Merge tag 'tty-4.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty

Pull tty fixes from Greg KH:
 "Here are three small HVC tty driver fixes to resolve a reported
  regression from 4.19-rc1.

  All of these have been in linux-next for a while with no reported
  issues"

* tag 'tty-4.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty:
  tty: hvc: hvc_write() fix break condition
  tty: hvc: hvc_poll() fix read loop batching
  tty: hvc: hvc_poll() fix read loop hang

Author: torvalds

drivers/tty/hvc/hvc_console.c

@@ -522,6 +522,8 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
 		return -EIO;
 
 	while (count > 0) {
+		int ret = 0;
+
 		spin_lock_irqsave(&hp->lock, flags);
 
 		rsize = hp->outbuf_size - hp->n_outbuf;
@@ -537,10 +539,13 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
 		}
 
 		if (hp->n_outbuf > 0)
-			hvc_push(hp);
+			ret = hvc_push(hp);
 
 		spin_unlock_irqrestore(&hp->lock, flags);
 
+		if (!ret)
+			break;
+
 		if (count) {
 			if (hp->n_outbuf > 0)
 				hvc_flush(hp);
@@ -623,6 +628,15 @@ static int hvc_chars_in_buffer(struct tty_struct *tty)
 #define MAX_TIMEOUT		(2000)
 static u32 timeout = MIN_TIMEOUT;
 
+/*
+ * Maximum number of bytes to get from the console driver if hvc_poll is
+ * called from driver (and can't sleep). Any more than this and we break
+ * and start polling with khvcd. This value was derived from from an OpenBMC
+ * console with the OPAL driver that results in about 0.25ms interrupts off
+ * latency.
+ */
+#define HVC_ATOMIC_READ_MAX	128
+
 #define HVC_POLL_READ	0x00000001
 #define HVC_POLL_WRITE	0x00000002
 
@@ -669,8 +683,8 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
 	if (!hp->irq_requested)
 		poll_mask |= HVC_POLL_READ;
 
+ read_again:
 	/* Read data if any */
-
 	count = tty_buffer_request_room(&hp->port, N_INBUF);
 
 	/* If flip is full, just reschedule a later read */
@@ -717,9 +731,23 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
 #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;
+
+	if (may_sleep) {
+		/* Keep going until the flip is full */
+		spin_unlock_irqrestore(&hp->lock, flags);
+		cond_resched();
+		spin_lock_irqsave(&hp->lock, flags);
+		goto read_again;
+	} else if (read_total < HVC_ATOMIC_READ_MAX) {
+		/* Break and defer if it's a large read in atomic */
+		goto read_again;
+	}
+
+	/*
+	 * Latency break, schedule another poll immediately.
+	 */
+	poll_mask |= HVC_POLL_READ;
 
  out:
 	/* Wakeup write queue if necessary */