Get started with driver development

This guide provides step-by-step instructions that walk you through the basic workflows of building, running, debugging, and updating drivers in a Fuchsia system using the Fuchsia SDK.

Complete the following sections:

  1. Prerequisites.
  2. Clone the SDK driver samples repository.
  3. Start the emulator.
  4. Build and load the sample driver.
  5. Build and run a tools component.
  6. Debug the sample driver.
  7. Modify and reload the sample driver.

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1. Prerequisites

Before you begin, complete the prerequisite steps below:

Check host machine requirements

This guide requires that your host machine meets the following criteria:

  • A Linux machine. macOS is not supported yet.
  • Has at least 15 GB of storage space.
  • Supports KVM (Kernel Virtual Machine) for running a QEMU-based emulator.
  • IPv6 is enabled.

Install dependencies

git and bazel need to be installed on the host machine. You need Bazel 5.1 or higher.

Do the following:

  1. Install Git.

  2. Install Bazel – the easiest install option is to download the Bazelisk binary and rename it to bazel in a convenient place on your path.

Generate Fuchsia-specific SSH keys

The ffx tool requires that Fuchsia-specific SSH keys are stored on the host machine for connecting to Fuchsia devices (including the Fuchsia emulator).

To check if your host machine already has Fuchsia SSH keys, do the following:

  1. Scan the $HOME/.ssh directory for Fuchsia SSH keys:

    ls $HOME/.ssh | grep fuchsia
    
  2. Verify that the following fuchsia_* files are present:

    $ ls $HOME/.ssh | grep fuchsia
    fuchsia_authorized_keys
    fuchsia_ed25519
    fuchsia_ed25519.pub
    

If you don’t see these files, you need to generate Fuchsia SSH keys on the host machine:

  1. Generate a new private and public SSH key pair:

    [[ -f "${HOME}/.ssh/fuchsia_ed25519" ]] || ssh-keygen -P "" -t ed25519 -f "${HOME}/.ssh/fuchsia_ed25519" -C "${USER}@$(hostname -f) Shared SSH Key for Fuchsia"
    
  2. Generate a fuchsia_authorized_keys file:

    [[ -f "${HOME}/.ssh/fuchsia_authorized_keys" ]] || ssh-keygen -y -f "${HOME}/.ssh/fuchsia_ed25519" > "${HOME}/.ssh/fuchsia_authorized_keys"
    
  3. Verify that Fuchsia SSH keys are generated:

    ls $HOME/.ssh | grep fuchsia
    

    This command prints output similar to the following:

    $ ls $HOME/.ssh | grep fuchsia
    fuchsia_authorized_keys
    fuchsia_ed25519
    fuchsia_ed25519.pub
    

2. Clone the SDK driver samples repository

Clone the SDK driver samples repository on your host machine. This repository contains sample driver components and the Bazel-based Fuchsia SDK.

The tasks include:

  • Bootstrap the SDK driver samples repository.
  • Verify that you can build the sample driver components and run ffx commands.

Do the following:

  1. In a terminal, change to your home directory:

    cd $HOME
    
  2. Clone the SDK driver samples repository:

    git clone https://fuchsia.googlesource.com/sdk-samples/drivers --recurse-submodules
    

    This creates a new directory named drivers, which clones the content of the SDK driver samples repository.

  3. Go to the new directory:

    cd drivers
    
  4. To verify the Fuchsia SDK environment setup, build the sample drivers:

    bazel build --config=fuchsia_x64 //src/qemu_edu
    

    The first build may take a few minutes to download dependencies, such as Bazel build rules, Clang, and Fuchsia IDK (which includes the ffx tool).

