Messages in this thread | | | Date | Fri, 6 Apr 2001 19:13:21 +0200 (MET DST) | From | "Maciej W. Rozycki" <> | Subject | Re: [Linux-fbdev-devel] Re: fbcon slowness [was NTP on 2.4.2?] |
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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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