Re: readsw/writesw readsl/writesl

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

>The only reason insl() exists is because the x86 has special
>instructions to perform that operation.
>
>It used to be an optimization when cpus were really slow.
>
>No cpu to my knowledge has special instructions to readsl et al.  and
>on no cpu would be faster than a hand coded loop.
>
>In fact I would instead vote to delete {in,out}s{b,w,l}() and friends.
>:-)

The problem with that approach is that the "s" versions must also take
care of byte swapping (or rather _not_ byteswapping while the non-"s"
do the byteswapping).

So we would need to have raw_{in,out}{b,w,l}. Currently, it's not
possible to implement {in,out}s{b,w,l} in an efficient way because of
that.

Then we would also need to expose the io_barrier for CPUs like PPC

etc...

I tend to think that makes us expose too much CPU-specific things, which
is why I'd rather have the {read,write}s{b,w,l} versions provided by the
arch so those can be done "the right way" in the arch code, and drivers
not care about some of the gory details.

Ben.