readl(), __readl(), raw_readl(), __raw_readl()

readl/writel:  strongly ordered wrt one another and other stores
               to cacheable RAM, byteswapping
__readl/__writel:  not ordered (needs mb/rmb/wmb to order with
                   other readl/writel and cacheable operations, or
                   io_*mb to order with one another)
raw_readl/raw_writel:  strongly ordered, no byteswapping
__raw_readl/__raw_writel:  not ordered, no byteswapping