tty: hvc: hvc_poll() break hv read loop

Avoid looping with the spinlock held while there is read data
being returned from the hv driver. Instead note if the entire
size returned by tty_buffer_request_room was read, and request
another read poll.

This limits the critical section lengths, and provides more
even service to other consoles in case there is a pathological
condition.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/tty/hvc/hvc_console.c

@@ -592,7 +592,7 @@ static u32 timeout = MIN_TIMEOUT;
 int hvc_poll(struct hvc_struct *hp)
 {
 	struct tty_struct *tty;
-	int i, n, poll_mask = 0;
+	int i, n, count, poll_mask = 0;
 	char buf[N_INBUF] __ALIGNED__;
 	unsigned long flags;
 	int read_total = 0;
@@ -618,7 +618,7 @@ int hvc_poll(struct hvc_struct *hp)
 
 	/* Now check if we can get data (are we throttled ?) */
 	if (tty_throttled(tty))
-		goto throttled;
+		goto out;
 
 	/* If we aren't notifier driven and aren't throttled, we always
 	 * request a reschedule
@@ -627,13 +627,13 @@ int hvc_poll(struct hvc_struct *hp)
 		poll_mask |= HVC_POLL_READ;
 
 	/* Read data if any */
-	for (;;) {
-		int count = tty_buffer_request_room(&hp->port, N_INBUF);
+
+	count = tty_buffer_request_room(&hp->port, N_INBUF);
 
 	/* If flip is full, just reschedule a later read */
 	if (count == 0) {
 		poll_mask |= HVC_POLL_READ;
-			break;
+		goto out;
 	}
 
 	n = hp->ops->get_chars(hp->vtermno, buf, count);
@@ -651,8 +651,9 @@ int hvc_poll(struct hvc_struct *hp)
 			 */
 			poll_mask |= HVC_POLL_READ;
 		}
-			break;
+		goto out;
 	}
+
 	for (i = 0; i < n; ++i) {
 #ifdef CONFIG_MAGIC_SYSRQ
 		if (hp->index == hvc_console.index) {
@@ -673,10 +674,11 @@ int hvc_poll(struct hvc_struct *hp)
 #endif /* CONFIG_MAGIC_SYSRQ */
 		tty_insert_flip_char(&hp->port, buf[i], 0);
 	}
+	if (n == count)
+		poll_mask |= HVC_POLL_READ;
+	read_total = n;
 
-		read_total += n;
-	}
- throttled:
+ out:
 	/* Wakeup write queue if necessary */
 	if (hp->do_wakeup) {
 		hp->do_wakeup = 0;