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

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Message-ID: <787b0d920609120021l7a4a2ce3ida379fbd69bd13b9@mail.gmail.com>
Date:	Tue, 12 Sep 2006 03:21:03 -0400
From:	"Albert Cahalan" <acahalan@...il.com>
To:	"Benjamin Herrenschmidt" <benh@...nel.crashing.org>
Cc:	jbarnes@...tuousgeek.org, alan@...rguk.ukuu.org.uk,
	davem@...emloft.net, jeff@...zik.org, paulus@...ba.org,
	torvalds@...l.org, linux-kernel@...r.kernel.org, akpm@...l.org,
	segher@...nel.crashing.org
Subject: Re: Opinion on ordering of writel vs. stores to RAM

On 9/12/06, Benjamin Herrenschmidt <benh@...nel.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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