Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining

["Bryan O'Sullivan" <bos@pathscale.com>]

On Tue, 2006-02-28 at 09:44 -0800, Jesse Barnes wrote:

> Something like this would be really handy.  Check out fbmem.c:fb_mmap for 
> a bad example of what can happen w/o it.

:-)

> In fact, I think it might make sense to export WC functionality via an 
> mmap flag (on an advisory basis since the platform may not support it or 
> there may be aliasing issues that prevent it); having an arch 
> independent routine to request it would make that addition easy to do in 
> generic code.

Yes.

>   (In particular I wanted this for the sysfs PCI interface.  
> Userspace apps can map PCI resources there and it would be nice if they 
> could map them with WC semantics if requested.)

They already sort of can.  It just happens that most arches ignore the
WC parameters.

> I don't think it addresses the flushing issue you seem to be concerned 
> about though.

Yes, I think I could have made my original wording a bit clearer.  I
don't care if writes have hit the device, merely that they do so in an
order that I control.

	<b

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