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Date: Fri, 6 Apr 2001 19:13:21 +0200 (MET DST)
From: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>
To: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
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        Linux Fbdev development list 
	<linux-fbdev-devel@lists.sourceforge.net>,
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Subject: Re: [Linux-fbdev-devel] Re: fbcon slowness [was NTP on 2.4.2?]
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Organization: Technical University of Gdansk
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On Fri, 6 Apr 2001, Ivan Kokshaysky wrote:

> >  Memory barriers are a separate issue.  On the alpha the
> > natural way to implement it would be in the page table fill code.
> > Memory barriers are o.k. but the really don't help the case when what
> > you want to do is read the latest value out of a pci register.  
> 
> You don't need memory barrier for that. "Write memory barriers" are
> used to ensure correct write order, and "memory barriers" are used
> to ensure that all pending reads/writes will complete before next read
> or write.

 You do.  PCI-space registers are volatile and they may change depending
on what was written (or read) previously.  A memory barrier before a PCI
read will ensure you get a value that is relevant to previous code
actions.  Without a barrier you may get pretty anything, depending on
which of previous writes managed to complete before. 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

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