Re: [RFC 0/5] ARM: dma-mapping: New dma_map_ops to control IOVA more precisely

[Hiroshi Doyu <hdoyu@nvidia.com>]

Hi Joerg,

On Tue, 18 Sep 2012 14:49:18 +0200
Joerg Roedel <joerg.roedel@amd.com> wrote:

> On Wed, Aug 29, 2012 at 09:55:30AM +0300, Hiroshi Doyu wrote:
> > The following APIs are needed for us to support the legacy Tegra
> > memory manager for devices("NvMap") with *DMA mapping API*.
> 
> Maybe I am not understanding the need completly. Can you elaborate on
> why this is needed for legacy Tegra?

Actually not for legacy but it's necessary to replace homebrewed
in-kernel API(not upstreamed) with the standard ones. The homebrewed
in-kernel API has been used for the abvoe nvmap as its backend. The
homebrewed ones are being replaced with the standard ones, IOMMU-API,
DMA-API and dma-buf, mainly for transition purpose. I found that some
missing features in DMA-API for that. I posted since other SoCs may
have the similiar requirements, (1) To specify IOVA address at
allocation, and (2) To have IOVA allocation and mapping separately.

> > New API:
> > 
> >  ->iova_alloc(): To allocate IOVA area.
> >  ->iova_alloc_at(): To allocate IOVA area at specific address.
> >  ->iova_free():  To free IOVA area.
> > 
> >  ->map_page_at(): To map page at specific IOVA.
> 
> This sounds like a layering violation. The situation today is as
> follows:
> 
> 	DMA-API   : Handle DMA-addresses including an address allocator
> 	IOMMU-API : Full control over DMA address space, no address
> 	            allocator
> 
> So what you want to do add to the DMA-API is already part of the
> IOMMU-API.
>
> Here is my suggestion what you can do instead of extending the DMA-API.
> You can use the IOMMU-API to initialize the device address space with
> any mappings at the IOVAs you need the mappings. In the end you allocate
> another free range in the device address space and use that to satisfy
> DMA-API allocations. Any reason why that could not work?

I guess that it would work. Originally I thought that using DMA-API
and IOMMU-API together in driver might be kind of layering violation
since IOMMU-API itself is used in DMA-API. Only DMA-API used in driver
might be cleaner. Considering that DMA API traditionally handling
*anonymous* {bus,iova} address only, introducing the concept of
specific address in DMA API may not be so encouraged, though.

It would be nice to listen how other SoCs have solved similar needs.

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