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

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Date:	Wed, 4 Jun 2008 12:25:49 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Trent Piepho <tpiepho@...escale.com>
Cc:	Matthew Wilcox <matthew@....cx>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Wednesday 04 June 2008 07:44, Trent Piepho wrote:
> On Tue, 3 Jun 2008, Matthew Wilcox wrote:

> > I don't understand why you keep talking about DMA.  Are you talking
> > about ordering between readX() and DMA?  PCI proides those guarantees.
>
> I guess you haven't been reading the whole thread.  The reason it started
> was because gcc can re-order powerpc (and everyone else's too) IO accesses
> vs accesses to cachable memory (but not spin-locks), which ends up only
> being a problem with coherent DMA.

I don't think it is only a problem with coherent DMA.

CPU0                         CPU1
mutex_lock(mutex);
writel(something, DATA_REG);
writel(GO, CTRL_REG);
started = 1;
mutex_unlock(mutex);
                             mutex_lock(mutex);
                             if (started)
                               /* oops, this can reach device before GO */
                               writel(STOP, CTRL_REG);
                               
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