Re: [PATCH 0/5] OMAP IOMMU fixes and IOMMU architecture questions

[Joerg Roedel <joro@8bytes.org>]

Hi Laurent,

On Tue, Apr 08, 2014 at 02:50:38PM +0200, Laurent Pinchart wrote:
> I agree with you, the two levels are already present, but there's still rough 
> edges that we need to soften. 
> 
> The ARM DMA API implementation requires someone to create the VA space mapping 
> by calling arm_iommu_create_mapping() and attach devices to mappings by 
> calling arm_iommu_attach_device().

Who is "someone" in this case?

> This must only be performed for devices that use the DMA API, devices
> that manage their IOMMU directly will call to the IOMMU API directly.
> 
> One obvious possibility is to call those functions in the IOMMU bus masters 
> device drivers. This is pretty easy, but makes the drivers IOMMU-aware (which 
> I'd like to avoid) and doesn't allow multiple bus masters to share a VA space 
> mapping. Another possibility is to place the calls in the IOMMU drivers. This 
> has the advantage of removing any IOMMU knowledge from bus masters device 
> drivers and allowing multiple bus masters per IOMMU, but makes IOMMU 
> management by the DMA API unconditional.

Why does that make IOMMU management by the DMA API unconditional? On x86
it works this way: The IOMMU drivers create DMA-API mappings for the
devices by default. So any driver can use the DMA-API just
out-of-the-box without being aware of an IOMMU. If the driver wants to
deal with the IOMMU directly, it creates its own iommu-domain and
attaches the device to it. The device uses the drivers domain then and
not the DMA-API domain setup by the IOMMU driver. On iommu-detach the
device is assigned back to its DMA-API domain.

> > The way this works on x86 is that a device driver can use the DMA-API per
> > default. If it wants to use the IOMMU-API it has to allocate a domain and
> > add the device it handles to this domain (it can't use DMA-API anymore
> > then).
> 
> Are drivers supposed to reimplement the DMA API internally in that case?

Usually not, but possible. Device drivers use the IOMMU-API when they
want to control the io-addresses themselves. This is the case for VFIO
for example, where a given range from a process address space is mapped
into a device address space. In those cases the device driver usally
does not need to implement its own address allocator.


Regards,

	Joerg