linux-kernel - Re: MMIO and gcc re-ordering (Was: [PATCH] [POWERPC] Improve (in|out)

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Date:	Fri, 23 May 2008 08:50:16 -0400
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Trent Piepho <tpiepho@...escale.com>
Cc:	David Miller <davem@...emloft.net>, scottwood@...escale.com,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: MMIO and gcc re-ordering (Was: [PATCH] [POWERPC] Improve
	(in|out)_beXX() asm code)


On Fri, 2008-05-23 at 08:36 -0400, Benjamin Herrenschmidt wrote:
> 
>         - mandate some kind of dma_sync_for_device/cpu on consistent memory.
> Almost no driver do that currently tho. They only do that for non
> consistent memory mapped with dma_map_*.
> 
>         - mandate the use of wmb,rmb,mb barriers for use between memory
> accesses and MMIOs for ordering them. (ie. fix drivers that don't do
> it). Advantage for powerpc is that I can remove (after some auditing of
> course) the added heavy barriers in the MMIO accessors themselves.

Note that the above is my preferred approach, and a lot of drivers
happen to already do this.

>         - stick a full memory clobber in all MMIO (and PIO) accessors on all
> archs.
> 
> Any other idea ? preference ?

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