Re: [RFC 0/6] iommu: generic api migration and grouping

[Arnd Bergmann <arnd@arndb.de>]

On Friday 03 June 2011, Ohad Ben-Cohen wrote:
> First stab at iommu consolidation:

Hi Ohad,

Great to see your progress here!
 
> - Migrate OMAP's iommu driver to the generic iommu API. With this in hand,
>   users can now start using the generic iommu layer instead of calling
>   omap-specific iommu API.
> 
>   New code that requires functionality missing from the generic iommu api,
>   will add that functionality in the generic framework (e.g. adding framework
>   awareness to multi page sizes, supported by the underlying hardware, will
>   avoid the otherwise-inevitable code duplication when mapping a memory
>   region).
>
>   OMAP-specific api that is still exposed in the omap iommu driver can
>   now be either moved to the generic iommu framework, or just removed (if not
>   used).
> 
>   This api (and other omap-specific primitives like struct iommu) needs to
>   be omapified (i.e. renamed to include an 'omap_' prefix). At this early
>   point of this patch set this is too much churn though, so I'll do that
>   in the following iteration, after (and if), the general direction is
>   accepted.

Sounds all good.

> - Migrate OMAP's iovmm (virtual memory manager) driver to the generic
>   iommu API. With this in hand, iovmm no longer uses omap-specific api
>   for mapping/unmapping operations. Nevertheless, iovmm is still coupled
>   with omap's iommu even with this change: it assumes omap page sizes,
>   and it uses omap's iommu objects to maintain its internal state.
> 
>   Further generalizing of iovmm strongly depends on our broader plans for
>   providing a generic virtual memory manager and allocation framework
>   (which, as discussed, should be separated from a specific mapper).
> 
>   iovmm has a mainline user: omap3isp, and therefore must be maintained,
>   but new potential users will either have to generalize it, or come up
>   with a different generic framework that will replace it.

I think the future of iovmm is looking not so good. Marek Szyprowski
is working on a generic version of the dma-mapping API (dma_map_ops)
based on the iommu API. As far as I can tell, once we have that in
place, we you can migrate omap3isp from iovmm to dma-mapping and
remove iovmm.

Of course if there are things missing from the dma-mapping API
that are present in iovmm, we should know about them so that we can
extend the dma-mapping API accordingly.

> - Migrate OMAP's iommu mainline user, omap3isp, to the generic API as well
>   (so it doesn't break). As with iovmm, omap3isp still depends on
>   omap's iommu, mainly because iovmm depends on it, but also for
>   iommu context saving and restoring.
> 
>   It is definitely desirable to completely remove omap3isp's dependency
>   on the omap-specific iommu layer, and that will be possible as the
>   required functionality will be added to generic framework.

ok.

> - Create a dedicated iommu drivers folder (and put the base iommu code there)
>
> - Move OMAP's and MSM's iommu drivers to that drivers iommu folder
> 
>   Putting all iommu drivers together will ease finding similarities
>   between different platforms, with the intention of solving problems once,
>   in a generic framework which everyone can use.
> 
>   I've only moved the omap and msm implementations for now, to demonstrate
>   the idea (and support the ARM diet :), but if this is found desirable,
>   we can bring in intel-iommu.c and amd_iommu.c as well.

Yes, very good idea.

	Arnd