[PATCH 05/13] iommu/io-pgtable-arm: Allow appropriate DMA API use

[Russell King - ARM Linux]

On Tue, Aug 04, 2015 at 03:47:13PM +0100, Robin Murphy wrote:
> Hi Laurent,
> 
> [ +RMK, as his patch is indirectly involved here ]
> 
> On 04/08/15 14:16, Laurent Pinchart wrote:
> >This is what I believe to be an API abuse. The dma_sync_single_for_device()
> >API is meant to pass ownership of a buffer to the device. Unless I'm mistaken,
> >once that's done the CPU isn't allowed to touch the buffer anymore until
> >dma_sync_single_for_cpu() is called to get ownership of the buffer back.

That's what I thought up until recently, but it's not strictly true - see
Documentation/DMA-API.txt which Robin quoted.

> [3]:Yes, there may generally be exceptions to that, but not in the context
> of this code. Unless the Renesas IPMMU does something I don't know about?

If an IOMMU does write to its page tables, then the only way to handle
those is using DMA-coherent memory, either via a coherent mapping or
allocated via dma_alloc_coherent().  The streaming DMA API is wholely
unsuitable for any mapping where both the CPU and DMA device both want
to simultaneously alter the contained data.

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