On Tue, Aug 11, 2015 at 3:52 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote: > On Tue, Aug 11, 2015 at 3:40 PM, Toshi Kani <toshi.kani@xxxxxx> wrote: >> 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*() variant >>> > > 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 we're >>> > > 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. > > Great. > >>> > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
