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

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Message-Id: <201204272052.34489.arnd@arndb.de>
Date:	Fri, 27 Apr 2012 20:52:34 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"uclinux-dist-devel@...ckfin.uclinux.org" 
	<uclinux-dist-devel@...ckfin.uclinux.org>,
	Mike Frysinger <vapier@...too.org>
Subject: Re: [PATCH 1/2] asm-generic: io: remove {read,write} string functions

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
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