RFC: extend iommu-map binding to support #iommu-cells > 1

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For context, please see the thread:
https://www.spinics.net/lists/arm-kernel/msg539066.html

The existing iommu-map binding did not account for the situation where
#iommu-cells == 2, as permitted in the ARM SMMU binding.  The 2nd cell
of the IOMMU specifier being the SMR mask.  The existing binding defines
the mapping as:
   Any RID r in the interval [rid-base, rid-base + length) is associated with
   the listed IOMMU, with the iommu-specifier (r - rid-base + iommu-base).

...and that does not work if iommu-base is 2 cells, the second being the
SMR mask.

While this can be worked around by always having length=1, it seems we
can get this cleaned up by updating the binding definition for iommu-map.

See patch below.  Thoughts?

Thanks,
Stuart

-------------------------------------------------------------------------

diff --git a/Documentation/devicetree/bindings/pci/pci-iommu.txt b/Documentation/devicetree/bindings/pci/pci-iommu.txt
index 56c8296..e81b461 100644
--- a/Documentation/devicetree/bindings/pci/pci-iommu.txt
+++ b/Documentation/devicetree/bindings/pci/pci-iommu.txt
@@ -38,8 +38,20 @@ Optional properties
   The property is an arbitrary number of tuples of
   (rid-base,iommu,iommu-base,length).

-  Any RID r in the interval [rid-base, rid-base + length) is associated with
-  the listed IOMMU, with the iommu-specifier (r - rid-base + iommu-base).
+  If the associated IOMMU has an #iommu-cells value of 1, any RID r in the
+  interval [rid-base, rid-base + length) is associated with the listed IOMMU,
+  with the iommu-specifier (r - rid-base + iommu-base).
+
+  ARM SMMU Note:
+    The ARM SMMU binding permits an #iommu-cells value of 2 and in this
+    case defines an IOMMU specifier to be: (stream-id,smr-mask)
+
+    In an iommu-map this means the iommu-base consists of 2 cells:
+        (rid-base,iommu,[stream-id,smr-mask],length).
+
+    In this case the RID to IOMMU specifier mapping is defined to be:
+    any RID r in the interval [rid-base, rid-base + length) is associated
+    with the listed IOMMU, with the iommu-specifier (r - rid-base + stream-id).

 - iommu-map-mask: A mask to be applied to each Requester ID prior to being
   mapped to an iommu-specifier per the iommu-map property.



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