Re: Opinion on ordering of writel vs. stores to RAM

["Albert Cahalan" <acahalan@gmail.com>]

On 9/12/06, Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> > If I see an io_to_io_barrier(), how am I to tell if it is
> > read to read, write to write, read to write, write to read,
> > read/write to read, read/write to write, read to read/write,
> > write to read/write, or read/write to read/write?
> >
> > Considering just reads and writes to MMIO, there are
> > 9 possible types of fence. SPARC seems to cover a
> > decent number of these distinctly; the instruction takes
> > an immediate value as flags.
>
> We need to decide wether a single one doing a full MMIO fence (and not
> memory) is enough or if the performance different justifies maybe having
> io_to_io_{wmb,rmb,mb}. I don't see any real use for more combinations.

Remember: it's more than just performance. It's documentation
in the code.
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