[+Tony] On Mon, Feb 11, 2019 at 02:34:31PM -0800, Linus Torvalds wrote: > On Mon, Feb 11, 2019 at 9:30 AM Will Deacon <will.deacon@xxxxxxx> wrote: > > > > + > > + 1. All readX() and writeX() accesses to the same peripheral are ordered > > + with respect to each other. For example, this ensures that MMIO register > > + writes by the CPU to a particular device will arrive in program order. > > Hmm. I'd like more people look at strengthening this one wrt across > CPUs and locking. > > Right now we document mmiowb(), but that "documentation" is really > just a fairy tale. Very *very* few drivers actually do mmiowb() on > their own. > > IOW, we should seriously just consider making the rule be that locking > will order mmio too. Because that's practically the rule anyway. I would /love/ to get rid of mmiowb() because I think it's both extremely difficult to use and also pretty much never needed. It reminds me a lot of smp_read_barrier_depends(), which we finally pushed into READ_ONCE for Alpha. > Powerpc already does it. IO within a locked region will serialize with the > lock. I thought ia64 was the hold out here? Did they actually have machines that needed this in practice? If so, I think we can either: (a) Add an mmiowb() to their spin_unlock() code, or (b) Remove ia64 altogether if nobody complains I know that Peter has been in favour of (b) for a while... Will
