[PATCH] powerpc/8xx: fix regression introduced by cache coherency rewrite

[Joakim Tjernlund]

Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote on 04/10/2009 22:28:38:
>
> > I have managed to update the TLB code to make proper use of dirty and accessed states.
> > Advantages are:
> >  - I/D TLB Miss never needs to write to the linux pte, saving a few cycles
>
> That's good, that leaves us with only 40x to fix now. Also we can remove
> atomic updates of PTEs for all non-hash. It's pointless on those CPUs
> anyway.
>
> >  - Accessed is only set by I/D TLB Error, should be a plus when SWAP is used.
>
> No need for that neither.

Since 8xx lacks HW support for ACCESSED, the only way is map
the page NoAccess and take a TLB Error on first access that sets
access bit (or bails to do_page_fault)

>
> ISI/DSI shouldn't touch the PTE. They should just fall back to C code
> which takes care of it all.l

Yes, that is what I do now(i.e I only read the pte). ISI and DSI is the
TLB Miss handlers on 8xx.

>
> >  - _PAGE_DIRTY is mapped to 0x100, the changed bit, and is set directly
> >     and there will be no extra DTLB Error to actually set the changed bit
> >     when a page has been made dirty.
> > - Proper RO/RW mapping of user space.
> >
> > Cons:
> >  - 4 more insn in TLB Miss handlers, but the since the linux pte isn't
> >    written it should still be a win.
> >
> > However, I did this on my 2.4 tree but I can port it to 2.6 if you guys
> > can test it for me.
>
> Why don't you use and test 2.6 ? :-)

Because porting my 8xx board to 2.6 isn't going to be easy so
I havn't yet. One day I might when we can't get away with 2.4 on our
old boards.