drm/i915: sanity check IER at wait_request time

We might sleep here anyway so I hope an extra uncached read is ok to
add.

In #20896 we found that vbetool clobbers the IER. In KMS mode this is
particularly bad since we don't set the interrupt regs late (in
EnterVT), so we'd fail to get *any* interrupts at all after X started
(since some distros have scripts that call vbetool at X startup
apparently).

So this patch checks IER at wait_request time, and re-enables
interrupts if it's been clobbered. In a proper config this check
should never be triggered.

This is really a distro issue, but having a sanity check is nice, as
long as it doesn't have a real performance hit.

Tested-by: Mateusz Kaduk <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>
[anholt: Moved the check inside of the sleeping case to avoid perf cost]
Signed-off-by: Eric Anholt <[e-mail]>

drivers/gpu/drm/i915/i915_gem.c

···
 i915_wait_request(struct drm_device *dev, uint32_t seqno)
 {
 	drm_i915_private_t *dev_priv = dev->dev_private;
+	u32 ier;
 	int ret = 0;
 
 	BUG_ON(seqno == 0);
 
 	if (!i915_seqno_passed(i915_get_gem_seqno(dev), seqno)) {
+		ier = I915_READ(IER);
+		if (!ier) {
+			DRM_ERROR("something (likely vbetool) disabled "
+				  "interrupts, re-enabling\n");
+			i915_driver_irq_preinstall(dev);
+			i915_driver_irq_postinstall(dev);
+		}
+
 		dev_priv->mm.waiting_gem_seqno = seqno;
 		i915_user_irq_get(dev);
 		ret = wait_event_interruptible(dev_priv->irq_queue,