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Subject: Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining
Date: Tue, 28 Feb 2006 13:07:50 -0500
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Thread-Topic: [PATCH] Define wc_wmb, a write barrier for PCI write combining
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From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Bryan O'Sullivan" <bos@pathscale.com>
Cc: "Jes Sorensen" <jes@sgi.com>, "Andrew Morton" <akpm@osdl.org>,
       "Andi Kleen" <ak@suse.de>,
       "linux-kernel" <linux-kernel@vger.kernel.org>
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On Tue, 28 Feb 2006, Bryan O'Sullivan wrote:

> On Tue, 2006-02-28 at 05:01 -0500, Jes Sorensen wrote:
>
>> Could you explain why the current mmiowb() API won't suffice for this?
>> It seems that this is basically trying to achieve the same thing.
>
> It's a no-op on every arch I care about:
>
> #define mmiowb()
>
> Which makes it useless.  Also, based on the comments in the qla driver,
> mmiowb() seems to have inter-CPU ordering semantics that I don't want.
> I'm thus hesitant to appropriate it for my needs.
>
> 	<b
>

When accessing PCI, you can cause any/all write combinations to
occur and any/all pending writes to get written to the devices
simply by executing a read. If the code requires that all previous
writes be written NOW, then the code should not hide that in
some macro. It should issue a read in its PCI space. Also, the
PCI bus is a FIFO. Nothing gets reordered. Everything will
get to the devices in the order written, but a byte or word on
a longword boundary may be subject to write-combining if all
the components are present in the FIFO.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction, book release in April.
_


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