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>

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index cb8578b7e4432c1aad04f79324be3d588c1e74e0..a4d80314e7f8cf5edb13bce97328a60f1f26f6ce 100644 (file)
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/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);