On Fri, Feb 24, 2006 at 08:20:50PM -0800, Bryan O'Sullivan wrote:
> On some platforms, a regular wmb() is not sufficient to guarantee that
> PCI writes have been flushed to the bus if write combining is in effect.
>
> This change introduces a new macro, wc_wmb(), that makes this guarantee.
>
> It does so by way of a new header file, <linux/system.h>. This header
> can be a site for oft-replicated code from <asm-*/system.h>, but isn't
> just yet.
>
> We also define a version of wc_wmb() with the required semantics
> on x86_64.
This does not do what you're trying to accomplish. sfence has no impact
on the flushing of the write combining buffers -- they're normally flushed
after a few cycles of no subsequent merges. If it is really necessary to
flush such a write, you will have to do a read from the target PCI device
(well, bus would do).
-ben
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