Messages in this thread | | | Date | Mon, 20 Apr 2009 12:53:04 +0200 | From | Ingo Molnar <> | Subject | Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars |
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* H. Peter Anvin <hpa@zytor.com> wrote:
> Roland Dreier wrote: > > > > Notice that it reads from addr+4 *before* it reads from addr, rather > > than after as in your example (and in fact your example depends on > > undefined compiler semantics, since there is no sequence point between > > the two operands of the | operator). Now, I don't know that hardware, > > so I don't know if it makes a difference, but the niu example I gave in > > my original email shows that given hardware with clear-on-read > > registers, the order does very much matter. > > > > At least for x86, the order should be low-high, because that is the > order that those two transactions would be seen on a 32-bit bus > downstream from the CPU if the CPU issued a 64-bit transaction. > > The only sane way to handle this as something other than per-driver > hacks would be something like: > > #include <linux/io64.h> /* Any 64-bit I/O OK */ > > #include <linux/io64lh.h> /* Low-high splitting OK */ > > #include <linux/io64hl.h> /* High-low splitting OK */ > > #include <linux/io64atomic.h> /* 64-bit I/O must be atomic */ > > ... i.e. letting the driver choose what fallback method it will accept.
Yeah - with the default being the natural low-high order.
The other argument is that if a driver really wants some rare, oddly different order it should better define its own method that is not named in the same (or in a similar) way as an existing generic API. Otherwise, confusion will ensue.
Ingo
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