[Intel-gfx] [PATCH] i915: sanity check IER at wait_request time

[Jesse Barnes]

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 <mateusz.kaduk at gmail.com>
Signed-off-by: Jesse Barnes <jbarnes at virtuousgeek.org>

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ee896d9..3b19f91 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1691,10 +1691,18 @@ static int
 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);
 
+	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);
+	}
+
 	if (!i915_seqno_passed(i915_get_gem_seqno(dev), seqno)) {
 		dev_priv->mm.waiting_gem_seqno = seqno;
 		i915_user_irq_get(dev);