xen/tpmfront: use xenbus_setup_ring() and xenbus_teardown_ring()

Simplify tpmfront's ring creation and removal via xenbus_setup_ring()
and xenbus_teardown_ring(), which are provided exactly for the use
pattern as seen in this driver.
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/char/tpm/xen-tpmfront.c

@@ -253,20 +253,12 @@ static int setup_ring(struct xenbus_device *dev, struct tpm_private *priv)
 	struct xenbus_transaction xbt;
 	const char *message = NULL;
 	int rv;
-	grant_ref_t gref;
 
-	priv->shr = (void *)__get_free_page(GFP_KERNEL|__GFP_ZERO);
-	if (!priv->shr) {
-		xenbus_dev_fatal(dev, -ENOMEM, "allocating shared ring");
-		return -ENOMEM;
-	}
-
-	rv = xenbus_grant_ring(dev, priv->shr, 1, &gref);
+	rv = xenbus_setup_ring(dev, GFP_KERNEL, (void **)&priv->shr, 1,
+			       &priv->ring_ref);
 	if (rv < 0)
 		return rv;
 
-	priv->ring_ref = gref;
-
 	rv = xenbus_alloc_evtchn(dev, &priv->evtchn);
 	if (rv)
 		return rv;
@@ -331,11 +323,7 @@ static void ring_free(struct tpm_private *priv)
 	if (!priv)
 		return;
 
-	if (priv->ring_ref)
-		gnttab_end_foreign_access(priv->ring_ref,
-				(unsigned long)priv->shr);
-	else
-		free_page((unsigned long)priv->shr);
+	xenbus_teardown_ring((void **)&priv->shr, 1, &priv->ring_ref);
 
 	if (priv->irq)
 		unbind_from_irqhandler(priv->irq, priv);