Subject: Re: Opinion on ordering of writel vs. stores to RAM
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Albert Cahalan <acahalan@gmail.com>
Cc: jbarnes@virtuousgeek.org, alan@lxorguk.ukuu.org.uk, davem@davemloft.net,
       jeff@garzik.org, paulus@samba.org, torvalds@osdl.org,
       linux-kernel@vger.kernel.org, akpm@osdl.org, segher@kernel.crashing.org
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Date: Tue, 12 Sep 2006 17:17:00 +1000
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> 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.

David ? It's your call here. What do you think ?

Ben.


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