drm/i915: make gpu hangman more resilient

- reset the stop_rings infrastructure while resetting the hw to
  avoid angering the hangcheck right away (and potentially declaring
  the gpu permanently wedged).

- ignore reset failures when hanging due to the hangman - we don't
  have reset code for all generations.

v2: Ensure that we only ignore reset failures when the hw reset is not
implemented and not when it failed.
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -770,6 +770,7 @@ static int gen6_do_reset(struct drm_device *dev, u8 flags)
 
 static int intel_gpu_reset(struct drm_device *dev, u8 flags)
 {
+	struct drm_i915_private *dev_priv = dev->dev_private;
 	int ret = -ENODEV;
 
 	switch (INTEL_INFO(dev)->gen) {
@@ -788,6 +789,17 @@ static int intel_gpu_reset(struct drm_device *dev, u8 flags)
 		break;
 	}
 
+	/* Also reset the gpu hangman. */
+	if (dev_priv->stop_rings) {
+		DRM_DEBUG("Simulated gpu hang, resetting stop_rings\n");
+		dev_priv->stop_rings = 0;
+		if (ret == -ENODEV) {
+			DRM_ERROR("Reset not implemented, but ignoring "
+				  "error for simulated gpu hangs\n");
+			ret = 0;
+		}
+	}
+
 	return ret;
 }