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From: Linus Torvalds (torvalds@osdl.org)
Date: Tue Dec 14 2004 - 15:15:13 EST


On Tue, 14 Dec 2004, Matthew Wilcox wrote:
>
> Hi Linus. On x86 and ia64, memcpy_fromio is protoyped as:
>
> static inline void memcpy_fromio(void *dst, volatile void __iomem *src, int count)
>
> ALSA does this (except on x86 and sparc32, so you don't see it):
>
> int copy_to_user_fromio(void __user *dst, const void __iomem *src, size_t count)
> [...]
> memcpy_fromio(buf, src, c);
>
> which provokes a warning from gcc that we're discarding a qualifier (the
> 'const') from src. Is ALSA just wrong? Or is the 'volatile' wrong?

Neither. The right thing for a read-only IO pointer is actually

        const volatile void __iomem *

which looks funny ("const volatile"?) but makes sense for prototypes,
exactly because a "const volatile" pointer is the most permissive kind of
pointer there is. And it actually does describe the thing perfectly: it is
"const" because we don't write to it ("const" in C does not mean that the
thing is constant, and never has, confusing name and some C++ semantic
changes aside) and obviously as an IO area it's both "volatile" and
"__iomem".

On x86, readb/w/l already gets that right, so I'll just fix
memcpy_fromio(). Other architectures can sort out themselves (ppc64 is
already correct, at least for eeh).

                Linus
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