Re: [PATCH v5 2/6] arch: unify ioremap prototypes and macro aliases

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Jul 1, 2015 at 1:09 AM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Wed, Jul 01, 2015 at 08:55:57AM +0200, Geert Uytterhoeven wrote:
>> On Wed, Jul 1, 2015 at 8:23 AM, Christoph Hellwig <hch@lst.de> wrote:
>> >> One useful feature of the ifdef mess as implemented in the patch is
>> >> that you could test for whether ioremap_cache() is actually
>> >> implemented or falls back to default ioremap().  I think for
>> >> completeness archs should publish an ioremap type capabilities mask
>> >> for drivers that care... (I can imagine pmem caring), or default to
>> >> being permissive if something like IOREMAP_STRICT is not set.  There's
>> >> also the wrinkle of archs that can only support certain types of
>> >> mappings at a given alignment.
>> >
>> > I think doing this at runtime might be a better idea.  E.g. a
>> > ioremap_flags with the CACHED argument will return -EOPNOTSUP unless
>> > actually implemented.  On various architectures different CPUs or
>> > boards will have different capabilities in this area.
>>
>> So it would be the responsibility of the caller to fall back from
>> ioremap(..., CACHED) to ioremap(..., UNCACHED)?
>> I.e. all drivers using it should be changed...
>
> Another important point here is to define what the properties of the
> mappings are.  It's no good just saying "uncached".
>
> We've recently been around this over the PMEM driver and the broken
> addition of ioremap_wt() on ARM...
>
> By "properties" I mean stuff like whether unaligned accesses permitted,
> any kind of atomic access (eg, xchg, cmpxchg, etc).
>
> This matters: on ARM, a mapping suitable for a device does not support
> unaligned accesses or atomic accesses - only "memory-like" mappings
> support those.  However, memory-like mappings are not required to
> preserve access size, number of accesses, etc which makes them unsuitable
> for device registers.

I'm proposing that we explicitly switch "memory-like" use cases over
to a separate set of "memremap()" apis, as these are no longer
"__iomem" [1].

> The problem with ioremap_uncached() in particular is that we have LDD
> and other documentation telling people to use it to map device registers,
> so we can't define ioremap_uncached() on ARM to have memory-like
> properties, and it doesn't support unaligned accesses.
>
> I have a series of patches which fix up 32-bit ARM for the broken
> ioremap_wt() stuff that was merged during this merge window, which I
> intend to push out into linux-next at some point (possibly during the
> merge window, if not after -rc1) which also move ioremap*() out of line
> on ARM but more importantly, adds a load of documentation about the
> properties of the resulting mapping on ARM.

Sounds good, I'll look for that before proceeding on this clean up.

[1]: https://lists.01.org/pipermail/linux-nvdimm/2015-June/001331.html

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