Part 11 · Senior Prep · Intermediate

config_db Wiring Whiteboard Q&A

Model answers on uvm_config_db set/get paths, virtual interface push from top, cfg object distribution, and debug when get fails.

config_db path questions

Q: Whiteboard top module vif push + agent get

diagram

[INT][SENIOR][UVM] MODEL ANSWER

Q: vif config_db whiteboard?

A:
  MECHANISM:  Top: uvm_config_db#(virtual apb_if)::set(null, "*", "vif", apb_if0);
              Agent build: get(this, "", "vif", vif) — fatal if fail.
  MOTIVATION:  Only elaboration-time top knows interface instances.
  PITFALL:    Typo in field name "vif" vs "apb_vif" — silent null at run_phase.
  EXAMPLE:    set before run_test(); get in agent build_phase before run_phase.

systemverilog

// top.sv
module tb_top;
  apb_if apb_if0(.pclk(clk), ...);
  initial begin
    uvm_config_db#(virtual apb_if)::set(null, "*", "vif", apb_if0);
    run_test("apb_base_test");
  end
endmodule

// agent build_phase
if (!uvm_config_db#(virtual apb_if)::get(this, "", "vif", vif))
  `uvm_fatal("AGT", "virtual interface not set")

Q: set(this, "env.agt", "cfg") vs set(null, "*") — when each?

diagram

[INT][SENIOR][UVM] MODEL ANSWER

Q: Scoped vs wildcard set?

A:
  MECHANISM:  Wildcard null,"*" for vif from top — one interface many agents.
              Scoped set(this,"env*","cfg") from test — avoid collisions.
  MOTIVATION:  Wildcard cfg causes last-write-wins across unrelated agents.
  PITFALL:    set(null,"*","cfg",cfg) for two agents — both get same mutable object.
  EXAMPLE:    Test sets per-agent path: set(this,"env.axi_agt","cfg",axi_cfg).

systemverilog

class base_test extends uvm_test;
  function void build_phase(uvm_phase phase);
    apb_cfg agt_cfg = apb_cfg::type_id::create("agt_cfg");
    agt_cfg.is_active = UVM_ACTIVE;
    uvm_config_db#(apb_cfg)::set(this, "env.agt", "cfg", agt_cfg);
    env = apb_env::type_id::create("env", this);
  endfunction
endclass

Key takeaways

vif: set(null,"*") from top; get in agent build_phase.
cfg: scoped paths from test — clone per agent.
Fatal on vif get fail — catch wiring bugs early.

Common pitfalls

get path does not match set hierarchy string — most common debug.
Setting cfg after child built — get returns false, defaults used silently.

Debug and print

Q: get returns false — debug checklist?

diagram

[INT][SENIOR][UVM] MODEL ANSWER

Q: config_db get fail debug?

A:
  STEPS:      1) uvm_config_db::dump(). 2) Compare set path vs get inst_name.
              3) Check type parameter matches exactly. 4) Set before get phase.
  MOTIVATION:  Silent default after failed get — TB runs with null vif until hang.
  PITFALL:    Assuming wildcard set reached agent with wrong field name.
  EXAMPLE:    set "apb_vif" get "vif" — dump shows mismatch instantly.

Q: Resources vs config_db — interview one-liner

diagram

[INT][SENIOR][UVM] MODEL ANSWER

Q: config_db vs uvm_resource_db?

A:
  MECHANISM:  config_db is scoped to component hierarchy with set/get audit trail;
              resource_db is global name pool — less hierarchy discipline.
  WHEN:       config_db for vif, cfg, virtual sequencer handles; resources for rare global.
  PITFALL:    Mixing both for same field — debug nightmare.
  EXAMPLE:    Stick to config_db in interview whiteboard unless asked resources.

bash

# enable config_db trace at sim launch
simv +UVM_CONFIG_DB_TRACE +UVM_TESTNAME=apb_base_test 2>&1 | tee cfg_trace.log
grep -E "set|get|vif" cfg_trace.log | head -30

Q: Whiteboard test → env → agent config flow

diagram

[INT][SENIOR][UVM] whiteboard config flow

  top:        set(null, "*", "vif", if)
  test:       set(this, "env", "cfg", top_cfg)
  env build:  get cfg; set(this, "agt", "cfg", agt_cfg_clone)
  agt build:  get cfg; get vif

  Draw 4 boxes with arrows — 90-second interview drill

systemverilog

// env distributes clone — whiteboard this function
function void apb_env::build_phase(uvm_phase phase);
  apb_cfg agt_cfg;
  super.build_phase(phase);
  uvm_config_db#(apb_cfg)::get(this, "", "cfg", cfg);
  agt_cfg = cfg.clone();
  agt_cfg.is_active = UVM_ACTIVE;
  uvm_config_db#(apb_cfg)::set(this, "agt", "cfg", agt_cfg);
  agt = apb_agent::type_id::create("agt", this);
endfunction

Key takeaways

get fail debug: dump, path match, type match, phase order.
config_db for TB wiring — resource_db only when asked.
Draw test→env→agent cfg flow — high-frequency whiteboard.

Common pitfalls

Cloning cfg forgotten — agents share mutable state.
config_db set in connect_phase — too late for build_phase get.

Practice this lessonQuestions tagged for this topic in the bank