On 06/26/2014 02:49 PM, Thierry Reding wrote:

From: Thierry Reding <redacted>

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

+When an "iommus" property is specified in a device tree node, the IOMMU will
+be used for address translation. If a "dma-ranges" property exists in the
+device's parent node it will be ignored. An exception to this rule is if the
+referenced IOMMU is disabled, in which case the "dma-ranges" property of the
+parent shall take effect.

I wonder how useful that paragraph is. The fact that someone disabled a
particular IOMMU's node doesn't necessarily mean that the HW can
actually do that; an IOMMU might always be active in HW and always
translate accesses by some master. In that case, the fallback to
dma-ranges wouldn't correlate with what the HW actually does. Perhaps
all we need is to add a note to that effect here?