Re: [Linaro-mm-sig] [PATCH 6/8] drivers: add Contiguous Memory Allocator

[Arnd Bergmann]

On Wednesday 06 July 2011, Nicolas Pitre wrote:
> On Wed, 6 Jul 2011, Russell King - ARM Linux wrote:
> 
> > Another issue is that when a platform has restricted DMA regions,
> > they typically don't fall into the highmem zone.  As the dmabounce
> > code allocates from the DMA coherent allocator to provide it with
> > guaranteed DMA-able memory, that would be rather inconvenient.
> 
> Do we encounter this in practice i.e. do those platforms requiring large 
> contiguous allocations motivating this work have such DMA restrictions?

You can probably find one or two of those, but we don't have to optimize
for that case. I would at least expect the maximum size of the allocation
to be smaller than the DMA limit for these, and consequently mandate that
they define a sufficiently large CONSISTENT_DMA_SIZE for the crazy devices,
or possibly add a hack to unmap some low memory and call
dma_declare_coherent_memory() for the device.

	Arnd
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