On Tue, Aug 11, 2015 at 3:52 PM, Luis R. Rodriguez <mcgrof(a)suse.com> wrote:
On Tue, Aug 11, 2015 at 3:40 PM, Toshi Kani <toshi.kani(a)hp.com>
> On Tue, 2015-08-11 at 23:30 +0200, Luis R. Rodriguez wrote:
>> On Wed, Jul 29, 2015 at 06:00:04PM -0600, Toshi Kani wrote:
>> > On Wed, 2015-07-29 at 23:43 +0200, Luis R. Rodriguez wrote:
>> > > On Wed, Jul 29, 2015 at 03:00:38PM -0600, Toshi Kani wrote:
>> > > > On Wed, 2015-07-29 at 11:33 -0700, Dan Williams wrote:
>> > > That would depend on the purpose of the region and the driver
>> > > developer should in theory know best. One issue with this of course is
>> > > that, as we've discovered, the semantics of on the ioremap*()
>> > > front regarding cache types is not clearly well defined, or at least
>> > > it may be only well defined implicitly on x86 only, so the driver
>> > > developer can only *hope* for the best across architectures. Our
>> > > ambiguity on our semantics on ioremap*() variants therefore means
>> > > driver developers can resonably be puzzled by what fallbacks to use.
>> > > That also means architectures maintainers should not whip driver
>> > > developers for any misuse. Such considerations are why although
>> > > now revisiting semantics for ioremap*() variants I was in hopes we
>> > > could be at least somewhat pedantic about memremap() semantics.
>> > I agree. However, there are a few exceptions like /dev/mem, which can
>> > map a target range without knowledge.
>> Still, the expectation to require support for overlapping ioremap() calls
>> would seem to be more of an exception than the norm, so I'd argue that
>> requiring callers who know they do need overlapping support to be explicit
>> about it may help us simplify expecations on semantics in other areas of
>> the kernel.
> Again, I agree. I am simply saying that the fallback in an overlapping case
> may need to remain supported for such exceptional cases, possibly with a
> separate interface.
>> > > For instance since memremap() only has 2 types right now can a
>> > > respective fallback be documented as an alternative to help with this
>> > > ? Or can we not generalize this ? One for MEMREMAP_WB and one for
>> > > MEMREMAP_WT ?
>> > Yes, if a target range can be only mapped by memremap(). However, there
>> > is no restriction that a range can be mapped with a single interface
>> > alone. For example, a range can be mapped with remap_pfn_range() to
>> > user space with any cache type. So, in theory, memremap() can overlap
>> > with any other types.
>> Shouldn't that be an issue or area of concern ? It seems the flakiness on
>> ioremap() and its wide array flexibility would spill over the any
>> semantics which folks would be trying to set out with memremap(). That
>> should not stop the evolution of memremap() though, just pointing out that
>> perhaps we should be a bit more restrictive over how things can criss
>> -cross and who areas can do it.
> reserve_pfn_range() allows the caller to specify if the fallback needs to be
> enabled or disabled with 'strict_prot'. track_pfn_remap() called from
> remap_pfn_range() enables it, and track_pfn_copy() disables it. I think we
> can do similar for the memremap and ioremap families as well. The fallback
> could be set disabled in the regular interfaces, and enabled in some
> interface as necessary. This also allows gradual transition, ex. disable in
> memremap while ioremap remains enabled for now.
Sounds sexy to me.
Cool, sounds like something we can tackle in 4.4 along with the
ioremap_cache removal cleanups.