Linus Torvalds wrote:
On Thu, 9 Mar 2006, Paul Mackerras wrote:
... and x86 mmiowb is a no-op. It's not x86 that I think is buggy.
x86 mmiowb would have to be a real op too if there were any multi-pathed
PCI buses out there for x86, methinks.
Basically, the issue boils down to one thing: no "normal" barrier will
_ever_ show up on the bus on x86 (ie ia64, afaik). That, together with any
situation where there are multiple paths to one physical device means that
mmiowb() _has_ to be a special op, and no spinlocks etc will _ever_ do the
serialization you look for.
Put another way: the only way to avoid mmiowb() being special is either
one of:
(a) have the bus fabric itself be synchronizing
(b) pay a huge expense on the much more critical _regular_ barriers
Now, I claim that (b) is just broken. I'd rather take the hit when I need
to, than every time.
I'm not very driver-minded; would it make sense to have io versions of
locks, which can provide critical sections for IO operations?
The number of (uncommented) memory barriers sprinkled around drivers
looks pretty scary...
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