Re: [RFC PATCH v3 7/7] arm: dma-mapping: plumb our iommu mapping ops into arch_setup_dma_ops

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Mon, Sep 22, 2014 at 06:43:37PM +0100, Will Deacon wrote:
> Hi Thierry,
> 
> On Mon, Sep 22, 2014 at 10:19:35AM +0100, Thierry Reding wrote:
> > On Fri, Sep 12, 2014 at 05:34:55PM +0100, Will Deacon wrote:
> > [...]
> > > +static bool arm_setup_iommu_dma_ops(struct device *dev, u64 dma_base, u64 size)
> > > +{
> > > +	struct dma_iommu_mapping *mapping;
> > > +
> > > +	mapping = arm_iommu_create_mapping(dev->bus, dma_base, size);
> > 
> > If I understand correctly this will be called for each device that has
> > an IOMMU master interface and will end up creating a new mapping for
> > each of the devices. Each of these mappings will translate to a domain
> > in the IOMMU API, which in turn is a separate address space.
> 
> Correct, although that's largely because I've bolted on the existing ARM
> IOMMU code.
> 
> > How do you envision to support use-cases where a set of devices need to
> > share a single domain? This is needed for example in DRM where SoCs
> > often have a set of hardware blocks (each with its own master interface)
> > that compose the display device. On Tegra for example there are two
> > display controllers that need access to the same IOVA domain so that
> > they can scan out framebuffers.
> 
> Yup. In this case, the iommu_dma_mapping passed to arch_setup_dma_ops
> contains a domain and an allocator for each IOMMU instance in the system.
> It would then be up to the architecture how it makes use of those, but
> the most obvious thing to do would be to attach devices mastering through
> an IOMMU instance to that per-instance domain.
> 
> The other use-case is isolation (one domain per device), which I guess
> matches what the ARM code is doing at the moment.

I think there are two cases here. You can have a composite device that
wants to manage a single domain (using its own allocator) for a set of
hardware devices. At the same time a set of devices (think 2D and 3D
engines) could want to use a multiple domains for process separation.
In that case I'd expect a logical DRM device to allocate one domain per
process and then associate the 2D and 3D engines with that same domain
on process switch.

What I proposed a while back was to leave it up to the IOMMU driver to
choose an allocator for the device. Or rather, choose whether to use a
custom allocator or the DMA/IOMMU integration allocator. The way this
worked was to keep a list of devices in the IOMMU driver. Devices in
this list would be added to domain reserved for DMA/IOMMU integration.
Those would typically be devices such as SD/MMC, audio, ... devices that
are in-kernel and need no per-process separation. By default devices
wouldn't be added to a domain, so devices forming a composite DRM device
would be able to manage their own domain.

Thierry

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