Mock DDK Migration

The fake_ddk driver testing library is being replaced by a new library, mock_ddk. The driver framework team needs help to migrate the 100+ driver tests in Fuchsia.

Goal & motivation

The Fake DDK has a number of ambiguities around what is being tested and made a number of assumptions about the driver structure that are increasingly invalid. The mock_ddk provides a more straightforward unit testing framework, with clear boundaries about what is able to be tested.

Technical background

Simple example

There are a number of fundamental differences between fake-ddk and mock-ddk, but here is how a very simple test might be migrated:

Fake DDK

TEST(FooDevice, BasicTest) {
  fake_ddk::Bind ddk;
  device = std::make_unique<FooDevice>(fake_ddk::FakeParent(), other_args);
  ASSERT_EQ(device->Init(), ZX_OK);
  // Do some testing here

Mock DDK

TEST(FooDevice, BasicTest) {
  std::shared_ptr<MockDevice> fake_parent = MockDevice::FakeRootParent();
  device = std::make_unique<FooDevice>(fake_parent.get(), other_args);
  ASSERT_EQ(device->Init(), ZX_OK);
  device.release(); // let go of the reference to the device
  // Do some testing here

  // The mock-ddk will automatically call DdkRelease() on any remaining children of
  // fake_parent upon destruction.

Overview of the Mock-DDK

The mock ddk exists simply as a set of zx_device_t’s that track the interactions a device has with the mocked driver host, and allow calls into the device. There is no global state - if the root “parent” device ever goes out of scope, all the zx_device_t’s will destruct and delete their accompanying device.

Here is an interaction model of how the mock-ddk interacts with a driver:

Figure: Interaction Model

Major changes from the fake-ddk


fake_ddk mock-ddk
All driver information is contained in global fake_ddk::Bind variable. Information for each device is stored in the zx_device_t for that device.
Multiple drivers not supported The zx_device_t maintains information about parents and children, so all devices descending from the same root parent can be discovered.
Does not hold a reference to the created device Just like the driver host, each zx_device_t stores the device context

See the section Getting the Device Context for how to get device context in mock-ddk.

Imitating Driverhost behavior

fake_ddk mock-ddk
* Replicates the driverhost behavior for Init and Remove/Unbind/Release * DriverHost behavior is kept to a minimum.
* There is one built-in behavior: each zx_device_t will call release() on its device context upon destruction.

No more fake_ddk::Bind::Ok() function

The Ok() function was not actually testing correct use of driverhost protocols. Although there is no drop in replacement for the Ok() function, the test writer can start and stop the driver just like the driver host does, to ensure the device state is initialized and shut down properly. An example of this test is provided below in the section An example lifecycle test.

Using the Mock DDK

Interactions with the Driverhost

The mock_ddk mocks out and makes available calls to and from the driverhost.

Calling into the device
(device ops)
Calling out to the driverhost
(Libdriver API)
Call device ops through the MockDevice. Functions are named as op name + Op
Call the init function using InitOp()
All calls in the libdriver API are recorded on the appropriate device, but no action is taken.
To test if device_init_reply() has been called, call InitReplyCalled()
or to wait on the call, WaitUntilInitReplyCalled().
An example lifecycle test

Fake DDK

fake_ddk::Bind bind;
TestDevice* device  = TestDevice::Create(fake_ddk::kFakeParent);

Mock DDK

auto parent = MockDevice::FakeRootParent();
// make sure the child device is there
ASSERT_EQ(1, parent->child_count());
auto* child = parent->GetLatestChild();
// If your device has an init function:
// Use this if init replies asynchronously:
EXPECT_EQ(ZX_OK,  child->WaitUntilInitReplyCalled());
// Otherwise, can just verify init replied:
// If your device has an unbind function:
// Use this if unbind replies asynchronously:
EXPECT_EQ(ZX_OK, child->WaitUntilUnbindReplyCalled());
// Otherwise, can just verify init replied:
// Mock-ddk will release all the devices on destruction, or you can do it manually.
Automatically Unbind and Release

The driverhost will always call unbind before releasing a driver, but that step must be done manually in the mock-ddk. If you have multiple drivers under test, it may be easier to automate the unbinding and releasing behavior. The Mock DDK has a helper function for this purpose:

auto parent = MockDevice::FakeRootParent();
TestDevice* test_device_0 = TestDevice::Create(parent.get());
TestDevice* test_device_1 = TestDevice::Create(test_device_0.zxdev());
// The state of the tree is now:
//         parent   <--  FakeRootParent
//           |
//         child    <--  test_device_0
//           |
//       grandchild <--  test_device_1

// You want to remove both test devices, by calling unbind and release in the right order?

