drm/i915: Fix PIPESTAT irq ack on i965/g4x

On i965/g4x IIR is edge triggered. So in order for IIR to notice that
there is still a pending interrupt we have to force and edge in ISR.
For the ISR/IIR pipe event bits we can do that by temporarily
clearing all the PIPESTAT enable bits when we ack the status bits.
This will force the ISR pipe event bit low, and it can then go back
high when we restore the PIPESTAT enable bits.

This avoids the following race:
1. stat = read(PIPESTAT)
2. an enabled PIPESTAT status bit goes high
3. write(PIPESTAT, enable|stat);
4. write(IIR, PIPE_EVENT)

The end result is IIR==0 and ISR!=0. This can lead to nasty
vblank wait/flip_done timeouts if another interrupt source
doesn't trick us into looking at the PIPESTAT status bits despite
the IIR PIPE_EVENT bit being low.

Before i965 IIR was level triggered so this problem can't actually
happen there. And curiously VLV/CHV went back to the level triggered
scheme as well. But for simplicity we'll use the same i965/g4x
compatible code for all platforms.

Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106033
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105225
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106030
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180611200258.27121-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_irq.c

@@ -1893,9 +1893,17 @@ static void i9xx_pipestat_irq_ack(struct drm_i915_private *dev_priv,
 
 		/*
 		 * Clear the PIPE*STAT regs before the IIR
+		 *
+		 * Toggle the enable bits to make sure we get an
+		 * edge in the ISR pipe event bit if we don't clear
+		 * all the enabled status bits. Otherwise the edge
+		 * triggered IIR on i965/g4x wouldn't notice that
+		 * an interrupt is still pending.
 		 */
-		if (pipe_stats[pipe])
-			I915_WRITE(reg, enable_mask | pipe_stats[pipe]);
+		if (pipe_stats[pipe]) {
+			I915_WRITE(reg, pipe_stats[pipe]);
+			I915_WRITE(reg, enable_mask);
+		}
 	}
 	spin_unlock(&dev_priv->irq_lock);
 }