Messages in this thread | | | Subject | Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining | From | Bryan O'Sullivan <> | Date | Fri, 24 Feb 2006 23:34:53 -0800 |
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On Sat, 2006-02-25 at 05:43 +0100, Andi Kleen wrote: > On Saturday 25 February 2006 05:20, 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. > > On what platforms?
On x86_64 in particular, if CONFIG_UNORDERED_IO is defined. Regular wmb() is implemented as a compiler memory barrier then, which isn't sufficient for PCI write combining.
> linux/system.h looks unnatural to me.
I used this for symmetry with <asm/system.h> where other barriers are defined. It could obviously go into io.h instead, since it's an MMIO-related barrier, but I didn't want to separate it from other barriers.
If you have a location you'd prefer, please let me know.
> > We also define a version of wc_wmb() with the required semantics > > on x86_64. > > Leaving i386 out in the cold?
Looks like it should be defined there, too, something like this perhaps:
#define wc_wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM)
Does that look right?
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