High Performance: Zero Latency Interrupts, Maskable Nested Interrupts

Generic Interrupt Handling

NuttX includes a generic interrupt handling subsystem that makes it convenient to deal with interrupts using only IRQ numbers. In order to integrate with this generic interrupt handling system, the platform specific code is expected to collect all thread state into a container, struct xcptcontext. This container represents the full state of the thread and can be saved, restored, and exchanged as a unit of thread.

While this state saving has many useful benefits, it does require processing time. It was reported to me that this state saving required about two microseconds on an STM32F4Discovery board. That added interrupt latency might be an issue in some circumstances.

In addition, critical sections that are required in various places throughout the RTOS can pause interrupt handling momentarily. This increases the latency for those interrupts which become pending during a critical section. As this is likely to occur for some instances of an interrupt and not others, the interrupt latency varies from time to time (experiences jitter). Like the added latency discussed above, that jitter might be an issue in some circumstances.

Terminology: The concepts discussed in this guide are not unique to NuttX. Other RTOSes have similar concepts but will use different terminology. The Nucleus RTOS, for example, uses the terms Native and Managed interrupts.

Bypassing the Generic Interrupt Handling

Most modern MCUs (such as the ARM Cortex-M family) receive and dispatch interrupts through a vector table. The vector table is a table in memory. Each entry in the table holds the address of an interrupt handler corresponding to different interrupts. When the interrupt occurs, the hardware fetches the corresponding interrupt handler address and gives control to the interrupt handler.

In the implementation of the generic interrupt handler, these vectored interrupts are not used as intended by the hardware designer. Rather, they are used to obtain an IRQ number and then to transfer control to the common, generic interrupt handling logic.

One way to achieve higher performance interrupts and still retain the benefits of the generic interrupt handling logic is to simply replace an interrupt handler address in the vector table with a different interrupt handler; one that does not vector to the generic interrupt handling logic logic, but rather to your custom code.

Often, the vector table is in ROM. So you can hard-code a special interrupt vector by modifying the ROM vector table so that the specific entry points to your custom interrupt handler. Or, if the architecture permits, you can use a vector table in RAM. Then you can freely attach and detach custom vector handlers by writing directly to the vector table. The ARM Cortex-M port provides interfaces to support this mode when the CONFIG_ARCH_RAMVECTORS option is enabled.

So what is the downside? There are two:

  • Your custom interrupt handler will not have collected its state into the struct xcptcontext container. Therefore, it cannot communicate with operating system. Your custom interrupt handler has been taken “out of the game” and can no longer work with the system.

  • If your custom interrupt is truly going to be high performance then you will also have to support nested interrupts! The custom interrupt must have a high priority and must be able interrupt the generic interrupt handling logic. Otherwise, it will be occasionally delayed when there is a collision between your custom interrupt and other, lower priority interrupts.

Getting Back into the Game

As mentioned, the custom interrupt handler cannot use most of the services of the OS since it has not created a struct xcptcontext container. So it needs a mechanism to “get back into the game” when it needs to interact with the operating system to, for example, post a semaphore, signal a thread, or send a message.

The ARM Cortex-M family supports a special way to do this using the PendSV interrupt:

  • The custom logic would connect with the PendSV interrupt using the standard irq_attach() interface.

  • In the custom interrupt handler, it would schedule the PendSV interrupt when it needs to communicate with the OS.

  • The PendSV interrupt is dispatched through the generic interrupt system so when the attached PendSV interrupt is handled, it will be in a context where it can perform any necessary OS interactions.

With the ARMv7_M architecture, the PendSV interrupt can be generated with:

up_trigger_irq(NVIC_IRQ_PENDSV);

On other architectures, it may be possible to do something like a software interrupt from the custom interrupt handler to accomplish the same thing.

The custom logic would be needed to communicate the events of interest between the high priority interrupt handler and PendSV interrupt handler. A detailed discussion of that custom logic is beyond the scope of this Wiki page.

The following table shows the priority levels of the Cortex-M family:

IRQ type               Priority
Dataabort              0x00
High prio IRQ1         0x20   (Zero-latency interrupt)
High prio IRQ2         0x30   (Can't call OS API in ISR)
SVC                    0x70
Disable IRQ            0x80
(critical-section)
Low  prio IRQ          0xB0
PendSV                 0xE0

Lower priority numbers mean a higher priority on this architecture.