    When finished successfully, it prints output similar to the following in the end:

    $ bazel build --config=fuchsia_x64 //src/qemu_edu
    ...
    INFO: Elapsed time: 131.746s, Critical Path: 26.89s
    INFO: 722 processes: 454 internal, 268 linux-sandbox.
    INFO: Build completed successfully, 722 total actions
    
  5. To verify that you can use the ffx tool in your environment, run the following command:

    tools/ffx version -v
    

    This command prints output similar to the following:

    $ tools/ffx version -v
    ffx:
      abi-revision: 0xA56735A6690E09D8
      api-level: 8
      build-version: 2022-06-09T20:02:48+00:00
      integration-commit-hash: dfddeea2221689c800ca1db7a7c7d1f2cb0bd99f
      integration-commit-time: Thu, 09 Jun 2022 20:02:48 +0000
    
    daemon:
      abi-revision: 0xA56735A6690E09D8
      api-level: 8
      build-version: 2022-06-09T20:02:48+00:00
      integration-commit-hash: dfddeea2221689c800ca1db7a7c7d1f2cb0bd99f
      integration-commit-time: Thu, 09 Jun 2022 20:02:48 +0000
    

    At this point, you only need to confirm that you can run this ffx command without any errors.

3. Start the emulator

Start the Fuchsia emulator on the host machine while configuring the emulator instance to use Fuchsia’s new driver framework (DFv2).

The tasks include:

  • Download Fuchsia's Workstation prebuilt image from Google Cloud Storage.
  • Start the Fuchsia emulator.
  • Set the emulator instance as your host machine’s default target device.
  • Start the Fuchsia package server.
  • Register the system package repository to the emulator instance.

Do the following:

  1. Download the latest Workstation image for the emulator:

    tools/ffx product-bundle get workstation_eng.qemu-x64
    

    This command may take a few minutes to download the image and product metadata.

    Once the download is finished, the ffx product-bundle get command creates a local Fuchsia package repository named workstation_eng.qemu-x64 on your host machine. This package repository hosts additional system packages for this Workstation prebuilt image. Later in Step 7 you’ll register this package repository to the emulator instance.

  2. Stop all emulator instances:

    tools/ffx emu stop --all
    
  3. Start the Fuchsia emulator:

    tools/ffx emu start workstation_eng.qemu-x64 --headless --kernel-args "driver_manager.use_driver_framework_v2=true" --kernel-args "driver_manager.root-driver=fuchsia-boot:///#meta/platform-bus.cm" --kernel-args "devmgr.enable-ephemeral=true"
    

    This command starts a headless emulator instance running the Workstation prebuilt image.

    When the instance is up and running, the command prints output similar to the following:

    $ tools/ffx emu start workstation_eng.qemu-x64 --headless --kernel-args "driver_manager.use_driver_framework_v2=true" --kernel-args "driver_manager.root-driver=fuchsia-boot:///#meta/platform-bus.cm" -- kernel-args "devmgr.enable-ephemeral=true"
    Logging to "/home/alice/.local/share/Fuchsia/ffx/emu/instances/fuchsia-emulator/emulator.log"
    Waiting for Fuchsia to start (up to 60 seconds).
    Emulator is ready.
    
  4. Verify that the new emulator instance is running:

    tools/ffx emu list
    

    This command prints output similar to the following:

    $ tools/ffx emu list
    [Active]  fuchsia-emulator
    
  5. Set the default target device:

    tools/ffx target default set fuchsia-emulator
    

    This command exits silently without output.

  6. Start the Fuchsia package server:

    tools/ffx repository server start
    

    This command prints output similar to the following:

    $ tools/ffx repository server start
    ffx repository server is listening on [::]:8083
    
  7. Register the system package repository (workstation_eng.qemu-x64) to the target device:

    tools/ffx target repository register -r workstation_eng.qemu-x64 --alias fuchsia.com
    

    This command exits silently without output.

4. Build and load the sample driver

The Fuchsia emulator (launched in the Start the emulator section above) is configured to create a virtual device named edu, which is an educational device for writing drivers. In the previous section, when the emulator started, Fuchsia’s driver framework detected this edu device in the system, but it wasn’t able to find a driver that could serve the edu device. So the edu device was left unmatched.

