Re: PATCH: support for Arctic/Beech/Ebony

Eugene Surovegin
2003-01-30T23:25:20Z

At 02:37 PM 1/30/2003, Joakim Tjernlund wrote:

> > > > > - You don't need mb() since ioremap will add the
> > > > > GUARDED attribute to the mapping.
> > > >
> > > > Well, this is not quite correct.
> > > >
> > > > GUARDED attribute does _NOT_ prevent reordering of loads and stores
> on PPC
> > > > (on 4xx at least).
> > > >
> > > > See comments in asm-ppc/system.h and also IBM app note "Software
> > > > consideration when migrating to the PowerPC 440GP from 405GP"
> > > >
> > >
> > >hmm, maybe it's specific to the 8xx family. 860 and 862 does not do
> > >speculative accesses on GUARDED memory.
> >
> > It does! But speculative accesses and access reordering are not the same
> > thing :)
>
>what do you mean, does or does not do speculative accesses?

Sorry for being unclear.

If I remember correctly, cache inhibited + guarded is enough for 8xx to
prevent both speculative access and load/store reordering.
But this is not true for ALL PPC processors, 440GP is an example when this
is not enough

> >
> > There is a subtle difference.
>
>Yes I understand now, but I discussed this with my HW contact at Motorola
>and he agreed
>that it was OK to remove the mb(). Maybe he was wrong or perhaps non
>speculative accesses
>on mpc 860/862 also disables reorder? I can not find any statement that
>confirms this.
>
>Howerver I have been running our boards without the mb()'s for months now,
>no problem with
>it so far.

Well, patch we are discussing is for 4xx, not for 8xx.

Eugene.