[eder-matheus]

gf12/ibex

• Remove SETUP_SLACK_MARGIN (180) and HOLD_SLACK_MARGIN (50). The flow
  only times the BC corner, so the margins over-repaired at BC without
  closing WC. Removing them cut area 18%, power 32% and flow runtime
  from 966 s to 307 s.
• Clock period 1020 -> 840 ps.
  Finish: setup WS -34 ps, TNS -9.4 ns (493 endpoints), hold clean, 0 DRC.

gf12/ca53

• Add constraint.sdc, which sources the platform SDC and redefines clk.
  Point SDC_FILE to it. The platform SDC is unchanged.
• Clock period 2000 -> 1550 ps.
  Finish: setup WS -98.6 ps, TNS -0.8 ns (16 endpoints, worst is a
  clock-gate enable path), 0 DRC. Hold WS -25 ps (-15 ps in CI before).

[gemini-code-assist]

Code Review

This pull request updates the SDC file path and constraints for the ca53 design, and adjusts the clock period and slack margins for the ibex design. A critical issue was identified in the new ca53 constraint file, where redefining the clock after sourcing the base SDC can invalidate existing clock-associated constraints in STA tools. It is recommended to set the clock period before sourcing the base SDC.

[gemini-code-assist]

Redefining a clock using create_clock after sourcing the base SDC (ca53_cpu.sdc) can be highly problematic in STA tools like OpenSTA. When create_clock is called on an existing clock port/name, the tool destroys the original clock object and creates a new one. This action typically clears or invalidates any clock-associated constraints (such as set_input_delay, set_output_delay, set_clock_uncertainty, or set_clock_latency) that were defined in ca53_cpu.sdc and bound to the original clock.

If ca53_cpu.sdc supports a parameterized clock period (e.g., by checking if clk_period is already defined), you should set clk_period before sourcing the SDC. If it does not, you may need to manually re-apply the input/output delays and other clock-dependent constraints after redefining the clock to ensure they are not silently lost.

set clk_period 1550
source $::env(PLATFORM_DIR)/ca53_cpu/sdc/ca53_cpu.sdc

[eder-matheus]

@maliberty this was a workaround to change the clock period without touching the platform files. Let me know if I should change directly there.

[openroad-ci]

🔍 QoR check

Metrics reflect the PR merge build — i.e. what will land on the target branch.

Commit bf0089a · Jenkins build #1 · Baseline: build · View build on dashboard

62 design(s) checked — 0 with regression(s), 0 without a comparable baseline.