drm/i915: Check for dev->primary->master before dereference.

I've hit the occasional oops inside i915_wait_ring() with an indication of
a NULL derefence of dev->primary->master.  Adding a NULL check is
consistent with the other potential users of dev->primary->master.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index ae83fe0ab374..a818b377e1f7 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -41,7 +41,6 @@
 int i915_wait_ring(struct drm_device * dev, int n, const char *caller)
 {
         drm_i915_private_t *dev_priv = dev->dev_private;
-        struct drm_i915_master_private *master_priv = dev->primary->master->driver_priv;
         drm_i915_ring_buffer_t *ring = &(dev_priv->ring);
         u32 acthd_reg = IS_I965G(dev) ? ACTHD_I965 : ACTHD;
         u32 last_acthd = I915_READ(acthd_reg);
@@ -58,8 +57,12 @@ int i915_wait_ring(struct drm_device * dev, int n, const char *caller)
                 if (ring->space >= n)
                         return 0;
 
-                if (master_priv->sarea_priv)
-                        master_priv->sarea_priv->perf_boxes |= I915_BOX_WAIT;
+                if (dev->primary->master) {
+                        struct drm_i915_master_private *master_priv = dev->primary->master->driver_priv;
+                        if (master_priv->sarea_priv)
+                                master_priv->sarea_priv->perf_boxes |= I915_BOX_WAIT;
+                }
+
 
                 if (ring->head != last_head)
                         i = 0;