In this section, we build and publish the qemu_edu sample driver (which is a Fuchsia component). Upon detecting a new driver, the driver framework will discover that this new qemu_edu driver is a match for the edu device. Once matched, the qemu_edu driver starts providing the edu device’s services (capabilities) to other components in the system – one of the services provided by the edu device is that it computes a factorial given an integer.

The tasks include:

  • View the drivers that are currently loaded in the emulator instance.
  • Build and publish the qemu_edu driver component.
  • Verify that the qemu_edu driver is loaded to the emulator instance.
  • View detailed information on the qemu_edu component.

Do the following:

  1. View the list of the currently loaded drivers:

    tools/ffx driver list --loaded
    

    This command prints output similar to the following:

    $ tools/ffx driver list --loaded
    fuchsia-boot:///#meta/block.core.cm
    fuchsia-boot:///#meta/bus-pci.cm
    fuchsia-boot:///#meta/fvm.cm
    fuchsia-boot:///#meta/hid-input-report.cm
    fuchsia-boot:///#meta/hid.cm
    fuchsia-boot:///#meta/intel-rtc.cm
    fuchsia-boot:///#meta/netdevice-migration.cm
    fuchsia-boot:///#meta/network-device.cm
    fuchsia-boot:///#meta/pc-ps2.cm
    fuchsia-boot:///#meta/platform-bus-x86.cm
    fuchsia-boot:///#meta/platform-bus.cm
    fuchsia-boot:///#meta/ramdisk.cm
    fuchsia-boot:///#meta/sysmem.cm
    fuchsia-boot:///#meta/virtio_block.cm
    fuchsia-boot:///#meta/virtio_ethernet.cm
    fuchsia-boot:///#meta/zxcrypt.cm
    fuchsia-pkg://fuchsia.com/virtual_audio#meta/virtual_audio_driver.cm
    
  2. Build and publish the qemu_edu driver component:

    bazel run --config=fuchsia_x64 //src/qemu_edu:pkg.component
    

    This command prints output similar to the following:

    $ bazel run --config=fuchsia_x64 //src/qemu_edu:pkg.component
    INFO: Analyzed target //src/qemu_edu:pkg.component (6 packages loaded, 162 targets configured).
    INFO: Found 1 target...
    Target //src/qemu_edu:pkg.component up-to-date:
      bazel-bin/src/qemu_edu/pkg.component_run_component.sh
    INFO: Elapsed time: 1.660s, Critical Path: 0.49s
    INFO: 21 processes: 12 internal, 8 linux-sandbox, 1 local.
    INFO: Build completed successfully, 21 total actions
    INFO: Build completed successfully, 21 total actions
    added repository bazel.pkg.component
    Registering fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm
    Successfully bound:
    Node 'root.sys.platform.platform-passthrough.PCI0.bus.00_06_0_', Driver 'fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm'.
    
  3. Verify that the qemu_edu driver is now loaded to the Fuchsia emulator instance:

    tools/ffx driver list --loaded
    

    This command prints output similar to the following:

    $ tools/ffx driver list --loaded
    fuchsia-boot:///#meta/block.core.cm
    fuchsia-boot:///#meta/bus-pci.cm
    fuchsia-boot:///#meta/fvm.cm
    fuchsia-boot:///#meta/hid-input-report.cm
    fuchsia-boot:///#meta/hid.cm
    fuchsia-boot:///#meta/intel-rtc.cm
    fuchsia-boot:///#meta/netdevice-migration.cm
    fuchsia-boot:///#meta/network-device.cm
    fuchsia-boot:///#meta/pc-ps2.cm
    fuchsia-boot:///#meta/platform-bus-x86.cm
    fuchsia-boot:///#meta/platform-bus.cm
    fuchsia-boot:///#meta/ramdisk.cm
    fuchsia-boot:///#meta/sysmem.cm
    fuchsia-boot:///#meta/virtio_block.cm
    fuchsia-boot:///#meta/virtio_ethernet.cm
    fuchsia-boot:///#meta/zxcrypt.cm
    fuchsia-pkg://fuchsia.com/virtual_audio#meta/virtual_audio_driver.cm
    fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm
    

    Notice that the qemu_edu driver is shown at the bottom of the loaded drivers list.

