0;
-	gb->iteration_count = 0;
-	gb_loopback_reset_stats(gb);
 }
 
 /* Time to send and receive one message */
@@ -397,11 +400,12 @@ static int gb_loopback_fn(void *data)
 	struct timeval tlat = {0, 0};
 	struct gb_loopback *gb = (struct gb_loopback *)data;
 
-	while (!kthread_should_stop()) {
-		if (!gb->type) {
-			msleep(1000);
-			continue;
-		}
+	while (1) {
+		if (!gb->type)
+			wait_event_interruptible(gb->wq, gb->type ||
+						 kthread_should_stop());
+		if (kthread_should_stop())
+			break;
 		if (gb->iteration_max) {
 			if (gb->iteration_count < gb->iteration_max) {
 				gb->iteration_count++;
@@ -429,7 +433,6 @@ static int gb_loopback_fn(void *data)
 		gb->ts = gb->te;
 		if (gb->ms_wait)
 			msleep(gb->ms_wait);
-
 	}
 	return 0;
 }
@@ -463,6 +466,7 @@ static int gb_loopback_connection_init(struct gb_connection *connection)
 	gb->size_max -= sizeof(struct gb_loopback_transfer_request);
 
 	gb_loopback_reset_stats(gb);
+	init_waitqueue_head(&gb->wq);
 	gb->task = kthread_run(gb_loopback_fn, gb, "gb_loopback");
 	if (IS_ERR(gb->task)) {
 		retval = PTR_ERR(gb->task);