Interrupts

An interrupt service routine (ISR) is a function that executes asynchronously in response to a hardware or software interrupt. An ISR normally preempts the execution of the current thread, allowing the response to occur with very low overhead. Thread execution resumes only once all ISR work has been completed.

Concepts

Any number of ISRs can be defined, subject to the constraints imposed by underlying hardware.

An ISR has the following key properties:

  • An interrupt request (IRQ) signal that triggers the ISR.
  • A priority level associated with the IRQ.
  • An interrupt handler function that is invoked to handle the interrupt.
  • An argument value that is passed to that function.

An IDT or a vector table is used to associate a given interrupt source with a given ISR. Only a single ISR can be associated with a specific IRQ at any given time.

Multiple ISRs can utilize the same function to process interrupts, allowing a single function to service a device that generates multiple types of interrupts or to service multiple devices (usually of the same type). The argument value passed to an ISR’s function allows the function to determine which interrupt has been signaled.

The kernel provides a default ISR for all unused IDT entries. This ISR generates a fatal system error if an unexpected interrupt is signaled.

The kernel supports interrupt nesting. This allows an ISR to be preempted in mid-execution if a higher priority interrupt is signaled. The lower priority ISR resumes execution once the higher priority ISR has completed its processing.

An ISR’s interrupt handler function executes in the kernel’s interrupt context. This context has its own dedicated stack area (or, on some architectures, stack areas). The size of the interrupt context stack must be capable of handling the execution of multiple concurrent ISRs if interrupt nesting support is enabled.

Important

Many kernel APIs can be used only by threads, and not by ISRs. In cases where a routine may be invoked by both threads and ISRs the kernel provides the k_is_in_isr() API to allow the routine to alter its behavior depending on whether it is executing as part of a thread or as part of an ISR.

Multi-level Interrupt handling

A hardware platform can support more interrupt lines than natively-provided through the use of one or more nested interrupt controllers. Sources of hardware interrupts are combined into one line that is then routed to the parent controller.

If nested interrupt controllers are supported, CONFIG_MULTI_LEVEL_INTERRUPTS should be set to 1, and CONFIG_2ND_LEVEL_INTERRUPTS and CONFIG_3RD_LEVEL_INTERRUPTS configured as well, based on the hardware architecture.

A unique 32-bit interrupt number is assigned with information embedded in it to select and invoke the correct Interrupt Service Routine (ISR). Each interrupt level is given a byte within this 32-bit number, providing support for up to four interrupt levels using this arch, as illustrated and explained below:

          9             2   0
    _ _ _ _ _ _ _ _ _ _ _ _ _         (LEVEL 1)
  5       |         A   |
_ _ _ _ _ _ _         _ _ _ _ _ _ _   (LEVEL 2)
  |   C                       B
_ _ _ _ _ _ _                         (LEVEL 3)
        D

There are three interrupt levels shown here.

  • ‘-‘ means interrupt line and is numbered from 0 (right most).
  • LEVEL 1 has 12 interrupt lines, with two lines (2 and 9) connected to nested controllers and one device ‘A’ on line 4.
  • One of the LEVEL 2 controllers has interrupt line 5 connected to a LEVEL 3 nested controller and one device ‘C’ on line 3.
  • The other LEVEL 2 controller has no nested controllers but has one device ‘B’ on line 2.
  • The LEVEL 3 controller has one device ‘D’ on line 2.

Here’s how unique interrupt numbers are generated for each hardware interrupt. Let’s consider four interrupts shown above as A, B, C, and D:

A -> 0x00000004
B -> 0x00000302
C -> 0x00000409
D -> 0x00030609

Note

The bit positions for LEVEL 2 and onward are offset by 1, as 0 means that interrupt number is not present for that level. For our example, the LEVEL 3 controller has device D on line 2, connected to the LEVEL 2 controller’s line 5, that is connected to the LEVEL 1 controller’s line 9 (2 -> 5 -> 9). Because of the encoding offset for LEVEL 2 and onward, device D is given the number 0x00030609.

Preventing Interruptions

In certain situations it may be necessary for the current thread to prevent ISRs from executing while it is performing time-sensitive or critical section operations.

A thread may temporarily prevent all IRQ handling in the system using an IRQ lock. This lock can be applied even when it is already in effect, so routines can use it without having to know if it is already in effect. The thread must unlock its IRQ lock the same number of times it was locked before interrupts can be once again processed by the kernel while the thread is running.

Important

The IRQ lock is thread-specific. If thread A locks out interrupts then performs an operation that allows thread B to run (e.g. giving a semaphore or sleeping for N milliseconds), the thread’s IRQ lock no longer applies once thread A is swapped out. This means that interrupts can be processed while thread B is running unless thread B has also locked out interrupts using its own IRQ lock. (Whether interrupts can be processed while the kernel is switching between two threads that are using the IRQ lock is architecture-specific.)

When thread A eventually becomes the current thread once again, the kernel re-establishes thread A’s IRQ lock. This ensures thread A won’t be interrupted until it has explicitly unlocked its IRQ lock.

