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Date: Tue, 30 Apr 2002 20:56:42 +0100
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From: arjan@fenrus.demon.nl
cc: linux-kernel@vger.kernel.org
To: Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
Subject: Re: How to write portable MMIO code?
In-Reply-To: <Pine.LNX.4.44.0204301112520.32217-100000@chaos.physics.uiowa.edu>
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In article <Pine.LNX.4.44.0204301112520.32217-100000@chaos.physics.uiowa.edu> you wrote:
>>   - should one in general (i.e., assuming the worst case) do wmb() on 
>> writes, and mb() on reads?

readl() and writel() are specified to contain the proper barriers already.

 
> I don't think mb() will help you. You're probably experiencing PCI posting 
> problems - when a writel() has executed, that doesn't necessarily mean 
> that the transaction has actually happened it may (and will) be buffered 
> for a potentially long time.
> 
> However, PCI won't reorder reads vs. writes, so you when you want to be 
> sure that a write() actually reached the hardware, do a dummy read() 
> afterwards, that'll flush the write buffer.

yup
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