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SubjectRe: [PATCH] Define wc_wmb, a write barrier for PCI write combining
On Tue, Feb 28, 2006 at 09:50:08AM -0800, Bryan O'Sullivan wrote:
> The last 32-bit write triggers the chip to put the packet on the wire.
> We make sure it happens after the earlier bulk write using a barrier.

The barrier you're looking for is wmb() in asm/system.h, which is defined
on both SMP and UP. On x86 you do not need the sfence as writes will show
up on the bus in program order, but you do need wmb() to prevent gcc from
reordering your code. Adding a new primative only makes things slower as
the sfence isn't necessary most of the time (it rightly has a dependancy on
CONFIG_UNORDERED_IO, which the jury is still out on).

-ben
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