Hi Thierry,

On Thu, Jun 26, 2014 at 09:49:42PM +0100, 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

Signed-off-by: Thierry Reding <redacted>

[...]

+Required properties:
+--------------------
+- #iommu-cells: The number of cells in an IOMMU specifier needed to encode an
+  address.
+
+Typical values for the above include:
+- #iommu-cells = <0>: Single master IOMMU devices are not configurable and
+  therefore no additional information needs to be encoded in the specifier.
+  This may also apply to multiple master IOMMU devices that do not allow the
+  association of masters to be configured.

A multiple-master capable IOMMU could be built with a single master, but
we'd still need #iommu-cells > 0 here. I appreciate this is just an example,
but the wording sounds like it's enforced.

+- #iommu-cells = <1>: Multiple master IOMMU devices may need to be configured
+  in order to enable translation for a given master. In such cases the single
+  address cell corresponds to the master device's ID.

Again, we will definitely need more than one cell in this case, as I fully
expect multiple StreamIDs for each master (e.g. Qualcomm mentioned on the
list the other day that they have a master emitting 43 unique IDs).

Anyway, the actual binding looks great, I just don't want people to think
they need to do something different because they don't fit your example
use-cases.

+Multiple-master IOMMU:
+----------------------
+
+	iommu {
+		/* the specifier represents the ID of the master */
+		#iommu-cells = <1>;
+	};
+
+	master {
+		/* device has master ID 42 in the IOMMU */
+		iommus = <&/iommu 42>;
+	};
+
+Multiple-master IOMMU with configurable DMA window:
+---------------------------------------------------
+
+	/ {
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		iommu {
+			/* master ID, address and length of DMA window */
+			#iommu-cells = <4>;
+		};
+
+		master {
+			/* master ID 42, 4 GiB DMA window starting at 0 */
+			iommus = <&/iommu  42  0  0x1 0x0>;
+		};
+	};

Could you also please include an example of a master with multiple IDs?

Will