[linux-fbdev] Re: readl() and friends and eieio on PPC

Paul Mackerras paulus at cs.anu.edu.au
Thu Aug 12 19:58:05 EST 1999

Richard Henderson <rth at cygnus.com> wrote:

> No, 2 Cycles to L1 cache.  One cycle to execute the store,
> which merely adds an entry to the store buffer.

Yes, of course, silly me.  Same on PPC.

> > (Ob. nitpicking: if a wmb takes 12 cycles, how come we can do a wmb
> > and 8 stores in 10 cycles? :-)
> Because it doesn't work like that.  wmb adds a magic token to the
> store buffer that prevents write combining and other such hw
> optimizations.  Timing

Then why is there such a big performance impact from the wmb's?


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