Writing Tests

This describes how to write tests in U-Boot and describes the possible options.

Test types

There are two basic types of test in U-Boot:

  • Python tests, in test/py/tests

  • C tests, in test/ and its subdirectories

(there are also UEFI tests in lib/efi_selftest/ not considered here.)

Python tests talk to U-Boot via the command line. They support both sandbox and real hardware. They typically do not require building test code into U-Boot itself. They are fairly slow to run, due to the command-line interface and there being two separate processes. Python tests are fairly easy to write. They can be a little tricky to debug sometimes due to the voluminous output of pytest.

C tests are written directly in U-Boot. While they can be used on boards, they are more commonly used with sandbox, as they obviously add to U-Boot code size. C tests are easy to write so long as the required facilities exist. Where they do not it can involve refactoring or adding new features to sandbox. They are fast to run and easy to debug.

Regardless of which test type is used, all tests are collected and run by the pytest framework, so there is typically no need to run them separately. This means that C tests can be used when it makes sense, and Python tests when it doesn’t.

This table shows how to decide whether to write a C or Python test:


C test

Python test

Fast to run?


No (two separate processes)

Easy to write?

Yes, if required test features exist in sandbox or the target system


Needs code in U-Boot?


No, provided the test can be executed and the result determined using the command line

Easy to debug?


No, since access to the U-Boot state is not available and the amount of output can sometimes require a bit of digging

Can use gdb?

Yes, directly

Yes, with –gdbserver

Can run on boards?

Some can, but only if compiled in and not dependent on sandboxau


Python or C

Typically in U-Boot we encourage C test using sandbox for all features. This allows fast testing, easy development and allows contributors to make changes without needing dozens of boards to test with.

When a test requires setup or interaction with the running host (such as to generate images and then running U-Boot to check that they can be loaded), or cannot be run on sandbox, Python tests should be used. These should typically NOT rely on running with sandbox, but instead should function correctly on any board supported by U-Boot.

Mixing Python and C

The best of both worlds is sometimes to have a Python test set things up and perform some operations, with a ‘checker’ C unit test doing the checks afterwards. This can be achieved with these steps:

  • Add the UT_TESTF_MANUAL flag to the checker test so that the ut command does not run it by default

  • Add a _norun suffix to the name so that pytest knows to skip it too

In your Python test use the -f flag to the ut command to force the checker test to run it, e.g.:

# Do the Python part
host load ...
bootm ...

# Run the checker to make sure that everything worked
ut -f bootstd vbe_test_fixup_norun

Note that apart from the UT_TESTF_MANUAL flag, the code in a ‘manual’ C test is just like any other C test. It still uses ut_assert…() and other such constructs, in this case to check that the expected things happened in the Python test.

How slow are Python tests?

Under the hood, when running on sandbox, Python tests work by starting a sandbox test and connecting to it via a pipe. Each interaction with the U-Boot process requires at least a context switch to handle the pipe interaction. The test sends a command to U-Boot, which then reacts and shows some output, then the test sees that and continues. Of course on real hardware, communications delays (e.g. with a serial console) make this slower.

For comparison, consider a test that checks the ‘md’ (memory dump). All times below are approximate, as measured on an AMD 2950X system. Here is is the test in Python:

def test_md(u_boot_console):
    """Test that md reads memory as expected, and that memory can be modified
    using the mw command."""

    ram_base = u_boot_utils.find_ram_base(u_boot_console)
    addr = '%08x' % ram_base
    val = 'a5f09876'
    expected_response = addr + ': ' + val
    u_boot_console.run_command('mw ' + addr + ' 0 10')
    response = u_boot_console.run_command('md ' + addr + ' 10')
    assert(not (expected_response in response))
    u_boot_console.run_command('mw ' + addr + ' ' + val)
    response = u_boot_console.run_command('md ' + addr + ' 10')
    assert(expected_response in response)

This runs a few commands and checks the output. Note that it runs a command, waits for the response and then checks it agains what is expected. If run by itself it takes around 800ms, including test collection. For 1000 runs it takes 19 seconds, or 19ms per run. Of course 1000 runs it not that useful since we only want to run it once.

There is no exactly equivalent C test, but here is a similar one that tests ‘ms’ (memory search):

/* Test 'ms' command with bytes */
static int mem_test_ms_b(struct unit_test_state *uts)
   u8 *buf;

   buf = map_sysmem(0, BUF_SIZE + 1);
   memset(buf, '\0', BUF_SIZE);
   buf[0x0] = 0x12;
   buf[0x31] = 0x12;
   buf[0xff] = 0x12;
   buf[0x100] = 0x12;
   run_command("ms.b 1 ff 12", 0);
   ut_assert_nextline("00000030: 00 12 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................");
   ut_assert_nextline("000000f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 12    ................");
   ut_assert_nextline("2 matches");

   ut_asserteq(2, env_get_hex("memmatches", 0));
   ut_asserteq(0xff, env_get_hex("memaddr", 0));
   ut_asserteq(0xfe, env_get_hex("mempos", 0));


   return 0;

This runs the command directly in U-Boot, then checks the console output, also directly in U-Boot. If run by itself this takes 100ms. For 1000 runs it takes 660ms, or 0.66ms per run.

So overall running a C test is perhaps 8 times faster individually and the interactions are perhaps 25 times faster.

It should also be noted that the C test is fairly easy to debug. You can set a breakpoint on do_mem_search(), which is what implements the ‘ms’ command, single step to see what might be wrong, etc. That is also possible with the pytest, but requires two terminals and –gdbserver.

Why does speed matter?

