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

From: Imre Deak
Date: Thu Sep 03 2009 - 03:37:29 EST

On Wed, Sep 02, 2009 at 05:10:44PM +0200, Deak Imre (Nokia-D/Helsinki) wrote:
> On Tue, Sep 01, 2009 at 03:43:48PM +0200, ext Laurent Pinchart wrote:
> > [...]
> > I might be missing something obvious, but I fail to see how VIVT caches could
> > work at all with multiple mappings. If a kernel-allocated buffer is DMA'ed to,
> > we certainly want to invalidate all cache lines that store buffer data. As the
> > cache doesn't care about physical addresses we thus need to invalidate all
> > virtual mappings for the buffer. If the buffer is mmap'ed in userspace I don't
> > see how that would be done.
> To my understanding buffers returned by dma_alloc_*, kmalloc, vmalloc
> are ok:
> The cache lines for direct mapping are flushed in dma_alloc_* and
> vmalloc. After this you are not supposed to access the buffers
> through the direct mapping until you're done with the DMA.
> For kmalloc you use the direct mapping in the first place, so the
> flush in dma_map_* will be enough.
> For user mappings I think you'd have to do an additional flush for
> the direct mapping, while the user mapping is flushed in dma_map_*.

Based on the the discussion so far this is my understanding on how
zero-copy DMA is possible on ARM. Could you please confirm / correct
these? :

- user space passes an arbitrary buffer:
- get_user_pages(user address range)
- DMA(user address range)
- user space reads from the buffer

- not supported according to Russell
- unhandled faults for cache ops on not-present PTEs, but patch
from Laurent fixes this

- mmap a kernel buffer to user space with cacheable mapping:
- user space writes to the buffer
- flush cache(user address range)
- DMA(kernel buffer)
- user space reads from the buffer

The additional flush cache is needed for VIVT/aliasing VIPT.
Instead of the flush cache:
- the mapping can be done with writethrough, non-writeallocate or
non-cacheable mapping, or
- for aliasing VIPT a non-aliasing user address is picked

DMA(address range) is:
- dma_map_*(address range)
- perform DMA to/from address range
- dma_unmap_*(address range)


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