On Tue, Mar 27, 2018 at 10:42:00AM +0100, Will Deacon wrote: > On Tue, Mar 27, 2018 at 09:56:47AM +0200, Arnd Bergmann wrote: > > On Tue, Mar 27, 2018 at 12:27 AM, Jason Gunthorpe <j...@ziepe.ca> wrote: > > > On Tue, Mar 27, 2018 at 09:01:57AM +1100, Benjamin Herrenschmidt wrote: > > >> On Mon, 2018-03-26 at 17:46 -0400, Sinan Kaya wrote: > > > > > > I even see patches adding wmb() based on actual observed memory > > > corruption during testing on Intel: > > > > > > https://patchwork.kernel.org/patch/10177207/ > > > > > > So you think all of this is unnecessary and writel is totally strongly > > > ordered, even on multi-socket Intel? > > > > This example adds a wmb() between two writes to a coherent DMA > > area, it is definitely required there. I'm pretty sure I've never seen > > any bug reports pointing to a missing wmb() between memory > > and MMIO write accesses, but if you remember seeing them in the > > list, maybe you can look again for some evidence of something going > > wrong on x86 without it? > > If this is just about ordering accesses to coherent DMA, then using > dma_wmb() instead will be much better performance on arm/arm64.
dma_wmb() is a NOP on x86, it was tested anyhow and didn't help this case.. Confusing, but probably not relevant to this discussion. Jason
