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

[Maciej W. R. <ma...@ds...>]

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: ma...@ds..., PGP key available        +