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

Toshi Kani
2015-07-07T23:12:15Z

On Tue, 2015-07-07 at 18:07 +0200, Luis R. Rodriguez wrote:
> On Tue, Jul 07, 2015 at 11:13:30AM +0100, Russell King - ARM Linux
> wrote:
:
> > On ARM, we (probably) have a lot of cases where ioremap() is used
> > multiple
> > times for the same physical address space, so we shouldn't rule out
> > having
> > multiple mappings of the same type.
>
> Why is that done? Don't worry if you are not sure why but only
> speculate of the
> practice's existence (sloppy drivers or lazy driver developers). FWIW
> for x86
> IIRC I ended up concluding that overlapping ioremap() calls with the
> same type
> would work but not if they differ in type. Although I haven't
> written a
> grammer rule to hunt down overlapping ioremap() I suspected its use
> was likely
> odd and likely should be reconsidered. Would this be true for ARM too
> ? Or are
> you saying this should be a feature ? I don't expect an answer now
> but I'm
> saying we *should* all together decide on this, and if you're
> inclined to
> believe that this should ideally be avoided I'd like to hear that. If
> you feel
> strongly though this should be a feature I would like to know why.

There are multiple mapping interfaces, and overlapping can happen among
them as well. For instance, remap_pfn_range() (and
io_remap_pfn_range(), which is the same as remap_pfn_range() on x86)
creates a mapping to user space. The same physical ranges may be
mapped to kernel and user spaces. /dev/mem is one example that may
create a user space mapping to a physical address that is already
mapped with ioremap() by other module. pmem and DAX also create
mappings to the same NVDIMM ranges. DAX calls vm_insert_mixed(), which
is particularly a problematic since vm_insert_mixed() does not verify
aliasing. ioremap() and remap_pfn_range() call reserve_memtype() to
verify aliasing on x86. reserve_memtype() is x86-specific and there is
no arch-generic wrapper for such check. I think DAX could get a cache
type from pmem to keep them in sync, though.

Thanks,
-Toshi