Re: [PATCH 1/2] asm-generic: io: remove {read,write} string functions

Arnd Bergmann
2012-04-27T20:53:42Z

On Friday 27 April 2012, H. Peter Anvin wrote:
> On 04/27/2012 10:14 AM, Will Deacon wrote:
> >
> > If you remove the architecture-specific drivers, there's really not a lot left
> > and, even then, we only need to convert those drivers which are intended to
> > be portable between architectures where the string functions are not
> > consistently available.
> >
> > By overheads, I assume you're referring to the IO_COND check on the address
> > space? I wouldn't expect this to be noticeable compared to the cost of the
> > I/O access and I'm not sure it's worth worrying about for the sake of the
> > small number of drivers affected.
> >
>
> It's not in time, but it adds bulk to the code. The point is that what
> is the benefit of not making these part of the general API? For
> architectures where the address space doesn't matter they could just
> alias it to the same functions, or use the generic versions.
>

The main reasons I can see for not making it a general purpose API are:

* It's a very confusing interface, because the endianess rules are different
from the non-string variants and counterintuitive.

* Almost all the users are ancient ARM specific drivers, the others are
either new ARM specific drivers or drivers that started out as ARM-only
and were ported later to other architectures (sh, avr32, mips, mn10300
and blackfin)

* On all these architectures, the PCI I/O space is memory mapped (or
non-existent), so the ioread* functions are trivial wrappers without
additional overhead.

* Most architectures don't implement them today, but all architectures
that support MMIO also implement the ioread string operations.

Arnd