-		if (ret == BLK_STS_RESOURCE) {
+		if ((ret == BLK_STS_RESOURCE) || (ret == BLK_STS_DEV_RESOURCE)) {

No need for the inner braces here.

+	if ((ret == BLK_STS_RESOURCE) || (ret == BLK_STS_DEV_RESOURCE))

Same here.

+/*
+ * This status is returned from driver to block layer if device related
+ * resource is run out of, but driver can guarantee that IO dispatch is
+ * triggered in future when the resource is available.
+ */
+#define BLK_STS_DEV_RESOURCE	((__force blk_status_t)13)

We'll need some good explanation on BLK_STS_RESOURCE vs
BLK_STS_DEV_RESOURCE I think.

Except for these nitpicks this looks fine to me.