On Thu, Aug 17, 2017 at 11:09 AM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote: > 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. Thanks Russell. I tend to forget things over time even though I have some memory of this being spelled out to me in the past :( Yours, Linus Walleij -- 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