  4. View the qemu_edu component information:

    tools/ffx component show qemu_edu.cm
    

    This command prints output similar to the following:

    $ tools/ffx component show qemu_edu.cm
                   Moniker: /bootstrap/universe-pkg-drivers:root.sys.platform.platform-passthrough.PCI0.bus.00_06_0_
                       URL: fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm
                      Type: CML dynamic component
           Component State: Resolved
     Incoming Capabilities: fuchsia.device.fs.Exporter
                            fuchsia.driver.compat.Service
                            fuchsia.logger.LogSink
                            pkg
               Merkle root: a4832605ffe6bf6ddad3aad0d3d36c435ee2e66f79d43cd0b818d2aae20f7755
           Execution State: Running
              Start reason: Instance is in a single_run collection
     Outgoing Capabilities: qemu-edu
    
  5. View device logs:

    tools/ffx log --filter qemu_edu
    

    This command prints output similar to the following:

    $ tools/ffx log --filter qemu_edu
    ...
    [176.540][core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://bazel.pkg.component/qemu_edu to a4832605ffe6bf6ddad3aad0d3d36c435ee2e66f79d43cd0b818d2aae20f7755 with TUF
    [176.542][bootstrap/driver_index][driver_index,driver][I] Registered driver successfully: fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm.
    [176.571][core/pkg-resolver][pkg-resolver][I] Fetching blobs for fuchsia-pkg://bazel.pkg.component/qemu_edu: []
    [176.573][core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://bazel.pkg.component/qemu_edu to a4832605ffe6bf6ddad3aad0d3d36c435ee2e66f79d43cd0b818d2aae20f7755 with TUF
    [176.577][bootstrap/driver_manager][driver_manager.cm][I]: [driver_runner.cc:858] Binding fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm to  00_06_0_
    [176.908][bootstrap/driver-hosts:driver-host-3][driver_host2.cm][I]: [driver_host.cc:289] Started 'fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm'
    

    Press CTRL+C to exit.

5. Build and run a tools component

The qemu_edu driver sample has a “tools” component named eductl, which can interact with the sample driver. Developers create these tools components for testing and debugging drivers during development.

In this case, the eductl component contacts the qemu_edu driver and passes an integer as input. The driver (using the resource of the edu virtual device) computes the integer's factorial and returns the result to the eductl component. The component then prints the result in the log.

The tasks include:

  • Build and run the eductl component.
  • Verify that the component can interact with the qemu_edu driver.

Do the following:

  1. Build and run the eductl component:

    bazel run --config=fuchsia_x64 //src/qemu_edu:eductl_pkg.eductl_component
    

    This command prints output similar to the following:

    $ bazel run --config=fuchsia_x64 //src/qemu_edu:eductl_pkg.eductl_component
    INFO: Analyzed target //src/qemu_edu:eductl_pkg.eductl_component (0 packages loaded, 14 targets configured).
    INFO: Found 1 target...
    Target //src/qemu_edu:eductl_pkg.eductl_component up-to-date:
      bazel-bin/src/qemu_edu/eductl_pkg.eductl_component_run_component.sh
    INFO: Elapsed time: 1.667s, Critical Path: 1.22s
    INFO: 23 processes: 7 internal, 15 linux-sandbox, 1 local.
    INFO: Build completed successfully, 23 total actions
    INFO: Build completed successfully, 23 total actions
    added repository bazel.eductl.pkg.eductl.component
    URL: fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl#meta/eductl.cm
    Moniker: /core/ffx-laboratory:eductl
    Creating component instance...
    Starting component instance...
    Success! The component instance has been started.
    
