Re: [PATCH] iommu: omap_iovmm: support non page-aligned buffers in iommu_vmap

["Roedel, Joerg" <Joerg.Roedel@amd.com>]

On Wed, Aug 31, 2011 at 12:56:26PM -0400, Laurent Pinchart wrote:
> True, but if we implement address rounding transparently in the IOMMU layer 
> Ohad's concern can be valid, depending on whether the device is trusted. If we 
> decide to push address rounding to the drivers that decision can be made on a 
> per-device basis. However, drivers are usually not aware of what granularity 
> the IOMMU works on, so that wouldn't be straightforward and clean.

Drivers usually just use the DMA-API, so they can not assume at all that
any protection happens. The problem also can't be really solved without
changing/breaking the DMA-API. It is defined to work on byte granularity
while the hardware only works with pages.

The only way to work around this it to implement the driver in a way so
that they only map page-aligned buffers where the size is also a
multiple of a page-size.

In this case you can assume the page-size you are working on because you
already assume in the driver that an iommu is in use.

	Joerg

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