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

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Date:	Wed, 19 Jan 2011 13:28:36 +0100
From:	Jonas Bonn <jonas.bonn@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Lars-Peter Clausen <lars@...afoo.de>, 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 19 January 2011 10:58, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday 18 January 2011 23:22:23 Lars-Peter Clausen wrote:
>> On 01/18/2011 10:37 PM, Arnd Bergmann wrote:
>> > ...
>> The lm32 architecture is a big-endian softcpu architecture. I'm currently working on
>> the MilkyMist SoC which uses it and all the SoC components have native endianess.
>
> ok.

I think this is the key concept that's being missed here:  devices
with _native_ endianess.  This is something that I've been grappling
with for the OpenRISC platform, as well.

The ioread/write accessors are supposed to be a platform-agnostic
interface and have a builtin assumption about the endianess of the
device/bus.  Device drivers that use ioread32 are explicity accessing
a little-endian device; ioread32be explicitly a big-endian device.
It's not really for the architecture to redefine these.  The same
applies to the readl/readw accessors as well, as far as I understand
it.

What's needed is another way of accessing device memory that doesn't
carry the endianess implication.  For the Wishbone bus, used by the
OpenRISC processor, I still haven't found a more satisfactory solution
than to define "wishbone accessors" that map to the underlying
io-accessors of the desired endianess.  These allow the endianess of
the bus to be set at compile time and hopefully for all wishbone
device drivers to do the right thing.

#ifdef CONFIG_WISHBONE_BIG_ENDIAN
#define wb_ioread8(p) ioread8(p)
#define wb_ioread16(p) ioread16be(p)
#define wb_ioread32(p) ioread32be(p)
#define wb_iowrite8(p) iowrite8(p)
#define wb_iowrite16(p) iowrite16be(p)
#define wb_iowrite32(p) iowrite32be(p)
#else
#define wb_ioread8(p) ioread8(p)
#define wb_ioread16(p) ioread16(p)
#define wb_ioread32(p) ioread32(p)
#define wb_iowrite8(p) iowrite8(p)
#define wb_iowrite16(p) iowrite16(p)
#define wb_iowrite32(p) iowrite32(p)
#endif /* CONFIG_WISHBONE_BIG_ENDIAN */

That said, there are architectures that do redefine the endianess of
the io-accessors to mean "native endianess", but this must have
implications for a large number of the drivers in the kernel tree and,
in my opinion, must be technically _incorrect_.  A driver written with
ioread32 shouldn't have the endianess of the access changed by the
architecture underneath it... or are others of a different opinion?

>> >
>> > * change the common ioread*/iowrite* functions to all be based on
>> >   the __raw_* I/O versions, not just the big-endian ones. The
>> >   space overhead you quoted is enough of a justification for that.
>>

Doing this breaks the endianess assumption built into the io-accessors.

/Jonas
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