drm/i915: close PM interrupt masking races in the rps work func

This patch closes the following race:

We get a PM interrupt A, mask it, set dev_priv->iir = PM_A and kick of the
work item. Scheduler isn't grumpy, so the work queue takes rps_lock,
grabs pm_iir = dev_priv->pm_iir and pm_imr = READ(PMIMR). Note that
pm_imr == pm_iir because we've just masked the interrupt we've got.

Now hw sends out PM interrupt B (not masked), we process it and mask
it.  Later on the irq handler also clears PMIIR.

Then the work item proceeds and at the end clears PMIMR. Because
(local) pm_imr == pm_iir we have
        pm_imr & ~pm_iir == 0
so all interrupts are enabled.

Hardware is still interrupt-happy, and sends out a new PM interrupt B.
PMIMR doesn't mask B (it does not mask anything), PMIIR is cleared, so
we get it and hit the WARN in the interrupt handler (because
dev_priv->pm_iir == PM_B).

That's why I've moved the
        WRITE(PMIMR, 0)
up under the protection of the rps_lock. And write an uncoditional 0
to PMIMR, because that's what we'll do anyway.

This races looks much more likely because we can arbitrarily extend
the window by grabing dev->struct mutex right after the irq handler
has processed the first PM_B interrupt.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -383,6 +383,7 @@ static void gen6_pm_rps_work(struct work_struct *work)
 	pm_iir = dev_priv->pm_iir;
 	dev_priv->pm_iir = 0;
 	pm_imr = I915_READ(GEN6_PMIMR);
+	I915_WRITE(GEN6_PMIMR, 0);
 	spin_unlock_irq(&dev_priv->rps_lock);
 
 	if (!pm_iir)
@@ -420,7 +421,6 @@ static void gen6_pm_rps_work(struct work_struct *work)
 	 * an *extremely* unlikely race with gen6_rps_enable() that is prevented
 	 * by holding struct_mutex for the duration of the write.
 	 */
-	I915_WRITE(GEN6_PMIMR, pm_imr & ~pm_iir);
 	mutex_unlock(&dev_priv->dev->struct_mutex);
 }