newbus bus access routines and bus_space_barrier()

[Warner Losh]

Usually they are needed, but we get away without them often because they are needed in a limited set of circumstances and we have memory barriers in our locking primitives.

Warner

On Oct 15, 2011, at 7:48 PM, Adrian Chadd wrote:

> Hi all,
> 
> I'm not clued up on the way of the bus API, so please excuse the
> newbie questions.
> 
> Nathan and I found that ath(4) wasn't working for a user because of a
> missing bus barrier. Ath trips it up because it does lots of loops of
> register reads/writes through the bus stream API rather than the
> normal bus API.
> It does this because it handles the register value swapping in
> hardware rather than in software.
> 
> The correct fix is to teach ath(4) to use bus_space_barrier() calls
> when doing stream calls, which I can do, but the newbus documentation
> points out that both normal and stream bus access doesn't enforce
> ordering, and barrier calls are needed. But I don't see lots of
> bus_space_barrier() calls everywhere. Why's that?
> 
> Thanks,
> 
> 
> Adrian
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