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. >> > > > ioremap() falls back to the cache type of an existing mapping to >> > > > avoid aliasing. >> > > >> > > Does that assume an existing ioremap*() call was used on a bigger >> > > range? Do you know if that happens to only be the case for x86 (I'd >> > > think so) or if its the same for other architectures ? >> > >> > In the /dev/mem example, it is remap_pfn_range(). I think other archs >> > have the same issue, but I do not know if they fall back in case of >> > overlapping call. >> >> What should happen if remap_pfn_range() was used with >> pgprot_writecombine() and later memremap() is used for instance with a >> smaller overlappin section, or perhaps bigger? > > With the fallback disabled, memremap() should fail in this case. Excellent. Luis -- 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
