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dm: core: Add a note about how livetree updates work

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The unflattening algorithm results in a single block of memory being
allocated for the whole tree. When writing new properties, these are
allocated new memory outside that block. When the block is freed, the
allocated properties remain.

Document how this works and the potential memory leak, as well as
mentioning that updating the livetree is actually supported now.

Signed-off-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Simon Glass 2022-07-30 15:52:07 -06:00 committed by Tom Rini
parent 5063ced278
commit 72b338aa2c
2 changed files with 15 additions and 5 deletions
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17
doc/develop/driver-model/livetree.rst
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@ -211,9 +211,18 @@ using it in new code.
Modifying the livetree
----------------------
This is not currently supported. Once implemented it should provide a much
more efficient implementation for modification of the device tree than using
the flat tree.
This is supported in a limited way, with ofnode_write_prop() and related
functions.
The unflattening algorithm results in a single block of memory being
allocated for the whole tree. When writing new properties, these are
allocated new memory outside that block. When the block is freed, the
allocated properties remain. This can result in a memory leak.
The solution to this leak would be to add a flag for properties (and nodes when
support is provided for adding those) that indicates that they should be
freed. Then the tree can be scanned for these 'separately allocated' nodes and
properties before freeing the memory block.
Internal implementation
@ -281,6 +290,6 @@ Live tree support was introduced in U-Boot 2017.07. There is still quite a bit
of work to do to flesh this out:
- tests for all access functions
- support for livetree modification
- more support for livetree modification
- addition of more access functions as needed
- support for livetree in SPL and before relocation (if desired)

3
include/dm/ofnode.h
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@ -1081,7 +1081,8 @@ int ofnode_device_is_compatible(ofnode node, const char *compat);
* ofnode_write_prop() - Set a property of a ofnode
*
* Note that the value passed to the function is *not* allocated by the
* function itself, but must be allocated by the caller if necessary.
* function itself, but must be allocated by the caller if necessary. However
* it does allocate memory for the property struct and name.
*
* @node: The node for whose property should be set
* @propname: The name of the property to set