[RFC] Describing arbitrary bus mastering relationships in DT

[arnd@arndb.de (Arnd Bergmann)]

On Monday 12 May 2014 10:19:16 Stephen Warren wrote:
> On 05/09/2014 04:56 AM, Dave Martin wrote:
> > On Fri, May 02, 2014 at 09:06:43PM +0200, Arnd Bergmann wrote:
> >> On Friday 02 May 2014 12:50:17 Stephen Warren wrote:
> ...
> >>> Now, perhaps there are devices which themselves control whether
> >>> transactions are sent to the IOMMU or direct to RAM, but I'm not
> >>> familiar with them. Is the GPU in that category, since it has its own
> >>> GMMU, albeit chained into the SMMU IIRC?
> >>
> >> Devices with a built-in IOMMU such as most GPUs are also easy enough
> >> to handle: There is no reason to actually show the IOMMU in DT and
> >> we can just treat the GPU as a black box.
> > 
> > It's impossible for such a built-in IOMMU to be shared with other
> > devices, so that's probably reasonable.
> 
> I don't believe that's true.
> 
> For example, on Tegra, the CPU (and likely anything that can bus-master
> the relevant bus) can send transactions into the GPU, which can then
> turn them around towards RAM, and those likely then go through the MMU
> inside the GPU.
> 
> IIRC, the current Nouveau support for Tegra even makes use of that
> feature, although I think that's a temporary thing that we're hoping to
> get rid of once the Tegra support in Nouveau gets more mature.

But the important point here is that you wouldn't use the dma-mapping
API to manage this. First of all, the CPU is special anyway, but also
if you do a device-to-device DMA into the GPU address space and that
ends up being redirected to memory through the IOMMU, you still wouldn't
manage the I/O page tables through the interfaces of the device doing the
DMA, but through some private interface of the GPU.

	Arnd