drm/i915: hangcheck disable parameter

Provide a parameter to disable hanghcheck. This is useful mostly for
developers trying to debug known problems, and probably should not be
touched by normal users.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index ae2b499..0b0de52 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -361,10 +361,12 @@
 
 	ring->irq_seqno = seqno;
 	wake_up_all(&ring->irq_queue);
-
-	dev_priv->hangcheck_count = 0;
-	mod_timer(&dev_priv->hangcheck_timer,
-		  jiffies + msecs_to_jiffies(DRM_I915_HANGCHECK_PERIOD));
+	if (i915_enable_hangcheck) {
+		dev_priv->hangcheck_count = 0;
+		mod_timer(&dev_priv->hangcheck_timer,
+			  jiffies +
+			  msecs_to_jiffies(DRM_I915_HANGCHECK_PERIOD));
+	}
 }
 
 static void gen6_pm_rps_work(struct work_struct *work)
@@ -1664,6 +1666,9 @@
 	uint32_t acthd, instdone, instdone1;
 	bool err = false;
 
+	if (!i915_enable_hangcheck)
+		return;
+
 	/* If all work is done then ACTHD clearly hasn't advanced. */
 	if (i915_hangcheck_ring_idle(&dev_priv->ring[RCS], &err) &&
 	    i915_hangcheck_ring_idle(&dev_priv->ring[VCS], &err) &&