Hi Arnd, On Mon, Feb 11, 2019 at 06:11:48PM +0100, Arnd Bergmann wrote: > On Mon, Feb 11, 2019 at 4:30 PM Will Deacon <will.deacon@xxxxxxx> wrote: > > > Given the lack of Intel response here, I went away to do some digging. > > As evidenced by the commit message, there is certainly an understanding > > amongst some developers that inX/outX() are strongly ordered on x86 and > > this was re-enforced by Linus in March last year: > > > > https://www.mail-archive.com/linuxppc-dev@xxxxxxxxxxxxxxxx/msg131212.html > > > > It was this information on which I based my patch. The Intel SDM is not > > quite as assertive in its claims. > > > > However, it has also occurred to me that this patch is actually missing > > the point. memory-barriers.txt should be documenting the *Linux* memory > > model, not the x86 one, and so the port accessors should be defined to > > have the same ordering semantics as the MMIO accessors. If this wasn't > > the case, then macros such as ioreadX() and iowriteX() would be unusable > > in portable driver code. > > My interpretation of the ioreadX() and iowriteX() semantics is that they > only guarantee readl()/writel() barrier semantics, even though they > may in fact provide stronger barriers for PIO on architectures that use > CONFIG_GENERIC_IOMAP (which falls back to inX()/outX()). > > > The inX/outX implementation in asm-generic would > > also be bogus, despite being widely used. > > They likely are. The asm-generic files tend to provide a generic > abstraction as much as that is possible, but without having access > to the architecture specific semantics, they raditionally don't know > what should be done here. We now have __io_pbw()/__io_paw()/ > __io_pbr()/__io_par() to let architectures get it right, but that is > a fairly recent addition, so nothing other than riscv defines them > today. > To make things worse, a lot of machines are unable to provide > __io_paw(), e.g. when all bus writes are posted. So I've just sent an RFC (you're on cc) that attempts to rewrite this part of memory-barriers.txt to reflect reality. Hopefully that can act as a starting point for discussion if we decide we want to change the documented behaviour and/or implementation. Will
