[patch 018/100] drm/i915: Check for dev->primary->master before dereference.

[Chris Wright]

-stable review patch.  If anyone has any objections, please let us know.
---------------------

From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

upstream commit: 98787c057fdefdce6230ff46f2c1105835005a4c

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Eric Anholt <eric@xxxxxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_dma.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- 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 * d
 		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;

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