On Tue, Feb 19, 2002 at 09:10:44AM -0800, David Mosberger wrote:
> On ia64, the fact that readl()/writel() are accessing uncached space
> ensures the CPU doesn't reorder the accesses. Furthermore, the
> accesses are performed through "volatile" pointers, which ensures that
> the compiler doesn't reorder them (and, as a side-effect, such
> pointers also generate ordered loads/stores, but this isn't strictly
> needed, due to accessing uncached space).
Making a variable volatile doesn't guarantee that the compiler won't
reorder references to it, AFAIK. And on some platforms, even uncached
I/O references aren't necessarily ordered.
To avoid the overhead of having I/O flushed on every memory barrier
and readX/writeX operation, we've introduced mmiob() on ia64, which
explicity orders I/O space accesses. Some ports have chosen to take
the performance hit in every readX/writeX, memory barrier, and
spinlock however (e.g. PPC64, MIPS).
Is this a reasonable approach? Is it acceptable to have a seperate
barrier operation for I/O space? If so, perhaps other archs would be
willing to add mmiob() ops?
Thanks,
Jesse
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Sat Feb 23 2002 - 21:00:21 EST