[PATCH] ARM: io: avoid writeback addressing modes for __raw_ accessors

On Tuesday 21 August 2012, Will Deacon wrote:

On Mon, Aug 20, 2012 at 07:45:20PM +0100, Nicolas Pitre wrote:

quoted

On Mon, 20 Aug 2012, Will Deacon wrote:

quoted

On Mon, Aug 20, 2012 at 07:04:01PM +0100, Nicolas Pitre wrote:

quoted

Nah... the const is wrong.  The way you wrote it means that addr may 
change but the pointed data is constant.  This is obviously wrong since 
we expect the pointed location to change even from a read.

That's the prototype for the read accessors though -- a bunch of other
architectures define them that way (including asm-generic), so I wonder what
the reasoning behind that was?

I'm still asserting that they're wrong.

I did a bit of digging around and `const volatile void *' is apparently used
because a function with such a parameter type can be passed any old pointer
without warnings. Torvalds says something about them here:

  http://readlist.com/lists/vger.kernel.org/linux-kernel/14/72300.html

Personally, I too think that the const is misleading and who on Earth would
be passing in pointers to const for an I/O region? However, it's an argument
I'd rather avoid so, for the sake of consistency, I'll cast away the const in
the asm block.

You could have a read-only register area, and I would regard it as sensible
to mark a pointer to it as "const", although there should not be any
optimizations based on that.

There is no need to cast away the constness of the pointer when passing
it into the inline assembly, as the "asm volatile" already implies that
gcc cannot remove the contents.

On a related topic, thank you very much for introducing the inline
assemblies here, as they finally solve a lingering problem that has
hit us in the past [1] and that could happen again in other drivers.

	Arnd

[1] http://old.nabble.com/ARM-unaligned-MMIO-access-with-attribute%28%28packed%29%29-td30827280.html