How can I unit test a specific DML method?

Question

I'm writing some common DML code that contains a fairly complex method, something like:

saved uint32 checksum_ini;
method calculate_checksum(bytes_t data) -> (uint32 sum) {
    uint32 result = checksum_ini;
    for (int i = 0; i < data.size; ++i) {
        result = f(result, data.data[i]);
    }
    return result;
}

My device calls the function indirectly by reading and writing some registers, which makes it cumbersome to unit test all corner cases of the checksum algorithm.

How can I efficiently write a unit test for my checksum implementation?

Answer 1

One approach is to create a dedicated test module, say test-checksum, containing a test device, say test_checksum_dev, that imports only your common code, and exposes the calculate_checksum method to Python, where it is easy to write tests. This is done in two steps: First, expose the method to C:

dml 1.4;
device test_checksum_dev;
import "checksum-common.dml";
// Make DML method calculate_checksum available as extern C symbol "calculate_checksum"
// The signature will be:
// uint64 calculate_checksum(conf_object_t *obj, bytes_t data)
export calculate_checksum as "calculate_checksum";

The second step is to expose it to Python. Create checksum.h:

#ifndef CHECKSUM_H
#define CHECKSUM_H

#include <simics/base/types.h>
#include <simics/pywrap.h>

extern uint32 calculate_checksum(conf_object_t *obj, bytes_t data);

#endif /* CHECKSUM_H */

(if you also add header %{ #include "checksum.h" %} to the DML file, you will get a hard check that signatures stay consistent).

Now add the header file to IFACE_FILES in your module makefile to create a Python wrapping:

SRC_FILES = test-checksum.dml
IFACE_FILES = checksum.h
include $(MODULE_MAKEFILE)

You can now call the DML method directly from your test:

SIM_load_module('test-checksum')
from simmod.test_checksum.checksum import calculate_checksum
obj = SIM_create_object('test_checksum_dev', 'dev', checksum_ini=0xdeadbeef)
assert calculate_checksum(obj, b'hello world') == (0xda39ba47).to_bytes(4, 'little')