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SubjectRe: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars

* 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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