On Thu, May 29, 2014 at 09:52:22AM -0600, Stephen Warren wrote: > On 05/23/2014 02:36 PM, Thierry Reding wrote: > > From: Thierry Reding <treding@xxxxxxxxxx> > > > > This commit introduces a generic device tree binding for IOMMU devices. > > Only a very minimal subset is described here, but it is enough to cover > > the requirements of both the Exynos System MMU and Tegra SMMU as > > discussed here: > > > > https://lkml.org/lkml/2014/4/27/346 > > > > Signed-off-by: Thierry Reding <treding@xxxxxxxxxx> > > --- > > Apologies for the noise, but apparently I mistyped one of the email > > addresses, should be fixed now. > > > > Changes in v2: > > - add notes about "dma-ranges" property (drop note from commit message) > > - document priorities of "iommus" property vs. "dma-ranges" property > > - drop #iommu-cells in favour of #address-cells and #size-cells > > I think this is a mistake. address-cells/size-cells are for transactions > flowing down the bus (from the CPU to date). Describing a connection > from a device to an IOMMU is something completely different, and should > therefore simply use an iommu-cells property to describe any necessary > information. If we start re-using properties for different things in > different contexts, how is anyone going to know what they mean, and how > will conflicts be resolved. For example, what if there's a single HW > module that both acts as a regular register bus with children (where > address-cells/size-cells defines how transactions reach the children > from the parent), and is also an IOMMU (where according to this binding > proposal, address-cells/size-cells represent some aspect of the IOMMU > feature). Using different properties for different things is the only > sane way to keep different concepts separate. Another alternative would > be to represent the single HW module as separate nodes in DT, but I > think that will only make our lives harder, and where I've done that in > the past, I've regretted it. There was some back-and-forth on this topic and the latest concensus when I wrote the second version was that #address-cells and #size-cells were to be used. But there was some bore back-and-forth after that, and it seems like Arnd no longer thinks that using #address-cells and #size-cells is a good idea either[0]. Arnd, can you take another look at this binding and see if there's anything else missing? If not I'll go through the document again and update all #address-cells/#size-cells references with #iommu-cells as appropriate and submit v3. Thanks, Thierry [0]: https://lkml.org/lkml/2014/5/20/609
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