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