Alternatively, a thread may temporarily disable a specified IRQ so its associated ISR does not execute when the IRQ is signaled. The IRQ must be subsequently enabled to permit the ISR to execute.

Important

Disabling an IRQ prevents all threads in the system from being preempted by the associated ISR, not just the thread that disabled the IRQ.

Offloading ISR Work

An ISR should execute quickly to ensure predictable system operation. If time consuming processing is required the ISR should offload some or all processing to a thread, thereby restoring the kernel’s ability to respond to other interrupts.

The kernel supports several mechanisms for offloading interrupt-related processing to a thread.

  • An ISR can signal a helper thread to do interrupt-related processing using a kernel object, such as a fifo, lifo, or semaphore.
  • An ISR can signal an alert which causes the system workqueue thread to execute an associated alert handler function. (See Alerts.)
  • An ISR can instruct the system workqueue thread to execute a work item. (See TBD.)

When an ISR offloads work to a thread, there is typically a single context switch to that thread when the ISR completes, allowing interrupt-related processing to continue almost immediately. However, depending on the priority of the thread handling the offload, it is possible that the currently executing cooperative thread or other higher-priority threads may execute before the thread handling the offload is scheduled.

Implementation

Defining a regular ISR

An ISR is defined at runtime by calling IRQ_CONNECT. It must then be enabled by calling irq_enable().

Important

IRQ_CONNECT() is not a C function and does some inline assembly magic behind the scenes. All its arguments must be known at build time. Drivers that have multiple instances may need to define per-instance config functions to configure each instance of the interrupt.

The following code defines and enables an ISR.

#define MY_DEV_IRQ  24       /* device uses IRQ 24 */
#define MY_DEV_PRIO  2       /* device uses interrupt priority 2 */
/* argument passed to my_isr(), in this case a pointer to the device */
#define MY_ISR_ARG  DEVICE_GET(my_device)
#define MY_IRQ_FLAGS 0       /* IRQ flags. Unused on non-x86 */

void my_isr(void *arg)
{
   ... /* ISR code */
}

void my_isr_installer(void)
{
   ...
   IRQ_CONNECT(MY_DEV_IRQ, MY_DEV_PRIO, my_isr, MY_ISR_ARG, MY_IRQ_FLAGS);
   irq_enable(MY_DEV_IRQ);
   ...
}

Since the IRQ_CONNECT macro requires that all its parameters be known at build time, in some cases this may not be acceptable. It is also possible to install interrupts at runtime with irq_connect_dynamic(). It is used in exactly the same way as IRQ_CONNECT:

void my_isr_installer(void)
{
   ...
   irq_connect_dynamic(MY_DEV_IRQ, MY_DEV_PRIO, my_isr, MY_ISR_ARG,
                       MY_IRQ_FLAGS);
   irq_enable(MY_DEV_IRQ);
   ...
}

Dynamic interrupts require the CONFIG_DYNAMIC_INTERRUPTS option to be enabled. Removing or re-configuring a dynamic interrupt is currently unsupported.

Defining a ‘direct’ ISR

Regular Zephyr interrupts introduce some overhead which may be unacceptable for some low-latency use-cases. Specifically:

  • The argument to the ISR is retrieved and passed to the ISR
  • If power management is enabled and the system was idle, all the hardware will be resumed from low-power state before the ISR is executed, which can be very time-consuming
  • Although some architectures will do this in hardware, other architectures need to switch to the interrupt stack in code
  • After the interrupt is serviced, the OS then performs some logic to potentially make a scheduling decision.

Zephyr supports so-called ‘direct’ interrupts, which are installed via IRQ_DIRECT_CONNECT. These direct interrupts have some special implementation requirements and a reduced feature set; see the definition of IRQ_DIRECT_CONNECT for details.

The following code demonstrates a direct ISR:

#define MY_DEV_IRQ  24       /* device uses IRQ 24 */
#define MY_DEV_PRIO  2       /* device uses interrupt priority 2 */
/* argument passed to my_isr(), in this case a pointer to the device */
#define MY_IRQ_FLAGS 0       /* IRQ flags. Unused on non-x86 */

ISR_DIRECT_DECLARE(my_isr)
{
   do_stuff();
   ISR_DIRECT_PM(); /* PM done after servicing interrupt for best latency */
   return 1; /* We should check if scheduling decision should be made */
}

void my_isr_installer(void)
{
   ...
   IRQ_DIRECT_CONNECT(MY_DEV_IRQ, MY_DEV_PRIO, my_isr, MY_IRQ_FLAGS);
   irq_enable(MY_DEV_IRQ);
   ...
}

Installation of dynamic direct interrupts is currently unsupported.

Implementation Details

Interrupt tables are set up at build time using some special build tools. The details laid out here apply to all architectures except x86, which are covered in the x86 Details section below.

