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

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>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36314
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36456
Reported-and-tested-by: Bo Wang <bo.b.wang@intel.com>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -1470,7 +1470,8 @@ 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);
+		struct drm_crtc *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 +1486,19 @@ 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);
+		if (crtc == NULL) {
+			/* We can arrive here never having been attached
+			 * to a CRTC, for instance, due to inheriting
+			 * random state from the BIOS.
+			 *
+			 * If the pipe is not running, play safe and
+			 * wait for the clocks to stabilise before
+			 * continuing.
+			 */
+			POSTING_READ(intel_dp->output_reg);
+			msleep(50);
+		} else
+			intel_wait_for_vblank(dev, to_intel_crtc(crtc)->pipe);
 	}
 
 	I915_WRITE(intel_dp->output_reg, DP & ~DP_PORT_EN);