Re: [PATCH 2/7] ARM: Samsung: update/rewrite Samsung SYSMMU (IOMMU) driver

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 19 April 2011, Marek Szyprowski wrote:

> > 
> > 1. change the runtime_pm subsystem to allow it to ignore some devices
> > in an easy way.
> > 
> > 2. change the device layout if the sysmmu. If the iommu device is
> > a child of the device that it is responsible for, I guess you don't
> > have this problem.
> > 
> > 3. Not represent the iommu as a device at all, just as a property
> > of another device.
> 
> Ok, we will handle this issue somehow. I consider this a minor issue and I
> would like to focus on the IOMMU/dma-mapping APIs first.

Yes, agreed.

> > That is a limitation of the current implementation. We might want to
> > change that anyway, e.g. to handle the mali IOMMU along with yours.
> > I believe the reason for allowing only one IOMMU type so far has been
> > that nobody required more than one. As I mentioned, the IOMMU API is
> > rather new and has not been ported to much variety of hardware, unlike
> > the dma-mapping API, which does support multiple different IOMMUs
> > in a single system.
> 
> Ok. I understand. IOMMU API is quite nice abstraction of the IOMMU chip.
> dma-mapping API is something much more complex that creates the actual
> mapping for various sets of the devices. IMHO the right direction will
> be to create dma-mapping implementation that will be just a client of
> the IOMMU API. What's your opinion?
 
Sounds good. I think we should put it into a new drivers/iommu, along
with your specific iommu implementation, and then we can convert the
existing ones over to use that.

Note that this also requires using dma-mapping-common.h, which we currently
don't on ARM.

> > The domain really reflects the user, not the device here, which makes more
> > sense if you think of virtual machines than of multimedia devices.
> >
> > I would suggest that you just use a single iommu_domain globally for
> > all in-kernel users.
> 
> There are cases where having a separate mapping for each device makes sense.
> It definitely increases the security and helps to find some bugs in
> the drivers.
> 
> Getting back to our video codec - it has 2 IOMMU controllers. The codec
> hardware is able to address only 256MiB of space. Do you have an idea how
> this can be handled with dma-mapping API? The only idea that comes to my
> mind is to provide a second, fake 'struct device' and use it for allocations
> for the second IOMMU controller.

Good question. 

How do you even decide which controller to use from the driver?
I would need to understand better what you are trying to do to
give a good recommendation.

	Arnd