[PATCH] drm/i915/dp: Be paranoid in case we disable a DP before it is attached

[Chris Wilson <chris@chris-wilson.co.uk>]

Given that the hardware may be left in a random condition by the BIOS,
it is conceivable that we then attempt to clear the DP_PIPEB_SELECT bit
without us ever enabling/attaching the DP encoder to a pipe. Thus
causing a NULL deference when we attempt to wait for a vblank on that
crtc.

Reported-and-tested-by: Bryan Christ <bryan.christ@gmail.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=36314
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
---
 drivers/gpu/drm/i915/intel_dp.c  |    3 +--
 drivers/gpu/drm/i915/intel_drv.h |    7 +++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 0daefca..819afca 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -1470,7 +1470,6 @@ intel_dp_link_down(struct intel_dp *intel_dp)
 
 	if (!HAS_PCH_CPT(dev) &&
 	    I915_READ(intel_dp->output_reg) & DP_PIPEB_SELECT) {
-		struct intel_crtc *intel_crtc = to_intel_crtc(intel_dp->base.base.crtc);
 		/* Hardware workaround: leaving our transcoder select
 		 * set to transcoder B while it's off will prevent the
 		 * corresponding HDMI output on transcoder A.
@@ -1485,7 +1484,7 @@ intel_dp_link_down(struct intel_dp *intel_dp)
 		/* Changes to enable or select take place the vblank
 		 * after being written.
 		 */
-		intel_wait_for_vblank(dev, intel_crtc->pipe);
+		intel_wait_for_crtc_vblank_safe(intel_dp->base.base.crtc);
 	}
 
 	I915_WRITE(intel_dp->output_reg, DP & ~DP_PORT_EN);
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index f5b0d83..aeb1b98 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -290,6 +290,13 @@ extern struct drm_display_mode *intel_crtc_mode_get(struct drm_device *dev,
 int intel_get_pipe_from_crtc_id(struct drm_device *dev, void *data,
 				struct drm_file *file_priv);
 extern void intel_wait_for_vblank(struct drm_device *dev, int pipe);
+static inline void intel_wait_for_crtc_vblank_safe(struct drm_crtc *crtc)
+{
+	if (crtc)
+		intel_wait_for_vblank(crtc->dev, to_intel_crtc(crtc)->pipe);
+	else
+		msleep(50);
+}
 extern void intel_wait_for_pipe_off(struct drm_device *dev, int pipe);
 extern struct drm_crtc *intel_get_load_detect_pipe(struct intel_encoder *intel_encoder,
 						   struct drm_connector *connector,
-- 
1.7.4.1