  2. View the device logs of the eductl component:

    tools/ffx log --filter eductl dump
    

    This command prints output similar to the following:

    $ tools/ffx log --filter eductl dump
    ...
    [367.076][core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl to 4fe2e38ed56693bf720565c3ee5e6f8314a64c601cae67288db5e8d30f1a9265 with TUF
    [367.080][core/pkg-resolver][pkg-resolver][I] Fetching blobs for fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl: []
    [367.081][core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl to 4fe2e38ed56693bf720565c3ee5e6f8314a64c601cae67288db5e8d30f1a9265 with TUF
    [367.166][core/ffx-laboratory:eductl][][I] Factorial(12) = 479001600
    [367.173][core/pkg-resolver][pkg-resolver][I] removing repository fuchsia-pkg://bazel.eductl.pkg.eductl.component
    [367.173][core/pkg-resolver][pkg-resolver][I] closing fuchsia-pkg://bazel.eductl.pkg.eductl.component
    [367.176][core/pkg-resolver][pkg-resolver][I] AutoClient for "http://10.0.2.2:8083/bazel.eductl.pkg.eductl.component/auto" stopping
    

    The output Factorial(12) = 479001600 shows that the eductl component passed 12 as input to the driver and received the result from the driver. (For the default input, see this eductl.cml file.)

6. Debug the sample driver

Use the Fuchsia debugger (zxdb) to step through the sample driver’s code as the driver is running on the emulator instance.

The tasks include:

  • Identify the driver host (which is a component) that is running the qemu_edu driver.
  • Start the Fuchsia debugger and connect it to the emulator instance.
  • Attach the debugger to the driver host.
  • Set a breakpoint on the driver’s code.
  • Run the tools component, which triggers the driver to execute its instructions.
  • Step through the driver’s code.

Do the following:

  1. View the list of the running driver hosts:

    tools/ffx driver list-hosts
    

    This command prints output similar to the following:

    $ tools/ffx driver list-hosts
    Driver Host: 4690
        fuchsia-boot:///#meta/block.core.cm
        fuchsia-boot:///#meta/bus-pci.cm
        fuchsia-boot:///#meta/fvm.cm
        fuchsia-boot:///#meta/hid.cm
        fuchsia-boot:///#meta/netdevice-migration.cm
        fuchsia-boot:///#meta/network-device.cm
        fuchsia-boot:///#meta/platform-bus-x86.cm
        fuchsia-boot:///#meta/platform-bus.cm
        fuchsia-boot:///#meta/ramdisk.cm
        fuchsia-boot:///#meta/sysmem.cm
        fuchsia-boot:///#meta/virtio_block.cm
        fuchsia-boot:///#meta/virtio_ethernet.cm
        fuchsia-boot:///#meta/zxcrypt.cm
        fuchsia-pkg://fuchsia.com/virtual_audio#meta/virtual_audio_driver.cm
    
    Driver Host: 7820
        fuchsia-boot:///#meta/intel-rtc.cm
    
    Driver Host: 7903
        fuchsia-boot:///#meta/pc-ps2.cm
    
    Driver Host: 50125
        fuchsia-pkg://bazel.pkg.component/qemu_edu#meta/qemu_edu.cm
    

    Make a note of the PID of the qemu_edu driver host (50125 in the example above).

  2. Start the Fuchsia debugger:

    tools/ffx debug connect
    

    This command prints output similar to the following:

    $ tools/ffx debug connect
    Connecting (use "disconnect" to cancel)...
    Connected successfully.
    👉 To get started, try "status" or "help".
    [zxdb]
    
  3. Attach the debugger to the qemu_edu driver host:

    [zxdb] attach PID
    

    Replace PID with the PID of the qemu_edu driver host identified in Step 1, for example:

    [zxdb] attach 50125
    

    This command prints output similar to the following:

    [zxdb] attach 50125
    Attached Process 1 state=Running koid=50125 name=driver_host2.cm
    Downloading symbols...
    Symbol downloading complete. 7 succeeded, 0 failed.
    [zxdb]
    
  4. Set a breakpoint at the driver’s ComputeFactorial function:

    [zxdb] break QemuEduDriver::ComputeFactorial
    

    This command prints output similar to the following:

