[RFC] ARM: dma-mapping: outer cache is invalidated twice

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Feb 15, 2011 at 01:32:32PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 15, 2011 at 02:14:55PM +0100, Per Forlin wrote:
> > outer_inv_range () is called twice for DMA_FROM_DEVICE.
> > The first time to "get rid of potential writebacks" and the second
> > time to "get rid of any stale speculative prefetches"
> 
> Correct.
> 
> > outer_inv_range() is a rather expensive operation. In the first case
> > isn't it enough to just call cache_sync()?
> 
> No.  If the CPU speculatively fetches data from the DMA buffer after
> it's been mapped for DMA, it will bring data into the L2 cache.  This
> data may or may not be up to date with the DMA buffer contents once
> DMA has completed.
> 
> As there is no way to know, we have to invalidate the L2 cache (and the
> L1 cache) after the DMA has completed to avoid any possibility of data
> corruption.

I should add: the solution to all of this is to have cache coherent DMA.

As the CPUs become more complex and start playing tricks like speculative
prefetching, we have seen cache maintainence for DMA becomes more expensive.
The only way to reduce the cost of that is to have cache coherency for DMA.

There is no way to safely avoid the double-invalidate for DMA_FROM_DEVICE.