Many development activities rely on running tests:

  • ‘git bisect run make qcheck’ can be used to find a failing commit

  • test-driven development relies on quick iteration of build/test

  • U-Boot’s continuous integration (CI) systems make use of tests. Running

    all sandbox tests typically takes 90 seconds and running each qemu test takes about 30 seconds. This is currently dwarfed by the time taken to build all boards

As U-Boot continues to grow its feature set, fast and reliable tests are a critical factor factor in developer productivity and happiness.

Writing C tests

C tests are arranged into suites which are typically executed by the ‘ut’ command. Each suite is in its own file. This section describes how to accomplish some common test tasks.

(there are also UEFI C tests in lib/efi_selftest/ not considered here.)

Add a new driver model test

Use this when adding a test for a new or existing uclass, adding new operations or features to a uclass, adding new ofnode or dev_read_() functions, or anything else related to driver model.

Find a suitable place for your test, perhaps near other test functions in existing code, or in a new file. Each uclass should have its own test file.

Declare the test with:

/* Test that ... */
static int dm_test_uclassname_what(struct unit_test_state *uts)
   /* test code here */

   return 0;
DM_TEST(dm_test_uclassname_what, UT_TESTF_SCAN_FDT);

Replace ‘uclassname’ with the name of your uclass, if applicable. Replace ‘what’ with what you are testing.

The flags for DM_TEST() are defined in test/test.h and you typically want UT_TESTF_SCAN_FDT so that the devicetree is scanned and all devices are bound and ready for use. The DM_TEST macro adds UT_TESTF_DM automatically so that the test runner knows it is a driver model test.

Driver model tests are special in that the entire driver model state is recreated anew for each test. This ensures that if a previous test deletes a device, for example, it does not affect subsequent tests. Driver model tests also run both with livetree and flattree, to ensure that both devicetree implementations work as expected.

Example commit: c48cb7ebfb4 (“sandbox: add ADC unit tests”) [1]

[1] https://gitlab.denx.de/u-boot/u-boot/-/commit/c48cb7ebfb4

Add a C test to an existing suite

Use this when you are adding to or modifying an existing feature outside driver model. An example is bloblist.

Add a new function in the same file as the rest of the suite and register it with the suite. For example, to add a new mem_search test:

/* Test 'ms' command with 32-bit values */
static int mem_test_ms_new_thing(struct unit_test_state *uts)
      /* test code here*/

      return 0;
MEM_TEST(mem_test_ms_new_thing, UT_TESTF_CONSOLE_REC);

Note that the MEM_TEST() macros is defined at the top of the file.

Example commit: 9fe064646d2 (“bloblist: Support relocating to a larger space”) [1]

[1] https://gitlab.denx.de/u-boot/u-boot/-/commit/9fe064646d2

Add a new test suite

Each suite should focus on one feature or subsystem, so if you are writing a new one of those, you should add a new suite.

Create a new file in test/ or a subdirectory and define a macro to register the suite. For example:

#include <common.h>
#include <console.h>
#include <mapmem.h>
#include <dm/test.h>
#include <test/ut.h>

/* Declare a new wibble test */
#define WIBBLE_TEST(_name, _flags)   UNIT_TEST(_name, _flags, wibble_test)

/* Tetss go here */

/* At the bottom of the file: */

int do_ut_wibble(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
  struct unit_test *tests = UNIT_TEST_SUITE_START(wibble_test);
  const int n_ents = UNIT_TEST_SUITE_COUNT(wibble_test);

  return cmd_ut_category("cmd_wibble", "wibble_test_", tests, n_ents, argc, argv);

Then add new tests to it as above.

Register this new suite in test/cmd_ut.c by adding to cmd_ut_sub[]:

/* Within cmd_ut_sub[]... */

U_BOOT_CMD_MKENT(wibble, CONFIG_SYS_MAXARGS, 1, do_ut_wibble, "", ""),

and adding new help to ut_help_text[]:

"ut wibble - Test the wibble feature\n"

If your feature is conditional on a particular Kconfig, then you can use #ifdef to control that.

Finally, add the test to the build by adding to the Makefile in the same directory:

obj-$(CONFIG_$(SPL_)CMDLINE) += wibble.o

Note that CMDLINE is never enabled in SPL, so this test will only be present in U-Boot proper. See below for how to do SPL tests.

As before, you can add an extra Kconfig check if needed:

ifneq ($(CONFIG_$(SPL_)WIBBLE),)
obj-$(CONFIG_$(SPL_)CMDLINE) += wibble.o

Example commit: 919e7a8fb64 (“test: Add a simple test for bloblist”) [1]

[1] https://gitlab.denx.de/u-boot/u-boot/-/commit/919e7a8fb64

Making the test run from pytest

All C tests must run from pytest. Typically this is automatic, since pytest scans the U-Boot executable for available tests to run. So long as you have a ‘ut’ subcommand for your test suite, it will run. The same applies for driver model tests since they use the ‘ut dm’ subcommand.

See test/py/tests/test_ut.py for how unit tests are run.

Add a C test for SPL

Note: C tests are only available for sandbox_spl at present. There is currently no mechanism in other boards to existing SPL tests even if they are built into the image.

SPL tests cannot be run from the ‘ut’ command since there are no commands available in SPL. Instead, sandbox (only) calls ut_run_list() on start-up, when the -u flag is given. This runs the available unit tests, no matter what suite they are in.

To create a new SPL test, follow the same rules as above, either adding to an existing suite or creating a new one.

An example SPL test is spl_test_load().

Writing Python tests

See U-Boot pytest suite for brief notes how to write Python tests. You should be able to use the existing tests in test/py/tests as examples.