    [zxdb] break QemuEduDriver::ComputeFactorial
    Created Breakpoint 1 @ QemuEduDriver::ComputeFactorial
       177 void QemuEduDriver::ComputeFactorial(ComputeFactorialRequestView request,
     ◉ 178                                      ComputeFactorialCompleter::Sync& completer) {
       179   // Write a value into the factorial register.
    [zxdb]
    
  5. In different terminal, run the tools component:

    bazel run --config=fuchsia_x64 //src/qemu_edu:eductl_pkg.eductl_component
    

    In the zxdb terminal, verify that the debugger is stopped at the driver’s ComputeFactorial function, for example:

    🛑 thread 2 on bp 1 qemu_edu::QemuEduDriver::ComputeFactorial(qemu_edu::QemuEduDriver*, fidl::WireServer<fuchsia_hardware_qemuedu::Device>::ComputeFactorialRequestView,    fidl::Completer<fidl::internal::WireCompleterBase<fuchsia_hardware_qemuedu::Device::ComputeFactorial> >::Sync&) • qemu_edu.cc:178
       176
       177 void QemuEduDriver::ComputeFactorial(ComputeFactorialRequestView request,
     ▶ 178                                      ComputeFactorialCompleter::Sync& completer) {
       179   // Write a value into the factorial register.
       180   uint32_t input = request->input;
    [zxdb]
    
  6. In the zxdb terminal, view the source code around the current breakpoint:

    [zxdb] list
    

    This command prints output similar to the following:

    [zxdb] list
       173       });
       174   return outgoing_.Serve(std::move(outgoing_dir));
       175 }
       176
       177 void QemuEduDriver::ComputeFactorial(ComputeFactorialRequestView request,
     ▶ 178                                      ComputeFactorialCompleter::Sync& completer) {
       179   // Write a value into the factorial register.
       180   uint32_t input = request->input;
       181
       182   mmio_->Write32(input, regs::kFactorialCompoutationOffset);
       183
       184   // Busy wait on the factorial status bit.
       185   while (true) {
       186     const auto status = regs::Status::Get().ReadFrom(&*mmio_);
       187     if (!status.busy())
       188       break;
    [zxdb]
    
  7. In the zxdb terminal, step through the code using the next command until the value of factorial is read from the device (that is, until the line 194 is reached):

    [zxdb] next
    

    The last next command prints output similar to the following:

    ...
    [zxdb] next
    🛑 thread 2 qemu_edu::QemuEduDriver::ComputeFactorial(qemu_edu::QemuEduDriver*, fidl::WireServer<fuchsia_hardware_qemuedu::Device>::ComputeFactorialRequestView, fidl::Completer<fidl::internal::WireCompleterBase<fuchsia_hardware_qemuedu::Device::ComputeFactorial> >::Sync&) • qemu_edu.cc:194
       192   uint32_t factorial = mmio_->Read32(regs::kFactorialCompoutationOffset);
       193
     ▶ 194   FDF_SLOG(INFO, "Replying with", KV("factorial", factorial));
       195   completer.Reply(factorial);
       196 }
    [zxdb]
    
  8. Print the factorial variable:

    [zxdb] print factorial
    

    This command prints output similar to the following:

    [zxdb] print factorial
    479001600
    [zxdb]
    
  9. To exit the zxdb terminal, type exit or press Ctrl-D.

7. Modify and reload the sample driver

Update the source code of the sample driver and reload it to the emulator instance.

The tasks include:

  • Restart the emulator instance to unload the qemu_edu driver.
  • Update the source code of the qemu_edu driver.
  • Load the updated driver.
  • Run the tools component to verify the change.

Do the following:

  1. Stop the emulator instance:

    tools/ffx emu stop
    

    This command stops the currently running emulator instance.

  2. Start a new instance of the Fuchsia emulator:

    tools/ffx emu start workstation_eng.qemu-x64 --headless --kernel-args "driver_manager.use_driver_framework_v2=true" --kernel-args "driver_manager.root-driver=fuchsia-boot:///#meta/platform-bus.cm" --kernel-args "devmgr.enable-ephemeral=true"
    

    This command starts a headless emulator instance running the Workstation prebuilt image.

