On Fri, Aug 11, 2017 at 11:16:20AM +0200, Linus Walleij wrote: > writel() should be guaranteeing that the values hit the hardware, wmb() is > spelled out "write memory barrier" I don't see what you're after here. Incorrect. writel() has a barrier which ensures that data written to memory (eg, dma coherent memory) is visible to the hardware prior to the write hitting the hardware. There is no barrier to ensure that the write hits the hardware in a timely manner - the write can be buffered by the buses, which will delay it before it hits its destination. PCI particularly buffers MMIO writes, and the requirement there has always been that if you need the write to hit the hardware in a timely fashion, you must perform a read-back to force the bus to deliver the write (since a read is not allowed to overlap a write.) The solution is never to use barrier() - barrier() is a _compiler_ barrier and does nothing for posted writes on hardware buses. -- 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 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
