README.md

TargetController

The TargetController component possesses full control of the connected hardware (debug tool and target). Execution of user-space device drivers takes place here. All interactions with the connected hardware go through the TargetController. It runs on a dedicated thread (see Applciation::startTargetController()). The source code for the TargetController component can be found in src/TargetController. The entry point is TargetControllerComponent::run().

Interfacing with the TargetController - The command-response mechanism

Other components within Bloom can interface with the TargetController via the provided command-response mechanism. Simply put, when another component within Bloom needs to interact with the connected hardware, it will send a command to the TargetController, and wait for a response. The TargetController will action the command and deliver the necessary response.

All TargetController commands can be found in src/TargetController/Commands, and are derived from the TargetController::Commands::Command base class. Responses can be found in src/TargetController/Responses, and are derived from the TargetController::Responses::Response base class.

NOTE: Components within Bloom do not typically concern themselves with the TargetController command-response mechanism. Instead, they use the TargetControllerService class, which encapsulates the command-response mechanism and provides a simplified means for interaction with the connected hardware. For more, see The TargetControllerService class section below.

Commands can be sent to the TargetController via the TargetController::CommandManager class.

For example, to read memory from the connected target, we would send the TargetController::Commands::ReadTargetMemory command:

auto tcCommandManager = TargetController::CommandManager();

auto readMemoryCommand = std::make_unique<TargetController::Commands::ReadTargetMemory>(
    someMemoryType, // Flash, RAM, EEPROM, etc
    someStartAddress,
    someNumberOfBytes
);

const auto readMemoryResponse = tcCommandManager.sendCommandAndWaitForResponse(
    std::move(readMemoryCommand),
    std::chrono::milliseconds(1000) // Response timeout
);

const auto& data = readMemoryResponse->data;

readMemoryResponse will be of type std::unique_ptr<TargetController::Responses::TargetMemoryRead>.

The CommandManager::sendCommandAndWaitForResponse<CommandType>(std::unique_ptr<CommandType> command, std::chrono::milliseconds timeout) member function is a template function. It will issue the command to the TargetController and wait for a response, or until a timeout has been reached. Because it is a template function, it is able to resolve the appropriate response type at compile-time (see the SuccessResponseType alias in some command classes). If the TargetController responds with an error, or the timeout is reached, CommandManager::sendCommandAndWaitForResponse() will throw an exception.

Atomic operations

In some instances, we need the TargetController to service a series of commands without any interruptions (servicing of other commands).

The TargetController allows for operations to be performed within "atomic sessions". Simply put, when the TargetController starts a new atomic session, any commands that are part of the session will be placed into a dedicated queue. When an atomic session is active, the TargetController will only process commands in the dedicated queue. All other commands will be processed once the atomic session has ended.

The TargetControllerService provides an RAII wrapper for atomic sessions. See Atomic sessions with the TargetControllerService for more.

The TargetControllerService class

The TargetControllerService class encapsulates the TargetController's command-response mechanism and provides a simplified means for other components to interact with the connected hardware. Iterating on the example above, to read memory from the target:

const auto tcService = Services::TargetControllerService();

const auto data = tcService.readMemory(
    someMemoryType, // Flash, RAM, EEPROM, etc
    someStartAddress,
    someNumberOfBytes
);

The TargetControllerService class does not require any dependencies at construction. It can be constructed in different threads and used freely to gain access to the connected hardware, from any component within Bloom.

All components within Bloom should use the TargetControllerService class to interact with the connected hardware. They should not directly issue commands via the TargetController::CommandManager, unless there is a very good reason to do so.

Atomic sessions with the TargetControllerService

The TargetControllerService::makeAtomicSession() member function returns an TargetControllerService::AtomicSession RAII object, which starts an atomic session with the TargetController, at construction, and ends the session at destruction. This allows us to perform operations within an atomic session, in an exception-safe manner:

auto tcService = Services::TargetControllerService();

{
    const auto atomicSession = tcService.makeAtomicSession();

    /*
     * These operations will take place in the atomic session - the TC will **NOT** service any other commands until
     * these commands have been processed (and the atomic session has ended).
     */
    tcService.writeMemory(...);
    tcService.readMemory(...);
    tcService.getProgramCounter();

    /*
     * Note: The TC does **NOT** support nested atomic sessions. Attempting to start another session in this block will
     * result in an exception being thrown.
     */
    {
        const auto nestedAtomicSession = tcService.makeAtomicSession(); // This will fail - a session is already active.
    }

    /*
     * Also note: When using TargetControllerService::makeAtomicSession(), the returned AtomicSession object is tied to
     * the TargetControllerService object that created it (in this example: tcService).
     *
     * So if you have **another** TargetControllerService object in this block, any operations performed via that
     * object will **NOT** be part of the atomic session, and, they will deadlock the TC. So don't ever do this.
     * You should never need more than one TargetControllerService object in a single block.
     */

    // Don't ever do this.
    auto anotherTcService = Services::TargetControllerService();

    // These operations will **NOT** be part of the atomic session, and they will cause a deadlock and timeout.
    anotherTcService.writeMemory(...);
    anotherTcService.readMemory(...);

    /*
     * One more thing: The AtomicSession object should **NEVER** outlive the TargetControllerService object that
     * created it.
     *
     * If this happens, the AtomicSession will have a dangling reference, which will result in UB.
     */
}

// At this point, the atomic session will have ended. The TC will now process any other commands in the queue.
tcService.readMemory(...); // Will not be part of the atomic session

Programming mode

When a component needs to write to the target's program memory, it must enable programming mode on the target. This can be done by issuing the EnableProgrammingMode command to the TargetController (see TargetControllerService::enableProgrammingMode()).

Once programming mode has been enabled, standard debugging operations such as program flow control and RAM access will become unavailable. The TargetController will reject any commands that involve these operations, until programming mode has been disabled. The Command::requiresDebugMode() virtual member function communicates a particular command's requirement for the target to not be in programming mode.

For example, the ResumeTargetExecution command returns true here, as it attempts to control program flow on the target, which can only be done when the target is not in programming mode:

class ResumeTargetExecution: public Command
{
public:
    // ...
    [[nodiscard]] bool requiresDebugMode() const override {
        return true;
    }
};

On the other hand, the ReadTargetMemory command will only return true if we're reading from RAM, as RAM is the only memory which isn't accessible when the target is in programming mode:

class ReadTargetMemory: public Command
{
public:
    Targets::TargetMemoryType memoryType;
    Targets::TargetMemoryAddress startAddress;
    Targets::TargetMemorySize bytes;

    // ...

    [[nodiscard]] bool requiresDebugMode() const override {
        return this->memoryType == Targets::TargetMemoryType::RAM;
    }
};

The TargetController will emit ProgrammingModeEnabled and ProgrammingModeDisabled events when it enables/disables programming mode. Components should listen for these events to ensure that they disable any means for the user to trigger debugging operations whilst programming mode is enabled. For example, the Insight component will disable much of its GUI components when programming mode is enabled.

It shouldn't be too much of a problem if a component attempts to perform a debug operation on the target whilst programming mode is enabled, as the TargetController will just respond with an error. But still, it would be best to avoid doing this where possible.

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TODO: Cover debug tool & target drivers.