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SubjectRe: [Linux-fbdev-devel] Re: fbcon slowness [was NTP on 2.4.2?]
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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