drm/i915: enable SDEIER later

On the preinstall stage we should just disable all the interrupts, but
we currently enable all the south display interrupts due to the way we
touch SDEIER at the IRQ handlers (note: they are still masked and our
IRQ handler is disabled). Instead of doing that, let's make the
preinstall stage just disable all the south interrupts, and do the
proper interrupt dance/ordering at the postinstall stage, including an
assert to check if everything is behaving as expected.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -2875,13 +2875,24 @@ static void ibx_irq_preinstall(struct drm_device *dev)
 
 	if (HAS_PCH_CPT(dev) || HAS_PCH_LPT(dev))
 		I915_WRITE(SERR_INT, 0xffffffff);
+}
 
-	/*
-	 * SDEIER is also touched by the interrupt handler to work around missed
-	 * PCH interrupts. Hence we can't update it after the interrupt handler
-	 * is enabled - instead we unconditionally enable all PCH interrupt
-	 * sources here, but then only unmask them as needed with SDEIMR.
-	 */
+/*
+ * SDEIER is also touched by the interrupt handler to work around missed PCH
+ * interrupts. Hence we can't update it after the interrupt handler is enabled -
+ * instead we unconditionally enable all PCH interrupt sources here, but then
+ * only unmask them as needed with SDEIMR.
+ *
+ * This function needs to be called before interrupts are enabled.
+ */
+static void ibx_irq_pre_postinstall(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	if (HAS_PCH_NOP(dev))
+		return;
+
+	WARN_ON(I915_READ(SDEIER) != 0);
 	I915_WRITE(SDEIER, 0xffffffff);
 	POSTING_READ(SDEIER);
 }
@@ -3077,6 +3088,8 @@ static int ironlake_irq_postinstall(struct drm_device *dev)
 
 	dev_priv->irq_mask = ~display_mask;
 
+	ibx_irq_pre_postinstall(dev);
+
 	GEN5_IRQ_INIT(DE, dev_priv->irq_mask, display_mask | extra_mask);
 
 	gen5_gt_irq_postinstall(dev);
@@ -3265,6 +3278,8 @@ static int gen8_irq_postinstall(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
+	ibx_irq_pre_postinstall(dev);
+
 	gen8_gt_irq_postinstall(dev_priv);
 	gen8_de_irq_postinstall(dev_priv);