On Wednesday 15 December 2010, Russell King - ARM Linux wrote:
> > The regular accessor function for I/O registers is readl, which handles
> > the access correctly with regard to atomicity, I/O ordering and byteorder.
>
> There's no possibility of those two being mis-ordered - they will be in
> program order whatever.
>
> What isn't guaranteed is the ordering between I/O accesses (accesses to
> device memory) and SDRAM accesses (normal memory) which can pass each other
> without additional barriers. Memory accesses can pass I/O accesses.
Yes, that's what I meant.
> If you don't need normal vs device access ordering, using readl_relaxed()/
> writel_relaxed() is preferred, and avoids the (apparantly rather high)
> performance overhead of having to issue barriers all the way down to the
> L2 cache.
Well, my point was that the authors should choose their I/O accessors
carefully. Using __raw_writel() without any explanations is a rather
bad default, it's not designed for that. Using writel() as a default
is usually a good choice, as we can assume it to do the right thing.
writel_relaxed() is also good where appropriate, because it tells
the reader that the driver author has thought about the I/O (vs. code)
ordering and concluded that it's safe to do.
> Lastly, I don't see where atomicity comes into it - __raw_writel vs writel
> have the same atomicity. Both are single access atomic provided they're
> naturally aligned. Misaligned device accesses are not predictable.
This is just what gcc turns it into today. In theory, a future gcc or
a future cpu might change that. If you mark a pointer as
'__attribute__((packed))', it probably already does, even for aligned
pointers, while it does not when using writel_{,relaxed}. The point is
that __raw_* means just that -- we don't give any guarantees on what
happens on the bus, so people should not use it.
Arnd
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