[RFC] handle access to non-present IO ports on 8xx

[Benjamin Herrenschmidt]

On Wed, 2005-06-01 at 13:50 -0300, Marcelo Tosatti wrote:

Hrm... removing a PCMCIA card triggers mchecks ? that is bad... With
"proper" PCMCIA controllers, those are swallowed properly when the card
is removed. The eating of the machine check is a bit too hackish to my
taste... Better is to "not do that" by making sure the legacy crap isn't
trying  to tap unexisting ports, but then, if PCMCIA is also a
problem...

> 1) why does the current PowerMac version covers only inb() and not outb() ?  
> I had to add outb() exception table entries for 8xx.

Not sure, maybe historical loss ? :) You should CC paulus

> 2) Is the same wanted for other embedded PPC's? 

It's up to you. It slows down those IOs, but on the other hand, inX/outX
aren't supposed to be very common anymore, at least not with "fast"
devices, and the IO itself is usually an order of magnitude slower than
doing those syncs...

> 3) How to make the misc.S exception entries and additional instructions 
> selectable only on the platform who need it? #ifdef does not sound 
> a good idea. 
> 
> Nevermind the "#ifdef CONFIG_ALL_PPC" crap - that needs to be done
> properly.
> 
> 
> Index: arch/ppc/kernel/misc.S
> ===================================================================
> RCS file: /mnt/test1/tslinux_mv21/linux-216/arch/ppc/kernel/misc.S,v
> retrieving revision 1.2
> diff -u -r1.2 misc.S
> --- arch/ppc/kernel/misc.S	22 Oct 2003 19:34:30 -0000	1.2
> +++ arch/ppc/kernel/misc.S	1 Jun 2005 17:59:30 -0000
> @@ -736,8 +736,23 @@
>  	subi	r4,r4,1
>  	blelr-
>  00:	lbz	r5,0(r3)
> -	eieio
> -	stbu	r5,1(r4)
> +01:	eieio
> +02:	stbu	r5,1(r4)
> +03:	twi 0, r5, 0
> +04:	isync
> +05:	nop

Hrm... the "twi/isync" stuff is not valid for a store. The reason for
doing twi here is to trigger a data dependency on the result of the load
(to make sure the load actually happens) and the isync to make sure that
twi has actually completed. For stores, you are stuffed... either your
HW don't do machine checks on stores, or you put a sync here and "hope"
you don't have write posting along the IO chain that would cause your
machine check to arrive too late...

Ben.