On Fri, Mar 30, 2018 at 10:29:58AM -0400, Sinan Kaya wrote: > The default implementation of mapping writeX() to __raw_writeX() is wrong. > writeX() has stronger ordering semantics. Compiler is allowed to reorder > __raw_writeX(). > > In the abscence of a write barrier or when using a strongly ordered > architecture, writeX() should at least have a compiler barrier in > it to prevent commpiler from clobbering the execution order. You want the barrier _before_ the call to __raw_writel() - you need to ensure that writes to memory are emitted by the compiler _before_ the write to the hardware - the write to the hardware may start DMA, and it may be reading data that the program thinks it previously wrote. Similarly, for readl(), you need the barrier after __raw_readl() to ensure that other reads in the program aren't scheduled before a potential DMA status register read. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up
