Re: memory barriers and dma_sync_single_for_*

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

Ar Sul, 2006-07-09 am 16:53 +0200, ysgrifennodd Stefan Richter:
> My question: Do I need wmb() within the following code?
> 
> 	dma_sync_single_for_cpu(..., DMA_TO_DEVICE);
> 	area->a = new_val_a;
> 	wmb();  /* necessary? */
> 	area->b = new_val_b;

Yes. Otherwise on some processors the I/O may be re-ordered

> 	dma_sync_single_for_device(..., DMA_TO_DEVICE);
> 
> Or are all changes to the area hidden from the device between 
> dma_sync_single_for_cpu and _for_device, thereby making the write order 
> a non-issue? Thanks,

Depends on the processor and setup. On x86-32 for example dma_sync_.. is
generally a NOP. In other environments nothing will be visible until the
sync.

Alan

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