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

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Message-Id: <200806101219.34995.jbarnes@virtuousgeek.org>
Date:	Tue, 10 Jun 2008 12:19:34 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Roland Dreier <rdreier@...co.com>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <matthew@....cx>,
	Trent Piepho <tpiepho@...escale.com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	scottwood@...escale.com, linuxppc-dev@...abs.org,
	alan@...rguk.ukuu.org.uk, linux-kernel@...r.kernel.org
Subject: Re: MMIO and gcc re-ordering issue

On Tuesday, June 10, 2008 12:05 pm Roland Dreier wrote:
>  > me too.  That's the whole basis for readX_relaxed() and its cohorts: we
>  > make our weirdest machines (like altix) conform to the x86 norm.  Then
>  > where it really kills us we introduce additional semantics to selected
>  > drivers that enable us to recover I/O speed on the abnormal platforms.
>
> Except as I pointed out before, Altix doesn't conform to the norm and
> many (most?) drivers are missing mmiowb()s that are needed for Altix.
> Just no one has plugged most devices into an Altix (or haven't stressed
> the driver in a way that exposes problems of IO ordering between CPUs).
>
> It would be a great thing to use the powerpc trick of setting a flag
> that is tested by spin_unlock()/mutex_unlock() and automatically doing
> the mmiowb() if needed, and then killing off mmiowb() entirely.

Yeah I think that's what Nick's guidelines would guarantee.  And Jes is 
already working on the spin_unlock change you mentioned, so mmiowb() should 
be history soon (in name only, assuming Nick also introduces the I/O barriers 
he talked about for ordering the looser accessors it would still be there but 
would be called io_wmb or something).

Jesse

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