// ReleaseFlaggedDevices performs the unbind and release of any device
// below the input device that has had device_async_remove called on it.

Getting Device Context

The mock-ddk only deals with the zx_device_t's that are associated with a device. However, if you have assigned a device context, by for example using the ddktl library, you may want to access corresponding the ddk::Device:

  auto parent = MockDevice::FakeRootParent();
  // May not get the device* back from bind:
  // Never fear! Recover device from parent:
  MockDevice* child = parent->GetLatestChild();
  TestDevice* test_dev =

Interactions with other drivers

Mocking out parent functionality uses mostly the same calls as the fake ddk, but setting mocks only affect the devices involved, instead of loading mocks into a global state.

Mocking Parent Protocols

Parent protocols are added to the parent before a child device is expected to access them with a call to device_get_protocol()

Fake DDK

 fake_ddk::Bind bind;
 const fake_ddk::Protocol kTestProto = {
   .ctx = reinterpret_cast<void*>(0x10),
   .ops = nullptr,

 bind.SetProtocol(8, &kTestProto);

Mock DDK

 auto parent = MockDevice::FakeRootParent();
 const void* ctx = reinterpret_cast<void*>(0x10),
 const void* ops = nullptr,

 parent->AddProtocol(8, ops, ctx);
Fragment protocols

Composite devices get protocols from multiple parent “fragments”. This is manifested in protocols being keyed by a name. Mock-ddk allows binding a name to a protocol, to indicate it comes from a fragment.

Fake DDK

 fake_ddk::Bind bind;
     fbl::Array<fake_ddk::FragmentEntry> fragments(new fake_ddk::FragmentEntry[2], 2);
     fragments[0].name = "fragment-1";
         fake_ddk::ProtocolEntry{0, fake_ddk::Protocol{nullptr, nullptr}});
         fake_ddk::ProtocolEntry{1, fake_ddk::Protocol{nullptr, nullptr}});
     fragments[1].name = "fragment-2";
    fake_ddk::ProtocolEntry{2, fake_ddk::Protocol{nullptr, nullptr}});

Mock DDK

 auto parent = MockDevice::FakeRootParent();
 void* ctx = reinterpret_cast<void*>(0x10),
 void* ops = nullptr,
 // Mock-ddk uses the same call as adding a
 // normal parent protocol:
 parent->AddProtocol(0, ops, ctx, "fragment-1");
 parent->AddProtocol(1, ops, ctx, "fragment-1");
 parent->AddProtocol(2, ops, ctx, "fragment-2");
Mocking FIDL connections

If the device serves a FIDL protocol, the test may want to call the fidl functions provided. This can be difficult as the fidl functions take a completer as an argument. You can create a client to communicate with the device class over a fidl channel.

Fake DDK

fake_ddk::Bind bind;
TestDevice* dev  = TestDevice::Create(fake_ddk::kFakeParent);
FidlMessenger fidl;
fidl.SetMessageOp((void *)dev,
   [](void* ctx,
      fidl_incoming_msg_t* msg,
      fidl_txn_t* txn) -> zx_status_t
          { return static_cast<Device*>(ctx)->DdkMessage(msg, txn)});
<fidl_client_function> (
    <fake_ddk>.local().get(), <args>);

Mock DDK

auto parent = MockDevice::FakeRootParent();
TestDevice* dev  =  TestDevice::Create(parent.get());
async::Loop loop_(&kAsyncLoopConfigNoAttachToCurrentThread);
auto endpoints = fidl::CreateEndpoints<fidl_proto>();
std::optional<fidl::ServerBindingRef<fidl_proto>> binding_;
binding_ = fidl::BindServer(loop_.dispatcher(),
rpmb_fidl_.Bind(std::move(endpoints->client), loop_.dispatcher());
Mocking Metadata

Metadata can be added to any ancestor of the device under test. Metadata is propagated to be available to all descendants.