  3. Use a text editor to open the source code of the sample driver, for example:

    nano src/qemu_edu/qemu_edu.cc
    
  4. In the QemuEduDriver::ComputeFactorial function, between the line uint32_t factorial = mmio_->Read32(regs::kFactorialCompoutationOffset); (Line 192) and the FDF_SLOG() call (Line 194), add the following line:

    factorial=12345;
    

    The function should look like below:

    void QemuEduDriver::ComputeFactorial(ComputeFactorialRequestView request,
                                         ComputeFactorialCompleter::Sync& completer) {
      // Write a value into the factorial register.
      uint32_t input = request->input;
    
      mmio_->Write32(input, regs::kFactorialCompoutationOffset);
    
      // Busy wait on the factorial status bit.
      while (true) {
        const auto status = regs::Status::Get().ReadFrom(&*mmio_);
        if (!status.busy())
          break;
      }
    
      // Return the result.
      uint32_t factorial = mmio_->Read32(regs::kFactorialCompoutationOffset);
      factorial = 12345;
      FDF_SLOG(INFO, "Replying with", KV("factorial", factorial));
      completer.Reply(factorial);
    }
    

    The function is now updated to return the value of 12345 only.

  5. Save the file and close the text editor.

  6. Rebuild and run the modified sample driver:

    bazel run --config=fuchsia_x64 //src/qemu_edu:pkg.component
    
  7. Run the tools component:

    bazel run --config=fuchsia_x64 //src/qemu_edu:eductl_pkg.eductl_component
    
  8. To verify that change, view the device logs of the tools component:

    tools/ffx log --filter eductl dump
    

    This command prints output similar to the following:

    $ tools/ffx log --filter eductl dump
    ...
    [43.349][core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl to 4fe2e38ed56693bf720565c3ee5e6f8314a64c601cae67288db5e8d30f1a9265 with TUF
    [43.354][core/pkg-resolver][pkg-resolver][I] Fetching blobs for fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl: []
    [43.355][core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://bazel.eductl.pkg.eductl.component/eductl to 4fe2e38ed56693bf720565c3ee5e6f8314a64c601cae67288db5e8d30f1a9265 with TUF
    [43.439][core/ffx-laboratory:eductl][][I] Factorial(12) = 12345
    [43.448][core/pkg-resolver][pkg-resolver][I] removing repository fuchsia-pkg://bazel.eductl.pkg.eductl.component
    [43.449][core/pkg-resolver][pkg-resolver][I] closing fuchsia-pkg://bazel.eductl.pkg.eductl.component
    [43.452][core/pkg-resolver][pkg-resolver][I] AutoClient for "http://10.0.2.2:8083/bazel.eductl.pkg.eductl.component/auto" stopping.
    

    The line in the logs shows that the qemu_edu driver returned the hardcoded value of 12345 as the factorial of 12 to the tools component.

Congratulations! You’re now all set with the Fuchsia driver development!

Next steps

Learn more about how the qemu_edu driver works in the Driver sample walkthrough: qemu_edu guide.

Appendices

Clean up the environment

If you run into a problem while following this guide and decide to start over from the beginning, consider running the commands below to clean up your development environment (that is, to clean up directories, build artifacts, downloaded files, symlinks, configuration settings, and more).

Remove the package repositories created in this guide:

tools/ffx repository remove workstation_eng.qemu-x64
tools/ffx repository server stop
rm -rf $HOME/.package_repos/sdk-samples

Remove all existing configurations and data of ffx:

tools/ffx daemon stop
rm -rf $HOME/.local/share/Fuchsia/ffx

Remove the drivers directory and its artifacts:

rm -rf $HOME/drivers

When Bazel fails to build, try the commands below:

bazel clean --expunge
bazel shutdown && rm -rf $HOME/.cache/bazel

Other clean up commands:

killall ffx
killall pm