Re: question on memory barrier

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

On Iau, 2005-08-25 at 11:14 +0200, moreau francis wrote:
> I'm compiling Linux kernel for a MIPS32 cpu. On my platform, writel seems
> expand to:
> 
>     static inline writel(u32 val, volatile void __iomem *mem)
>     {
>             volatile u32 *__mem;
>             u32 __val;
> 
>             __mem = (void *)((unsigned long)(mem));
>             __val = val;
> 
>             *__mem = __val;
>     }
> 
> I don't see the magic in it since "volatile" keyword do not handle memory
> ordering constraints...Linus wrote on volatile keyword, see

For the case and the platform the volatile is sufficient to force
ordering. That or the arch code author made a mistake. But I think its
sufifcient for MIPS. The volatile prevents

	*foo = 1; 
	*foo = 2;

or even

	*foo;

from being collapsed together or eliminated