[PATCH 2/6] 8xx: get rid of _PAGE_HWWRITE dependency in MMU.

[Joakim Tjernlund]

Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote on 08/10/2009 02:04:56:
>
>
> > Yes it does. If one adds HWEXEC it will fail, right?
>
> Why ? We can just filter out DSISR, we don't really care why it failed
> as long as we know whether it was a store or not.
>
> > Also this count as a read and you could easily end up
> > in the protection case(in 2.4 you do)
>
> I'm not sure what you mean by "the protection case" Again, the C code
> shouldn't care.

it does, and it should. How else should you know if you try
to read a NA space?

See:
	/* Protection fault on exec go straight to failure on
		 * Hash based MMUs as they either don't support per-page
		 * execute permission, or if they do, it's handled already
		 * at the hash level. This test would probably have to
		 * be removed if we change the way this works to make hash
		 * processors use the same I/D cache coherency mechanism
		 * as embedded.
		 */
		if (error_code & DSISR_PROTFAULT)
			goto bad_area;

...
/* a read */
	} else {
		/* protection fault */
		if (error_code & 0x08000000)
			goto bad_area;

>
> > hey, I think you have to show how then :) I am not
> > good at ppc shift, mask, rotate insn.
>
> They are so fun ! :-0

Great that someone thinks so :)

>
> > >
> > > > Because if you go to C with a protection fault, you are in trouble.
> > >
> > > Why ?
> >
> > In 2.4 you end up in read protection fault an get a SEGV back :)
>
> We probably should ignore the DSISR bits then. So it just goes to
> generic C code which then fixes up ACCESSED etc... and returns.

Not really I think.

>
> > Now I only handle DIRTY and the rest in C. Figured it is
> > much faster and really simplie now, stay tuned.
>
> You should not even have to handle DIRTY at all in asm. At least in 2.6.
> I can't vouch for what 2.4 generic code does... You should really port
> your board over :-)

2.4 and 2.6 have the same handling in asm.
hmm, maybe I should just call C, but 8xx isn't a speed monster so every
cycle counts :)
It works if I trap to C for DIRTY too.
Before thinking on porting my old board, I want 2.4 to enjoy
the new TLB code too :)

  Jocke