Re: I/O write ordering

James Bottomley
2004-09-22T15:11:57Z

On Wed, 2004-09-22 at 10:51, Benjamin Herrenschmidt wrote:
> Read & write barriers are different for IO. Actually, as I stated earlier,
> the real issue isn't much with IO barriers proper (which are mostly dealt
> with within the implementation of the IO accessors), but in IO vs. memory
> accesses, at least on PPC. Cacheable and non-cacheable accesses go through
> completely different path on the CPU and can be completely re-ordered one
> to each other unless more heavy barriers are used. That has been a pain
> for ages for us since Linux doesn't provide an abstraction to order those,
> it would be nice to get that in place now ;)

Just to be sure we're talking about the same thing; by cacheable and
non-cacheable are you thinking about what PCI calls consistent and
streaming (because your consistent memory is implemented as
non-cacheable) or is this really the fact that PCI memory space as seen
by the CPU is all uncacheable?

James