mapping uncached memory

[Russell King - ARM Linux]

On Wed, Mar 17, 2010 at 10:02:09AM +0200, Budhee Jamaich wrote:
> On Wed, Mar 17, 2010 at 1:54 AM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> >> 3. use dma_alloc_coherent in some way
> >>
> >> cons: documentation says we still need to use cache clean/inv
> 
> > Please provide a pointer to that documentation.
> 
> 
> from Documentation/DMA-API.txt:
> 
> "void *
> dma_alloc_coherent(struct device *dev, size_t size,
>                              dma_addr_t *dma_handle, gfp_t flag)
> 
> Consistent memory is memory for which a write by either the device or
> the processor can immediately be read by the processor or device
> without having to worry about caching effects.  (You may however need
> to make sure to flush the processor's write buffers before telling
> devices to read that memory.)"
> 
> 
> 
> That last sentence - what does it really say ? That I still need to
> manually clean/invalidate the caches myself ?

No - if it did, it would contradict the previous sentence.  What it's
referring to is that on weakly ordered CPUs, you may need barriers.