[POWERPC] Improved documentation of device tree 'ranges'
This clarifies how 'ranges' properties are used, and in particular,
how to get an identity mapping.
Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/Documentation/powerpc/booting-without-of.txt b/Documentation/powerpc/booting-without-of.txt
index e9a3cb1..7327f37 100644
--- a/Documentation/powerpc/booting-without-of.txt
+++ b/Documentation/powerpc/booting-without-of.txt
@@ -711,13 +711,14 @@
like address space bits, you'll have to add a bus translator to the
prom_parse.c file of the recent kernels for your bus type.
-The "reg" property only defines addresses and sizes (if #size-cells
-is non-0) within a given bus. In order to translate addresses upward
+The "reg" property only defines addresses and sizes (if #size-cells is
+non-0) within a given bus. In order to translate addresses upward
(that is into parent bus addresses, and possibly into CPU physical
addresses), all busses must contain a "ranges" property. If the
"ranges" property is missing at a given level, it's assumed that
-translation isn't possible. The format of the "ranges" property for a
-bus is a list of:
+translation isn't possible, i.e., the registers are not visible on the
+parent bus. The format of the "ranges" property for a bus is a list
+of:
bus address, parent bus address, size
@@ -735,6 +736,10 @@
1/1 format, unless the processor supports physical addresses greater
than 32-bits, in which case a 2/1 format is recommended.
+Alternatively, the "ranges" property may be empty, indicating that the
+registers are visible on the parent bus using an identity mapping
+translation. In other words, the parent bus address space is the same
+as the child bus address space.
2) Note about "compatible" properties
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