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[POWERPC] Improved documentation of device tree 'ranges'

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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>
This commit is contained in:
Stephen Neuendorffer 2007-12-04 12:08:57 +11:00 committed by Paul Mackerras
parent 773f76b160
commit e1fd18656c
1 changed files with 9 additions and 4 deletions
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13
Documentation/powerpc/booting-without-of.txt
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@ -711,13 +711,14 @@ define a bus type with a more complex address format, including things
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 @@ fit in a single 32-bit word. New 32-bit powerpc boards should use a
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
-------------------------------------