Arjan van de Ven wrote:
>
> > with PCI, and the PCI posting rules, there is no "one" serializing
> > instruction, you need to know the specifics of the device in
question to
> > cause the flush. So at least there is no universal possible
> > implementation of volatile as you suggest ;-)
> >
>
> A serializing volatile makes it possible to write portable code to
> access pci mmio. You'd just follow a write with a read or whatever the
> rules say.
yeah except that the compiler cannot know what to read; reading back the
same memory location is NOT correct nor safe. It's device specific, for
some devices it'll be safe, for others you have to read some OTHER
location.
I didn't suggest the compiler could or should do it, just that it would
be possible (for the _user_) to write portable ISO C code to access PCI
mmio registers, if volatile's implementation serialized access.
With the current implementation of volatile in gcc, it is impossible -
you need to resort to inline assembly for some architectures, which is
not an ISO C feature.
And I'm not suggesting that it would be a good idea to use volatile even
if it was corrected - it would have to take a worst-case approach and
thus would generate very bad code.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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