On 09/03/15 20:09, Robin Murphy wrote:
[...]
I think ideally you'd call dma_map_page when you first create the page
table, dma_sync_single_for_device on any update, and dma_unmap_page when you
tear it down, and you'd also use the appropriate DMA addresses everywhere
instead of physical addresses.
No.
dma_map_page() ownership changes CPU->DMA
dma_sync_single_for_cpu() ownership changes DMA->CPU
dma_sync_single_for_device() ownership changes CPU->DMA
dma_unmap_page() ownership changes DMA->CPU
It's invalid to miss out the pairing that give those ownership changes.
Thanks for the clarification - the wording in DMA-API.txt rather implies
that in the DMA_TO_DEVICE case you only have to sync the updated data
/after/ writing it. For the sake of purely getting pages flushed, would
it be more reasonable then to call dma_map_single() followed immediately
by dma_unmap_single_attrs() with DMA_ATTR_SKIP_CPU_SYNC? Since we know
the IOMMU can never write back to memory (ones that can are a different
issue) it would be nice to be able to skip the extra invalidations
somehow, without too heinously abusing the API.
Scratch that, I'm being a massive idiot (again). Of course the actual
invalidations will only happen if they need to, based on the DMA
direction. The overhead of dma_sync_*_for_cpu() and dma_unmap() is then
only a handful of function calls, which is probably an acceptable price
to pay for making sure things work as correctly as possible.
Robin.
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