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

[Daniel Vetter <daniel.vetter@ffwll.ch>]

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.

v2: Chris Wilson pointed out some dead code and a now misleading
comment to kill.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_irq.c |   10 ++--------
 1 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 2fdd9f9..d5a2188 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -377,12 +377,12 @@ static void gen6_pm_rps_work(struct work_struct *work)
 	drm_i915_private_t *dev_priv = container_of(work, drm_i915_private_t,
 						    rps_work);
 	u8 new_delay = dev_priv->cur_delay;
-	u32 pm_iir, pm_imr;
+	u32 pm_iir;
 
 	spin_lock_irq(&dev_priv->rps_lock);
 	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)
@@ -415,12 +415,6 @@ static void gen6_pm_rps_work(struct work_struct *work)
 	gen6_set_rps(dev_priv->dev, new_delay);
 	dev_priv->cur_delay = new_delay;
 
-	/*
-	 * rps_lock not held here because clearing is non-destructive. There is
-	 * 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);
 }
 
-- 
1.7.6