[RFC 06/18] arm: msm: implement proper dmb() for 7x27

[Jamie Lokier]

Russell King - ARM Linux wrote:
> > It's not an ARMv7, otherwise it wouldn't be using the mcr version of
> > dmb().  Does that make the mapping ok, since it's been ok for years on
> > < ARMv7?  Or are we trying to get away from doing that on all ARMs?
> 
> Technically, it also applies to ARMv6 as well.
> 
> > Actually it is only used on two very specific CPUs.  Perhaps it can be
> > confirmed as Not A Problem(tm) on those, with a comment to say why
> > it's ok in the mapping call?
> 
> The fact of the matter is that cache lines will be allocated for
> empty_zero_page.  If this CPU is ARMv6 or ARMv7, with either an aliasing
> or non-aliasing VIPT cache, you will get cache lines allocated for this
> page which will overlap the strongly ordered mapping.
> 
> That in turn can turn the strongly ordered mapping into a cached mapping
> which is definitely not what you want.
> 
> If your CPU speculatively prefetches, and it prefetches some data via a
> cached mapping, the same thing can happen.

Fair enough.  Is it like this?

   1. Data ends up in cache lines from access via the cached mapping.
   2. Access via the strongly ordered mapping may still look at the cache,
      because it's easier that way and it's not supposed to have any data.
   3. Cache effectively intercepts the access.
   4. Bus does not see strongly ordered access.

> 'Memory, uncached' is used for DMA mappings on ARMv7 and soon to be on
> ARMv6 as well to comply with the architecture requirements.  Strongly
> ordered is used for a certain set of IOP3xx registers because that is
> what is stipulated in the device documentation.
> 
> What is expressly not permitted for ARMv7 (and ARMv6) is having two or
> more mappings of the same physical address with differing memory types
> or sharability settings.
> 
> (Technically, that extends to cacheability modes as well - but if ARM Ltd
> think that the kernel's going to comply with that, I think they're in
> cloud cuckoo land.  Well, we could do _if_ (eg) ARM Ltd bring in hardware
> DMA coherency as a mandatory architecture requirement.)

Isn't the above sequence of events you described earlier even _more_
likely to occur when there are simultaneous cacheable and
non-cacheable mappings?

I understand the reason for the cloud cuckoo land comment :-) But I'm
thinking, an implementation which has no problem with cacheable and
non-cacheable overlapping mappings is very unlikely to have a problem
with cacheable and strongly-ordered mappings.  Is there a reason to
believe otherwise?

-- Jamie