linux-kernel - Re: [PATCH] asm-generic/io.h: Fix io{read,write}{16,32}be for big endian systems

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Message-Id: <201101181944.07146.arnd@arndb.de>
Date:	Tue, 18 Jan 2011 19:44:07 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Lars-Peter Clausen" <lars@...afoo.de>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic/io.h: Fix io{read,write}{16,32}be for big endian systems

On Tuesday 18 January 2011 19:11:01 Lars-Peter Clausen wrote:
> Currently io{read,write}{16,32} expand to
>         *addr = cpu_to_leXX(cpu_to_beXX(val))
> and
>         val = beXX_to_cpu(leXX_to_cpu(*addr))
> 
> While it should rather be:
>         *addr = cpu_to_be{16,32}(val)
> and
>         val = be{16,32}_to_cpu(*addr)
> 
> The current implementation works on litte-endian targets(where cpu_to_leXX is a
> noop), but breaks on on big-endian targets, this patch fixes it.
> 
> Signed-off-by: Lars-Peter Clausen <lars@...afoo.de>

The existing code looks broken on big-endian indeed, thanks
for the report!

However, you cannot use __raw_readl in general, because that
function knows nothing about the various kinds of I/O accesses
(potentially) handled by readl. Think of architectures where
readl is not a pointer dereference but something else.

The right solution is probably to use swab16/swab32 for the
big-endian functions. This also corrects the iowrite functions
which really should be using cpu_to_be32 instead of be32_to_cpu
(although they are always defined to be the same afaict.

	Arnd
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