config_db Gets in build_phase

The config_db contract at build time

config_db is the only supported way to pass virtual interfaces, config objects, and scalar knobs into components during build_phase. Gets must match prior sets on path, name, and type.

function void build_phase(uvm_phase phase);
  super.build_phase(phase);

  if (!uvm_config_db#(apb_cfg)::get(this, "", "cfg", cfg))
    `uvm_fatal("NOCFG", $sformatf("apb_cfg missing at %s", get_full_name()))

  if (!uvm_config_db#(virtual apb_if)::get(this, "", "vif", vif))
    `uvm_fatal("NOVIF", $sformatf("apb_if missing at %s", get_full_name()))
endfunction

[PHASE][UVM] config_db triple match

SET side:  uvm_config_db#(T)::set(cntxt, inst_path, field_name, value)
GET side:  uvm_config_db#(T)::get(cntxt, inst_path, field_name, value)

All three must align:
  1) type T
  2) inst_path ("" = this component)
  3) field_name string

Key takeaways

• Fail fast with uvm_fatal on missing mandatory config — silent defaults hide integration bugs.

• Set from test/top with broad paths; get locally with inst_path "".

• Virtual interface type in get must match set exactly (including virtual keyword).

Common pitfalls

• Set at "env.agt" but get at "" — path mismatch, get fails.

• Set virtual apb_if but get apb_if — type parameter mismatch.

• Using uvm_config_db::set without context — wrong propagation scope.

Hierarchical set patterns

Env-level components push config down to children using scoped instance paths.

Env pushes, agent pulls

class my_env extends uvm_env;
  env_cfg cfg;
  function void build_phase(uvm_phase phase);
    super.build_phase(phase);
    void'(uvm_config_db#(env_cfg)::get(this, "", "cfg", cfg));
    uvm_config_db#(env_cfg)::set(this, "*", "cfg", cfg);
    agt = my_agent::type_id::create("agt", this);
  endfunction
endclass

[PHASE] path patterns

top → all:     set(null, "*", "vif", vif)
test → env:    set(this, "env", "cfg", cfg)
env → agents:  set(this, "agt*", "cfg", agt_cfg)
agent local:   get(this, "", "cfg", cfg)

Debug helper

function void debug_config_db(string field);
  uvm_config_db#(int)::dump();
  if (!uvm_config_db#(apb_cfg)::get(this, "", field, cfg))
    `uvm_warning("CFG",
      $sformatf("get failed: comp=%s field=%s", get_full_name(), field))
endfunction

• Call dump() once when a get fails — shows all entries and types.

• Log get_full_name() in fatal messages for instant path triage.

• Prefer one cfg object over dozens of scalar config_db fields.

[PHASE][UVM] config triage matrix

get returns 0:
  → type mismatch? check #(T) on both sides
  → path mismatch? compare set inst_path vs get context
  → set too late? setter must run before getter's build_phase
  → wrong field name? typo in "cfg" vs "config"

Common pitfalls

• Storing config only in plusargs without pushing to config_db before build.

• Multiple competing sets with different types on the same field name.

• Using get without checking return value — null cfg used silently.