CAmkES Timer Tutorial


This exercise is to set up a timer driver in CAmkES and use it to delay for 2 seconds.


We’ll be working within apps/hello-camkes-timer for this tutorial.

make arm_hello-camkes-timer_defconfig



Start in hello-camkes-timer.camkes.

Instantiate some components. You’re already given one component instance

  • client. You need to instantiate two more - a timer driver, and a component instance representing the timer hardware itself. Look in components/Timer/Timer.camkes for the definitions of both of these components.

Note the lines connection seL4RPCCall hello_timer(from client.hello, to timer.hello); and timer.sem_value = 0;. They assume that the name of the timer ‘‘driver’’ will be timer. If you wish to call your driver something else, you’ll have to change these lines.


Connect timer driver to timer hardware. The timer hardware component exposes two interfaces which must be connected to the timer driver. One of these represents memory-mapped registers. The other represents an interrupt.


Configure the timer hardware component instance with device-specific info. The physical address of the timer’s memory-mapped registers, and its irq number must both be configured.


Now open components/Timer/src/timer.c.

We’ll start by completing the irq_handle function, which is called in response to each timer interrupt. Note the name of this function. It follows the naming convention <interface>_handle, where <interface> is the name of an IRQ interface connected with seL4HardwareInterrupt. When an interrupt is received on the interface <interface>, the function <interface>_handle will be called.

The implementation of the timer driver itself isn’t directly in this file. The driver is implemented in a CAmkES-agnostic way in a library called libplatsupport.

This task is to call the timer_handle_irq function from libplatsupport, to inform the driver that an interrupt has occurred.


Acknowledge the interrupt. CAmkES generates the seL4-specific code for ack-ing an interrupt and provides a function <interface>_acknowldege for IRQ interfaces (specifically those connected with seL4HardwareInterrupt).


Now we’ll complete hello__init - a function which is called once before the component’s interfaces start running.

We need to initialise a timer driver from libplatsupport for this device, and store a handle to the driver in the global variable timer_drv.


Note that this task is to understand the existing code. You won’t have to modify any files.

Implement the timer_inf RPC interface. This interface is defined in interfaces/timer.camkes, and contains a single method, sleep, which should return after a given number of seconds. in components/Timer/Timer.camkes, we can see that the timer_inf interface exposed by the Timer component is called hello. Thus, the function we need to implement is called hello_sleep.


Tell the timer to interrupt after the given number of seconds. The timer_oneshot_relative function from libplatsupport will help. Note that it expects its time argument to be given in nanoseconds.

Note the existing code in hello_sleep. It waits on a binary semaphore. irq_handle will be called on another thread when the timer interrupt occurs, and that function will post to the binary semaphore, unblocking us and allowing the function to return after the delay.


Build and run with:

make simulate

Expect the following output with a 2 second delay between the last 2 lines:

Starting the client
------Sleep for 2 seconds------
After the client: wakeup