[POWERPC] convert string i/o operations to C

[Segher Boessenkool]

> But it couldn't hurt, right? There has to be an application note  
> per-CPU on the correct sequence of operations for such an access (I  
> seem to have collected a directory full for firmware development),

The architecture describes the rules already, not many CPUs
have "faster"/"better" sequences.

> it seems a little odd to pick and choose one instruction over  
> another for one thing, and then say you need to do it to support  
> the 601 of all things, and run this code against the G3/G4/G5 which  
> perhaps doesn't care or is more intelligent about it (or is  
> guaranteed to have a more intelligent host bridge at least).

The comment you're referring to is old; it doesn't talk about
synchronisation requirements, but focuses on having the CPU
trap on exactly these instructions when an access causes a
(asynchronous) machine check.  Asynchronous exceptions don't
necessarily return the instruction pointer where the real
failure was, so it's no surprise different CPUs have a different
idea about it.  It's pretty safe to assume (but not guaranteed)
that it will always be somewhere between the load and the insn
after the isync, inclusive, though.


Segher