Re: [Linaro-mm-sig] [RFC 0/2] DMA-mapping & IOMMU - physically contiguous allocations

[Hiroshi Doyu <hdoyu@nvidia.com>]

Hi Inki/Marek,

On Tue, 16 Oct 2012 02:50:16 +0200
Inki Dae <inki.dae@samsung.com> wrote:

> 2012/10/15 Marek Szyprowski <m.szyprowski@samsung.com>:
> > Hello,
> >
> > Some devices, which have IOMMU, for some use cases might require to
> > allocate a buffers for DMA which is contiguous in physical memory. Such
> > use cases appears for example in DRM subsystem when one wants to improve
> > performance or use secure buffer protection.
> >
> > I would like to ask if adding a new attribute, as proposed in this RFC
> > is a good idea? I feel that it might be an attribute just for a single
> > driver, but I would like to know your opinion. Should we look for other
> > solution?
> >
> 
> In addition, currently we have worked dma-mapping-based iommu support
> for exynos drm driver with this patch set so this patch set has been
> tested with iommu enabled exynos drm driver and worked fine. actually,
> this feature is needed for secure mode such as TrustZone. in case of
> Exynos SoC, memory region for secure mode should be physically
> contiguous and also maybe OMAP but now dma-mapping framework doesn't
> guarantee physically continuous memory allocation so this patch set
> would make it possible.

Agree that the contigous memory allocation is necessary for us too.

In addition to those contiguous/discontiguous page allocation, is
there any way to _import_ anonymous pages allocated by a process to be
used in dma-mapping API later?

I'm considering the following scenario, an user process allocates a
buffer by malloc() in advance, and then it asks some driver to convert
that buffer into IOMMU'able/DMA'able ones later. In this case, pages
are discouguous and even they may not be yet allocated at
malloc()/mmap().

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