[U-Boot] [PATCH] mpc83xx: Make it boot again

[Scott Wood]

On Wed, 17 Nov 2010 18:26:02 +0100
Joakim Tjernlund <joakim.tjernlund at transmode.se> wrote:

> Scott Wood <scottwood at freescale.com> wrote on 2010/11/17 18:05:37:
> >
> > On Wed, 17 Nov 2010 17:57:53 +0100
> > Joakim Tjernlund <joakim.tjernlund at transmode.se> wrote:
> >
> > > After adding some more stuff in start.S I find that a lwz isn't
> > > enough. An extra isync fixes this though
> > >
> > >  lwz r4, LBLAWAR1(r3)
> > >  isync
> > >
> > > So something is missing but what? I guess isync isn't it either but
> > > it works for now.
> >
> > As I said before, the sequence we follow in the normal I/O accessors is:
> >
> > lwz   r4, LBLAWAR1(r3)
> > twi   0, r4, 0
> > isync
> 
> That works too. I do wonder if twi ..,r4,.. is correct though?
> Even a
>  lwz r4, LBLAWAR1(r3)
>  nop
> works.

Just because you can get away with something doesn't mean that it's
theoretically sufficient.  We got away with nothing at all until
recently.

The "load, conditional branch, isync" sequence is documented in the
architecture manual (1.7.1), "even if the effects of the 'dependency'
are independent of the value loaded".

-Scott