drm/i915: Wait for gen3 reset status to be asserted

After we assert the reset request (and wait for 20us), when the device
has been fully reset it asserts the reset-status bit. Before we stop
requesting the reset and allow the device to return to normal, we should
wait for the reset to be completed. (Similar to how we wait for the
device to return to normal after deasserting the reset request.)

v2: Rename i915_reset_completed() probe to not cause as much confusion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207222824.29864-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

drivers/gpu/drm/i915/intel_uncore.c

@@ -1550,24 +1550,31 @@ static void i915_stop_engines(struct drm_i915_private *dev_priv,
 		gen3_stop_engine(engine);
 }
 
-static bool i915_reset_complete(struct pci_dev *pdev)
+static bool i915_in_reset(struct pci_dev *pdev)
 {
 	u8 gdrst;
 
 	pci_read_config_byte(pdev, I915_GDRST, &gdrst);
-	return (gdrst & GRDOM_RESET_STATUS) == 0;
+	return gdrst & GRDOM_RESET_STATUS;
 }
 
 static int i915_do_reset(struct drm_i915_private *dev_priv, unsigned engine_mask)
 {
 	struct pci_dev *pdev = dev_priv->drm.pdev;
+	int err;
 
-	/* assert reset for at least 20 usec */
+	/* Assert reset for at least 20 usec, and wait for acknowledgement. */
 	pci_write_config_byte(pdev, I915_GDRST, GRDOM_RESET_ENABLE);
 	usleep_range(50, 200);
+	err = wait_for(i915_in_reset(pdev), 500);
+
+	/* Clear the reset request. */
 	pci_write_config_byte(pdev, I915_GDRST, 0);
+	usleep_range(50, 200);
+	if (!err)
+		err = wait_for(!i915_in_reset(pdev), 500);
 
-	return wait_for(i915_reset_complete(pdev), 500);
+	return err;
 }
 
 static bool g4x_reset_complete(struct pci_dev *pdev)