Re: [PATCH v2 08/25] arch: introduce memremap()

[Toshi Kani]

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:
> > > On Wed, Jul 29, 2015 at 11:27 AM, Luis R. Rodriguez <mcgrof@xxxxxxxx> 
> > > wrote:
> > > > On Wed, Jul 29, 2015 at 08:50:04AM +0200, Christoph Hellwig wrote:
> > > > > On Mon, Jul 27, 2015 at 04:26:03PM -0700, Dan Williams wrote:
> > > > > > Oh, because all we have at this point is ioremap_cache() which
> > > > > > silently falls back.  It's not until the introduction of
> > > > > > arch_memremp() where we update the arch code to break that 
> > > > > > behavior.
> > > > > 
> > > > > Ok, makes sense.  Might be worth to document in the changelog.
> > > > > 
> > > > > > That said, I think it may be beneficial to allow a fallback if 
> > > > > > the user cares.  So maybe memremap() can call plain ioremap() if
> > > > > > MEMREMAP_STRICT is not set and none of the other mapping types 
> > > > > > are satisfied.
> > > > > 
> > > > > Is there a real use case for it?  Fallback APIs always seem 
> > > > > confusing and it might make more sense to do this in the caller(s) 
> > > > > that actually need it.
> > > > 
> > > > It seems semantics-wise we are trying to separate these two really, 
> > > > so I agree with this. Having a fallback would onloy make things more
> > > > complicated for any sanitizer / checker / etc, and I don't think the 
> > > > practical gains of having a fallback outweight the gains of having a 
> > > > clear semantic separation on intended memory type and interactions 
> > > > with it.
> > > > 
> > > 
> > > Yup, consider it dropped.  Drivers that want fallback behavior can do
> > > it explicitly.
> > 
> > I agree in general.  However, for the drivers to be able to fall back
> > properly, they will need to know the cache type they can fall back with. 
> > 
> 
> 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.

> 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.

> > 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. 

> While at it, Dan, will / should memremap() support overlapping calls ?
> What is the expectation on behaviour ?
> 
> PS: I did see you reply about this being about lacking an arch 
> implementation for a memremap() type, not a cache type, but as the driver 
> uses one memremap() type the goal for a region is just as important as the 
> resulting type.

Agreed.  Drivers cannot tell if a fallback is due to lacking implementation
of overlapping, unless they check with #ifdef ioremap_xxx.

Thanks,
-Toshi
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