OpenTitan binary and test rules

The bazel rules defined in //rules/opentitan/... define how to build and test C and C++ binaries on the OpenTitan platform. The test rules also permit dispatching test binaries/images produced by other languages or rule sets.

Execution Environments

A core concept of the new rule structure is that of an execution environment (or exec_env). An execution environment is a unique hardware and software configuration. Our exec_envs include the sim_verilator, sim_dv and fpga_cw310 environments (among others). In addition to simply referring to the simulation or emulation platforms, the exec_envs also include the ROM, OTP configuration and linker maps for a given environment. For example, the fpga_cw310_test_rom environment is used to execute test programs on the CW310 board with the test program linked in flash memory at the ROM_EXT stage and executed after the test_rom. There may be several additional environments for the fpga_cw310 that execute the test program under different conditions (ie: under the real ROM or under the ROM_EXT).

An exec_env defines everything about building and dispatching testing binaries for that environment. In addition to the resources needed to build and dispatch binaries (e.g. the test_rom, linker scripts, signing keys, etc), the exec_env also supplies some starlark functions to help the various environments perform the build and test actions.

  • Building
    • An exec_env-specific provider, such as Cw310BinaryInfo or SimVerilatorBinaryInfo.
    • An exec_env-specific transform function that converts binaries into the environment’s required form (e.g. vmem files for DV or Verilator).
  • Testing
    • An exec_env-specific test_dispatch function that configures the test environment and dispatches the binary to that environment.

The exec_envs are provided to the opentitan_binary and opentitan_test rules in order to allow them to perform their function.

The opentitan_binary rule

The opentitan_binary rule is very similar to bazel’s built-in cc_binary rule: it accepts a list of sources, dependencies and other compiler configuration parameters and produces a linked binary. Unlike the built-in cc_binary, the opentitan_binary rule transitions the C toolchain to the opentitan platform and accepts a list of exec_envs describing for which targets to build binaries.

Example: building a binary for 3 exec_envs

    name = "hello_world",
    srcs = ["hello_world.c"],
    exec_env = [
    deps = [

The opentitan_binary rule builds and links the binary for each provided exec_env. The rule emits several providers which are meant to be consumed by downstream rules. These include:

  • A per-exec_env provider (ie: Cw310BinaryInfo).
  • A DefaultInfo provider with the default binary form as specified by each exec_env.
  • An OutputGroupInfo containing a field for every file produced in the build. These fields are of the form {exec_env_name}_{artifact_type}. For example, fpga_cw310_binary, fpga_cw310_mapfile, fpga_cw310_disassembly, etc.

Because the opentitan_binary rule emits several providers, any rule that accepts the outputs of opentitan_binary must be prepared to deal with the providers it emits. You can use a filegroup to capture individual files from the OutputGroupInfo provider should you need to provide a file to a rule that does not understand the per-exec_env providers.

The opentitan_test rule

The opentitan_test rule is actually a macro exported by defs.bzl. There is a real _opentitan_test rule in cc.bzl, but users are expected to use the macro. The reason for using a macro is to instantiate an individual test rule per exec_env so that tests can be filtered by rule name or test tags.

Like the binary rule, the test rule builds and links a binary, but then dispatches that binary according to the exec_env-specific dispatch function and the test parameters in the exec_env.

Example: instantiating a test for 3 exec_envs

    name = "uart_smoketest",
    srcs = ["uart_smoketest.c"],
    cw310 = cw310_params(timeout = "long"),
    exec_env = {
        "//hw/top_earlgrey:fpga_cw310_test_rom": None,
        "//hw/top_earlgrey:sim_dv": None,
        "//hw/top_earlgrey:sim_verilator": None,
    deps = [

The test macro will instatiate individual tests for each exec_env and name the bazel targets with the “basename” of the exec_env. In the example above, the tests will be:

  • uart_smoketest_fpga_cw310_test_rom
  • uart_smoketest_sim_dv
  • uart_smoketest_sim_verilator

While the binary rule accepts a list of exec_envs, the test macro requires a dictionary. This is because tests can specify additional parameters via a params block (ie: cw310_params(...)). A dict value of None instructs the macro to fetch the params block from the default-named argument for the exec_env (ie: the fpga_cw310 exec_env gets it’s param block from the cw310 parameter). Any other dict str-value instructs the macro to find a parameter with the same name as the value. This allows us to dispatch a test to multiple fpga_cw310_... exec_envs with different parameter blocks (as needed).