Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")

["David Xiao" <dxiao@broadcom.com>]

On Tue, 2009-08-11 at 02:31 -0700, Catalin Marinas wrote:
> On Thu, 2009-08-06 at 22:59 -0700, David Xiao wrote:
> > The V7 speculative prefetching will then probably apply to DMA coherency
> > issue in general, both kernel and user space DMAs. Could this be
> > addressed by inside the dma_unmap_sg/single() calling dma_cache_maint()
> > when the direction is DMA_FROM_DEVICE/DMA_BIDIRECTIONAL, to basically
> > invalidate the related cache lines in case any filled by prefetching?
> > Assuming dma_unmap_sg/single() is called after each DMA operation is
> > completed. 
> 
> Theoretically, with speculative prefetching on ARMv7 and the FROM_DEVICE
> case we need to invalidate the corresponding D-cache lines both before
> and after the DMA transfer, i.e. in both dma_map_sg and dma_unmap_sg,
> otherwise there is a risk of stale data in the cache.
> 
The dma_map_sg() code is already calling dma_cache_maint() to invalidate
the cache lines in the DMA_FROM_DEVICE/DMA_BIDIRECTIONAL direction
cases. And the suggestion was to do something similar in dma_unmap_sg()
case to deal with the speculative prefetching on ARMv7, and Russel has
other postings talking about the details of this in terms of
feasibility/etc.

Furthermore, duplicate MMU mappings in the kernel bring more twists to
this problem as explained in this email chain as well, especially in the
case of DMA-coherent memory (dma_alloc_coherent()).

David