Fake DDK

fake_ddk::Bind bind;
const char kSource[] = "test";
                 kSource, sizeof(kSource));

Mock DDK

auto parent = MockDevice::FakeRootParent();
const char kSource[] = "test";
                   kSource, sizeof(kSource));
Load Firmware

Load firmware is an deprecated function, but is included for the drivers that still need it:

Fake DDK

No Functionality.

Mock DDK

auto parent = MockDevice::FakeRootParent();
auto result = TestDevice::Bind(parent.get());
TestDevice* test_device = result.value();
constexpr std::string_view kFirmwarePath = "test path";
std::vector<uint8_t> kFirmware(200, 42);
test_device->zxdev()->SetFirmware(kFirmware, kFirmwarePath);

How to help

Picking a task

Bugs have been filed for all remaining uses of Fake DDK (e.g., fxb/115045). The label is df-mock-ddk-migration.

You can also check the allowlist in src/devices/testing/fake_ddk/

Doing a task

  1. If it isn't already, assign the bug to yourself.
  2. Change build rules and includes to target mock-ddk instead of fake_ddk

    $ sed -i 's%testing/fake_ddk%testing/mock-ddk%' path/to/
    $ sed -i 's%<lib/fake_ddk/fake_ddk.h>%"src/devices/testing/mock-ddk/mock-device.h"%'
  3. Remove the driver folder from the fake_ddk allowlist in src/devices/testing/fake_ddk/

  4. Change usage of fake_ddk::Bind to auto fake_parent = MockDevice::FakeRootParent();

    1. Note that you may need to be more careful with the scope of fake_parent since it does not involve any global variables.
    2. If your test creates a class that inherits from fake_ddk::Bind, you should find that mock-ddk supports the features that necessitated creating the subclass. If not, please contact garratt@.
  5. Change usage of fake_ddk::kFakeParent and fake_ddk::FakeParent() to fake_parent.get()

  6. Remove usage of fake_ddk::Bind::Ok() (see explanation above.) Instead, check the specific device state to ensure initialization and shutdown operate as expected. The example: An example lifecycle test may be a good start.

  7. Make sure you do not explicitly delete the test device, except by calling the ReleaseOp. Doing so violates how the mock-ddk (and the driverhost) operates, and will result in a double free error. (see section Imitating Driverhost behavior above)

  8. Port Bind::SetProtocoland Bind:SetFragments to MockDevice::AddProtocol. Note that MockDevice::AddProtocol takes the ops and the context separately.

  9. Mocking the metadata should be unchanged, although it is called on the device’s parent instead of fake_ddk::Bind.

  10. Port the instances of fake_ddk::FidlMessenger to the mock-ddk equivalent.

  11. This might be the first time someone has looked at this driver’s unit test in a while. If the test seems lacking, (for example it only contains the one “lifecycle” test) please file a bug and add the label “improve_driver_unit_tests”. In the bug, point out a few potential tests that could be written.

  12. Add the test target to your build and test it. The test should not require specific hardware.

Common Issues:

  • Not calling Init/Unbind
    • Call Init using the MockDevice::InitOp()
    • Call Unbind using the MockDevice::UnbindOp(), or call device_async_remove() and call mock_ddk::ReleaseFlaggedDevices
  • Deleting the device directly
    • Solution: release the Device from the current scope after calling DdkAdd()

Overriding fake_ddk::Bind::DeviceAdd

Some more complex tests subclass fake_ddk::Bind in order to override the DeviceAdd method. Typically this is done to intercept some information from the device_add_args, such as an inspect VMO.

In Mock DDK, you can instead access the device using GetLatestChild.

Completing a task

  • Upload the change with the bug number in a Fixed: tag.


Change List Example of when a test...
fxr/560643 Created subclass of fake_ddk::Bind
fxr/557553 Used fake_ddk::FidlMessenger
fxr/560246 Uses SetMetadata and SetProtocol
fxr/552027 Called fake_ddk::Bind::Ok()


If at any point you need help or have questions, please reach out to garratt@ or tq-df-eng@.