[POWERPC] hvc_console: Polling mode timer backoff

Add a back-off mechanism to hvc_console's polling logic.   This change
drops the timers/second ratio from ~90 to ~1/2 while the console is
idle.
This change is most noticeable when watching /proc/timer_stats output.

This only affects when the hvc_console is running in poll mode, i.e.
power4 and cell systems.

I've tested on Power4, Michael Ellerman has both contributed to the
patch and tested on cell.

Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

drivers/char/hvc_console.c

@@ -47,8 +47,6 @@
 #define HVC_MAJOR	229
 #define HVC_MINOR	0
 
-#define TIMEOUT		(10)
-
 /*
  * Wait this long per iteration while trying to push buffered data to the
  * hypervisor before allowing the tty to complete a close operation.
@@ -550,6 +548,18 @@ static int hvc_chars_in_buffer(struct tty_struct *tty)
 	return hp->n_outbuf;
 }
 
+/*
+ * timeout will vary between the MIN and MAX values defined here.  By default
+ * and during console activity we will use a default MIN_TIMEOUT of 10.  When
+ * the console is idle, we increase the timeout value on each pass through
+ * msleep until we reach the max.  This may be noticeable as a brief (average
+ * one second) delay on the console before the console responds to input when
+ * there has been no input for some time.
+ */
+#define MIN_TIMEOUT		(10)
+#define MAX_TIMEOUT		(2000)
+static u32 timeout = MIN_TIMEOUT;
+
 #define HVC_POLL_READ	0x00000001
 #define HVC_POLL_WRITE	0x00000002
 
@@ -642,9 +652,14 @@ static int hvc_poll(struct hvc_struct *hp)
  bail:
 	spin_unlock_irqrestore(&hp->lock, flags);
 
-	if (read_total)
+	if (read_total) {
+		/* Activity is occurring, so reset the polling backoff value to
+		   a minimum for performance. */
+		timeout = MIN_TIMEOUT;
+
 		tty_flip_buffer_push(tty);
-	
+	}
+
 	return poll_mask;
 }
 
@@ -688,8 +703,12 @@ int khvcd(void *unused)
 		if (!hvc_kicked) {
 			if (poll_mask == 0)
 				schedule();
-			else
-				msleep_interruptible(TIMEOUT);
+			else {
+				if (timeout < MAX_TIMEOUT)
+					timeout += (timeout >> 6) + 1;
+
+				msleep_interruptible(timeout);
+			}
 		}
 		__set_current_state(TASK_RUNNING);
 	} while (!kthread_should_stop());