Interrupts

Prerequisites

  1. Set up your machine.
  2. Notification tutorial

Initialising

# For instructions about obtaining the tutorial sources see https://docs.sel4.systems/Tutorials/#get-the-code
#
# Follow these instructions to initialise the tutorial
# initialising the build directory with a tutorial exercise
./init --tut interrupts
# building the tutorial exercise
cd interrupts_build
ninja

Outcomes

  • Understand the purpose of the IRQControl capability.
  • Be able to obtain capabilities for specific interrupts.
  • Learn how to handle interrupts and their relation with notification objects.

Background

IRQControl

The root task is given a single capability from which capabilities to all irq numbers in the system can be derived, seL4_CapIRQControl. This capability can be moved between CSpaces and CSlots but cannot be duplicated. Revoking this capability results in all access to all irq capabilities being removed.

IRQHandlers

IRQHandler capabilities give access to a single irq and are standard seL4 capabilities: they can be moved and duplicated according to system policy. IRQHandlers are obtained by invoking the IRQControl capability, with architecture specific parameters. Below is an example of obtaining an IRQHandler.

// Get a capability for irq number 7 and place it in cslot 10 in a single-level cspace.
error = seL4_IRQControl_Get(seL4_IRQControl, 7, cspace_root, 10, seL4_WordBits);

There are a variety of different invocations to obtain irq capabilities which are hardware dependent, including:

Receiving interrupts

Interrupts are received by registering a capability to a notification object with the IRQHandler capability for that irq, as follows:

seL4_IRQHandler_SetNotification(irq_handler, notification);

On success, this call will result in signals being delivered to the notification object when an interrupt occurs. To handle multiple interrupts on the same notification object, you can set different badges on the notification capabilities bound to each IRQHandler. When an interrupt arrives, the badge of the notification object bound to that IRQHandler is bitwise orred with the data word in the notification object. Recall the badging technique for differentiating signals from the notification tutorial.

Interrupts can be polled for using seL4_Poll or waited for using seL4_Wait. Either system call results in the data word of the notification object being delivered as the badge of the message, and the data word cleared.

seL4_IRQHandler_Clear can be used to unbind the notification from an IRQHandler.

Handling interrupts

Once an interrupt is received and processed by the software, you can unmask the interrupt using seL4_IRQHandler_Ack on the IRQHandler. seL4 will not deliver any further interrupts after an IRQ is raised until that IRQHandler has been acked.

Exercises

In this tutorial you will set up interrupt handling for a provided timer driver on the zynq7000 ARM platform. This timer driver can be located inside the projects/sel4-tutorials/zynq_timer_driver folder from the root of the projects directory, i.e. where the .repo folder can be found and where the initial repo init command was executed. The tutorial has been set up with two processes: timer.c, the timer driver and RPC server, and client.c, which makes a single request.

On successful initialisation of the tutorial, you will see the following:

timer client: hey hey hey
timer: got a message from 61 to sleep 2 seconds
<<seL4(CPU 0) [decodeInvocation/530 T0xe8265600 "tcb_timer" @84e4]: Attempted to invoke a null cap #9.>>
main@timer.c:78 [Cond failed: error]
	Failed to ack irq

The timer driver we are using emits an interrupt in the TTC0_TIMER1_IRQ number.

Exercise Invoke irq_control, which contains the seL4_IRQControl capability, the place the IRQHandler capability for TTC0_TIMER1_IRQ into the irq_handler CSlot.

    /* TODO invoke irq_control to put the interrupt for TTC0_TIMER1_IRQ in
       cslot irq_handler (depth is seL4_WordBits) */

On success, you should see the following output, without the error message that occurred earlier, as the irq_handle capability is now valid:

Undelivered IRQ: 42

This is a warning message from the kernel that an IRQ was recieved for irq number 42, but no notification capability is set to sent a signal to.

Exercise Now set the notification capability (ntfn) by invoking the irq handler.

     /* TODO set ntfn as the notification for irq_handler */

Now the output will be:

Tick

Only one interrupt is delivered, as the interrupt has not been acknowledged. The timer driver is programmed to emit an interrupt every millisecond, so we need to count 2000 interrupts before replying to the client.

Exercise Acknowledge the interrupt after handling it in the timer driver.

        /* TODO ack the interrupt */

Now the timer interrupts continue to come in, and the reply is delivered to the client.

timer client wakes up

That’s it for this tutorial.

Getting help

Stuck? See the resources below.

Tutorial included from github repo edit