linux-kernel - Re: MMIO and gcc re-ordering issue

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Date:	Tue, 27 May 2008 13:33:56 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Trent Piepho <tpiepho@...escale.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: MMIO and gcc re-ordering issue

Trent Piepho wrote:
> Is there an issue with anything _besides_ coherent DMA?
> 
> Could one have a special version of the accessors for drivers that
> want to assume they are strictly ordered vs coherent DMA memory?
> That would be much easier to get right, without slowing _everything_
> down.

It's better to be safe by default and then optimize the fast paths than 
to be relaxed by default and hang the machine in some piece of code that 
runs once a month.  "Premature optimization is the root of all evil", 
and what not.

> One could even go as far as to allow a driver to "#define 
> WANT_STRICT_IO" and then it would get the strict versions.  Add that
> to any driver that uses DMA and then worry about vetting those
> drivers.

See above -- if you must have a #define, then it should be WANT_RELAXED_IO.

-Scott

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