Re: [PATCH v2 3/4] arch: introduce memremap()

[Dan Williams]

On Sat, May 30, 2015 at 2:00 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Saturday 30 May 2015, Dan Williams wrote:
>>
>> +/*
>> + * memremap() is "ioremap" for cases where it is known that the resource
>> + * being mapped does not have i/o side effects and the __iomem
>> + * annotation is not applicable.
>> + */
>> +
>> +static inline void *memremap(resource_size_t offset, size_t size)
>> +{
>> +       return (void __force *) ioremap(offset, size);
>> +}
>> +
>> +static inline void *memremap_nocache(resource_size_t offset, size_t size)
>> +{
>> +       return (void __force *) ioremap_nocache(offset, size);
>> +}
>> +
>> +static inline void *memremap_cache(resource_size_t offset, size_t size)
>> +{
>> +       return (void __force *) ioremap_cache(offset, size);
>> +}
>> +
>
> There are architectures on which the result of ioremap is not necessarily
> a pointer, but instead indicates that the access is to be done through
> some other indirect access, or require special instructions. I think implementing
> the memremap() interfaces is generally helpful, but don't rely on the
> ioremap implementation.

Is it enough to detect the archs where ioremap() does return an
otherwise usable pointer and set ARCH_HAS_MEMREMAP, in the first take
of this introduction?  Regardless, it seems that drivers should have
Kconfig dependency checks for archs where ioremap can not be used in
this manner.

> Adding both cached an uncached versions is also dangerous, because you
> typically get either undefined behavior or a system checkstop when a
> single page is mapped both cached and uncached at the same time. This
> means that doing memremap() or memremap_nocache() on something that
> may be part of the linear kernel mapping is a bug, and we should probably
> check for that here.

Part of the reason for relying on ioremap() was to borrow its internal
checks to fail attempts that try to remap ranges that are already in
the kernel linear map.  Hmm, that's a guarantee x86 ioremap gives, but
maybe that's not universal?

> We can probably avoid having both memremap() and memremap_nocache(),
> as all architectures define ioremap() and ioremap_nocache() to be the
> same thing.
>

Ok
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