[PATCH] OMAP: fix DMA vs memory ordering

[tony@atomide.com (Tony Lindgren)]

Hi,

* Russell King - ARM Linux <linux@arm.linux.org.uk> [120414 06:27]:
> Using coherent DMA memory with the OMAP DMA engine results in
> unpredictable behaviour due to memory ordering issues; as things stand,
> there is no guarantee that data written to coherent DMA memory will be
> visible to the DMA hardware.
> 
> This is because the OMAP dma_write() accessor contains no barriers,
> necessary on ARMv6 and above.  The effect of this can be seen in comments
> in the OMAP serial driver, which incorrectly talks about cache flushing
> for the coherent DMA stuff.
> 
> Rather than adding barriers to the accessors, add it in the DMA support
> code just before we enable DMA, and just after we disable DMA.  This
> avoids having barriers for every DMA register access.

Good catch.

I guess what you're saying is we don't want to do mb() for each completed
transfer that calls the driver specific callback function?

Other than that:

Acked-by: Tony Lindgren <tony@atomide.com>