docs(upf): document known limitations in UPF module

[shuvro-git]

Summary

This PR documents the known limitations of the UPF module
in the README.md file, which previously had an empty
Limitations section.

What was verified

All limitations listed were verified directly from the
codebase using the following methods:

• grep searches returning zero output confirming absent commands
• Runtime testing confirming invalid command name errors
• Function signature analysis confirming missing parameters
• find command confirming no validation framework exists

Limitations documented

Unsupported UPF standard commands

• upf_version
• create_supply_net
• create_supply_port
• connect_supply_net
• add_power_state
• create_pst
• add_pst_state

Partially supported commands

• set_isolation — missing options: -isolation_supply_set,
  -source, -sink
• set_level_shifter — implemented but option descriptions
  are empty in README
• set_level_shifter_cell — all options undocumented (KIV)

Missing infrastructure

• No validation framework for UPF rule compliance
• No power state table verification
• No cross-domain signal integrity checks

Related

This work is preparatory research for a GSoC 2026 proposal
to implement full UPF standard support in OpenROAD.

[gemini-code-assist]

Code Review

This PR adds a comprehensive 'Limitations' section to the UPF module's README.md, which was previously empty. It also includes minor formatting fixes by removing trailing whitespace. My review focuses on improving the clarity and professionalism of the new documentation section by suggesting removal of implementation-specific verification notes and refining the language to be more user-friendly. The changes are a valuable addition to the project's documentation.

[gemini-code-assist]

Good work on documenting these limitations. For better readability in the final README.md, I suggest removing the parenthetical verification details (which are already well-covered in the PR description) and making the language more formal and consistent. Here's a suggested revision for the whole section:

### Unsupported UPF standard commands The following standard UPF commands are not implemented: - `upf_version` — UPF version declaration - `create_supply_net` — named supply net creation - `create_supply_port` — supply port creation - `connect_supply_net` — supply net connectivity - `add_power_state` — power state definition - `create_pst` — power state table creation - `add_pst_state` — power state table entries ### Partially supported commands - `set_isolation` — The following options are not supported: `-isolation_supply_set`, `-source`, `-sink`. - `set_level_shifter` — While the command is implemented, its option descriptions are not yet documented in this README. - `set_level_shifter_cell` — The options (`-level_shifter`, `-cell_name`, `-input_port`, `-output_port`) are not documented. ### Missing infrastructure - No validation framework for UPF rule compliance. - No power state table verification. - No cross-domain signal integrity checks.