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Date: Tue, 07 Jul 2015 17:10:58 -0600
From: Toshi Kani <toshi.kani@...com>
To: "Luis R. Rodriguez" <mcgrof@...e.com>,
Russell King - ARM Linux <linux@....linux.org.uk>
Cc: Christoph Hellwig <hch@....de>,
Andy Lutomirski <luto@...capital.net>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Julia Lawall <julia.lawall@...6.fr>,
Dan Williams <dan.j.williams@...el.com>,
Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ross Zwisler <ross.zwisler@...ux.intel.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Juergen Gross <jgross@...e.com>, X86 ML <x86@...nel.org>,
"linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Stefan Bader <stefan.bader@...onical.com>,
Linux MM <linux-mm@...ck.org>,
Ralf Baechle <ralf@...ux-mips.org>,
Henrique de Moraes Holschuh <hmh@....eng.br>,
Michael Ellerman <mpe@...erman.id.au>,
Tejun Heo <tj@...nel.org>, Paul Mackerras <paulus@...ba.org>,
mcgrof@...not-panic.com,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 2/6] arch: unify ioremap prototypes and macro aliases
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
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