commit 9f4f07ceb1716d8796089fcef91621c5f07c872a
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Jan 8 10:42:15 2009 -0800

    drm/i915: don't enable vblanks on disabled pipes
    
    In some cases userland may be confused and try to wait on vblank events from
    pipes that aren't actually enabled.  We shouldn't allow this, so return
    -EINVAL if the pipe isn't on.
    
    Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
    Signed-off-by: Eric Anholt <eric@anholt.net>

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 69b9a42..81ac2bd 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -400,6 +400,12 @@ int i915_enable_vblank(struct drm_device *dev, int pipe)
 {
 	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
 	unsigned long irqflags;
+	int pipeconf_reg = (pipe == 0) ? PIPEACONF : PIPEBCONF;
+	u32 pipeconf;
+
+	pipeconf = I915_READ(pipeconf_reg);
+	if (!(pipeconf & PIPEACONF_ENABLE))
+		return -EINVAL;
 
 	spin_lock_irqsave(&dev_priv->user_irq_lock, irqflags);
 	if (IS_I965G(dev))