Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument

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On Wed, Jan 8, 2020 at 9:36 AM Christophe Leroy <christophe.leroy@xxxxxx> wrote:
> Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :
> > On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > I'll add to this one also changes to ioreadX_rep() and add another
> > patch for volatile for reads and writes. I guess your review will be
> > appreciated once more because of ioreadX_rep()
> >
> volatile should really only be used where deemed necessary:
> It is said: " ...  accessor functions might use volatile on
> architectures where direct I/O memory access does work. Essentially,
> each accessor call becomes a little critical section on its own and
> ensures that the access happens as expected by the programmer."

The I/O accessors are one of the few places in which 'volatile' generally
makes sense, at least for the implementations that do a plain pointer
dereference (probably none of the ones in question here).

In case of readl/writel, this is what we do in asm-generic:

static inline u32 __raw_readl(const volatile void __iomem *addr)
        return *(const volatile u32 __force *)addr;

The __force-cast that removes the __iomem here also means that
the 'volatile' keyword could be dropped from the argument list,
as it has no real effect any more, but then there are a few drivers
that mark their iomem pointers as either 'volatile void __iomem*' or
(worse) 'volatile void *', so we keep it in the argument list to not
add warnings for those drivers.

It may be time to change these drivers to not use volatile for __iomem
pointers, but that seems out of scope for what Krzysztof is trying
to do. Ideally we would be consistent here though, either using volatile
all the time or never.


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