On Wed, Jan 08, 2020 at 09:44:36AM +0100, Arnd Bergmann wrote:

On Wed, Jan 8, 2020 at 9:36 AM Christophe Leroy [off-list ref] wrote:

quoted

Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :

quoted

On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven [off-list ref] 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:

https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html

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;
}

SuperH is another example:
1. ioread8_rep(void __iomem *addr, void *dst, unsigned long count)
   calls mmio_insb()

2. static inline void mmio_insb(void __iomem *addr, u8 *dst, int count)
   calls __raw_readb()

3. #define __raw_readb(a)          (__chk_io_ptr(a), *(volatile u8  __force *)(a))

Even if interface was not marked as volatile, in fact its implementation
was casting to volatile.

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.

Indeed. I guess there are no objections around const so let me send v2
for const only.

Best regards,
Krzysztof