Any invocation of IRQ_CONNECT will declare an instance of struct _isr_list which is placed in a special .intList section:

struct _isr_list {
    /** IRQ line number */
    s32_t irq;
    /** Flags for this IRQ, see ISR_FLAG_* definitions */
    s32_t flags;
    /** ISR to call */
    void *func;
    /** Parameter for non-direct IRQs */
    void *param;
};

Zephyr is built in two phases; the first phase of the build produces zephyr_prebuilt.elf which contains all the entries in the .intList section preceded by a header:

struct {
    void *spurious_irq_handler;
    void *sw_irq_handler;
    u32_t num_isrs;
    u32_t num_vectors;
    struct _isr_list isrs[];  <- of size num_isrs
};

This data consisting of the header and instances of struct _isr_list inside zephyr_prebuilt.elf is then used by the gen_isr_tables.py script to generate a C file defining a vector table and software ISR table that are then compiled and linked into the final application.

The priority level of any interrupt is not encoded in these tables, instead IRQ_CONNECT also has a runtime component which programs the desired priority level of the interrupt to the interrupt controller. Some architectures do not support the notion of interrupt priority, in which case the priority argument is ignored.

Vector Table

A vector table is generated when CONFIG_GEN_IRQ_VECTOR_TABLE is enabled. This data structure is used natively by the CPU and is simply an array of function pointers, where each element n corresponds to the IRQ handler for IRQ line n, and the function pointers are:

  1. For ‘direct’ interrupts declared with IRQ_DIRECT_CONNECT, the handler function will be placed here.
  2. For regular interrupts declared with IRQ_CONNECT, the address of the common software IRQ handler is placed here. This code does common kernel interrupt bookkeeping and looks up the ISR and parameter from the software ISR table.
  3. For interrupt lines that are not configured at all, the address of the spurious IRQ handler will be placed here. The spurious IRQ handler causes a system fatal error if encountered.

Some architectures (such as the Nios II internal interrupt controller) have a common entry point for all interrupts and do not support a vector table, in which case the CONFIG_GEN_IRQ_VECTOR_TABLE option should be disabled.

Some architectures may reserve some initial vectors for system exceptions and declare this in a table elsewhere, in which case CONFIG_GEN_IRQ_START_VECTOR needs to be set to properly offset the indices in the table.

SW ISR Table

This is an array of struct _isr_table_entry:

struct _isr_table_entry {
    void *arg;
    void (*isr)(void *);
};

This is used by the common software IRQ handler to look up the ISR and its argument and execute it. The active IRQ line is looked up in an interrupt controller register and used to index this table.

x86 Details

The x86 architecture has a special type of vector table called the Interrupt Descriptor Table (IDT) which must be laid out in a certain way per the x86 processor documentation. It is still fundamentally a vector table, and the gen_idt tool uses the .intList section to create it. However, on APIC-based systems the indexes in the vector table do not correspond to the IRQ line. The first 32 vectors are reserved for CPU exceptions, and all remaining vectors (up to index 255) correspond to the priority level, in groups of 16. In this scheme, interrupts of priority level 0 will be placed in vectors 32-47, level 1 48-63, and so forth. When the gen_idt tool is constructing the IDT, when it configures an interrupt it will look for a free vector in the appropriate range for the requested priority level and set the handler there.

There are some APIC variants (such as MVIC) where priorities cannot be set by the user and the position in the vector table does correspond to the IRQ line. Systems like this will enable CONFIG_X86_FIXED_IRQ_MAPPING.

On x86 when an interrupt or exception vector is executed by the CPU, there is no foolproof way to determine which vector was fired, so a software ISR table indexed by IRQ line is not used. Instead, the IRQ_CONNECT call creates a small assembly language function which calls the common interrupt code in _interrupt_enter() with the ISR and parameter as arguments. It is the address of this assembly interrupt stub which gets placed in the IDT. For interrupts declared with IRQ_DIRECT_CONNECT the parameterless ISR is placed directly in the IDT.

On systems where the position in the vector table corresponds to the interrupt’s priority level, the interrupt controller needs to know at runtime what vector is associated with an IRQ line. gen_idt additionally creates an _irq_to_interrupt_vector array which maps an IRQ line to its configured vector in the IDT. This is used at runtime by IRQ_CONNECT to program the IRQ-to-vector association in the interrupt controller.

For dynamic interrupts, the build must generate some 4-byte dynamic interrupt stubs, one stub per dynamic interrupt in use. The number of stubs is controlled by the CONFIG_X86_DYNAMIC_IRQ_STUBS option. Each stub pushes an unique identifier which is then used to fetch the appropriate handler function and parameter out of a table populated when the dynamic interrupt was connected.

Suggested Uses

Use a regular or direct ISR to perform interrupt processing that requires a very rapid response, and can be done quickly without blocking.

Note

Interrupt processing that is time consuming, or involves blocking, should be handed off to a thread. See Offloading ISR Work for a description of various techniques that can be used in an application.

Configuration Options

Related configuration options:

Additional architecture-specific and device-specific configuration options also exist.

APIs

The following interrupt-related APIs are provided by irq.h:

The following interrupt-related APIs are provided by kernel.h: