RFC: devicetree: use stable path-based hashes in DT object names #77799
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RFC
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Add a new "hash" attribute to all Devicetree EDT nodes. The hash is calculated on the full path of the node; this means that its value remains stable across rebuilds. The hash is checked for uniqueness among nodes in the same EDT. This computed token is then added to `devicetree_generated.h` and made accessible to Zephyr code via a new DT_HASH(node_id) macro. Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
Add a new set of macros that allow customizing the symbol name when exporting symbols. This is useful when the symbol name that extensions need to look up is different from the identifier used in the base image. Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
This new option allows to generate device names from the hash of the devicetree node path, instead of the device's ordinal number. The resulting names are stable across rebuilds. Add a new test case to test this new option. Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
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Problem statement
Currently Zephyr uses a simple but effective sequential numbering scheme for the various C identifiers associated with Devicetree nodes: each node is numbered incrementally and results in a
dts_ord_nnn name and C-level identifier. Since these numbers are allocated at every build, any node added or removed in the DT will result in a different mapping between identifiers and devices.This is a problem for extensions that need to access devices and DT nodes, as these names would not be constant and the extension would have to be rebuilt along with the main application on any Devicetree change, even those not directly affecting the extension.
Proposed solution
This PR proposes to add a Kconfig option,
CONFIG_LLEXT_EXPORT_DEV_IDS_BY_HASH, to use a stable hash computed at build time from the full DT path of the node for identifiers exported to exensions. The resulting symbol names are of the formdts_hash_xxx with xxx being the full hash of the DT node (with SHA256, a 64-char hex string). The build fails if a hash collision is detected, but with 256-bit hashes of text strings it should be an exceedingly unlikely event.A new Twister test has been added to check this new functionality and ensure stability of the generated hashes.
Conclusion
Stable DT names, or some way to have a stable look-up, is going to be important for LLEXT moving forward. Looking for feedback on both the proposed solution and implementation!