On Wed, Nov 9, 2022, at 01:28, Akira Yokosawa wrote: > From quick glance of io_ordering.rst's git history, contents of this file > is never updated since the beginning of Git history (v2.6.12.rc2). > > Which strongly suggests that you can ignore io_ordering.rst. > I managed to track the origin of the document further to a bitkeeper pull request and a patch on the ia64 mailing list: https://lore.kernel.org/all/200304091927.h39JRob0010157@xxxxxxxxxxxxxxxxx/ https://marc.info/?l=git-commits-head&m=104992658231313&w=1 While the document that was added here (and survives to this day) talks about MMIO (writel), the code changes in this patch appear to be only about PIO (outl), so I suspect that it already mixed things up at the time. > PS: > Do we need to keep that outdated document??? > > I think Documentation/driver-api/device-io.rst is the one properly > maintained. I think we need to have one location that properly documents what can and cannot go wrong with posted writel() vs spinlock. At the moment, device-io.rst refers to this one saying: "Note that posted writes are not strictly ordered against a spinlock, see Documentation/driver-api/io_ordering.rst." The same document recently gained a section on ioremap_np() that explains some of this better, and it also contradicts io_ordering.rst by saying "A driver author must issue a read from the same device to ensure that writes have occurred in the specific cases the author cares." which I think implies that the "problem" example in io_ordering.rst is actually fine as long as "my_status" and "ring_ptr" are part of the same device: while the writel() can still leak past the spin_unlock(), it won't ever bypass the spin_lock()+readl() on another CPU, right? Arnd