As you can see, the zero-latency interrupts have higher priority than the critical section and SVC, but with the tradeoff that High prio IRQ can’t call OS APIs in ISR.

Maskable Nested Interrupts

The ARM Cortex-M family supports a feature called BASEPRI that can be used to disable interrupts at a priority level below a certain level. This feature can be used to support maskable nested interrupts.

Maskable nested interrupts differ from zero-latency interrupts in that they obey the interrupt masking mechanisms of the system. For example, setting the BASEPRI register to a specific threshold will block all interrupts of a lower or equal priority. However, high-priority interrupts (such as Non-Maskable Interrupts or zero-latency interrupts) are unaffected by these masks.

This is useful when you have a high-priority interrupt that needs to be able to interrupt the system, but you also have lower-priority interrupts that you want to be able to mask.

The following table shows the priority levels of the Cortex-M family:

IRQ type                Priority
Dataabort               0x00
SVC                     0x70
Disable IRQ             0x80
(critical-section)
High prio IRQ1          0x90    (Maskable nested interrupt)
High prio IRQ2          0xA0    (Can call OS API in ISR)
Low  prio IRQ           0xB0
PendSV                  0xE0

Lower priority numbers mean a higher priority on this architecture.

As you can see, the priority levels of the maskable nested interrupts are between the critical section and the low-priority interrupts. In this case, High prio IRQ can call OS APIs in ISR.

Nested Interrupt Handling

Some general notes about nested interrupt handling are provided in Nested Interrupts. In this case, handling the nested custom interrupt is simpler because the generic interrupt handler is not re-entered. Rather, the generic interrupt handler must simply be made to co-exist with the custom interrupt interrupt handler.

Modifications may be required to the generic interrupt handling logic to accomplish. A few points need to be made here:

  • The MCU should support interrupt prioritization so that the custom interrupt can be scheduled with a higher priority.

  • The generic interrupt handlers currently disable interrupts during interrupts. Instead, they must be able to keep the custom interrupt enabled throughout interrupt process but still prevent re-entrancy by other standard interrupts (This can be done by setting an interrupt base priority level in the Cortex-M family).

  • The custom interrupt handler can now interrupt the generic interrupt handler at any place. Is the logic safe in all cases to be interrupted? Sometimes interrupt handlers place the MCU in momentarily perverse states while registers are being manipulated. Make sure that it is safe to take interrupts at any time (or else keep the interrupts disabled in the critical times).

  • Will the custom interrupt handler have all of the resources it needs in place when it occurs? Will it have a valid stack pointer? (In the Cortex-M implementation, for example, the MSP may not be valid when the custom interrupt handler is entered).

Some of these issues are complex and so you should expect some complexity in getting the nested interrupt handler to work.

Cortex-M3/4 Implementation

Such high priority, nested interrupt handler has been implemented for the Cortex-M3/4 families.

The following paragraphs will summarize that implementation.

Configuration Options

CONFIG_ARCH_HIPRI_INTERRUPT

The OS disables interrupts by setting the BASEPRI register to NVIC_SYSH_DISABLE_PRIORITY so that most interrupts will not have execution priority. SVCall must have execution priority in all cases.

In the normal cases, interrupts are not nest-able and all interrupts run at an execution priority between NVIC_SYSH_PRIORITY_MIN and NVIC_SYSH_PRIORITY_MAX (with NVIC_SYSH_PRIORITY_MAX reserved for SVCall).

If, in addition, CONFIG_ARCH_HIPRI_INTERRUPT is defined, then special high priority interrupts are supported. These are not “nested” in the normal sense of the word. These high priority interrupts can interrupt normal processing but execute outside of OS (although they can “get back into the game” via a PendSV interrupt).

Disabling the High Priority Interrupt

In the normal course of things, interrupts must occasionally be disabled using the up_irq_save() inline function to prevent contention in use of resources that may be shared between interrupt level and non-interrupt level logic. Now the question arises, if we are using the BASEPRI to disable interrupts and have high priority interrupts enabled (CONFIG_ARCH_HIPRI_INTERRUPT=y), do we disable all interrupts except SVCall (we cannot disable SVCall interrupts)? Or do we only disable the “normal” interrupts?

If we are using the BASEPRI register to disable interrupts, then the answer is that we must disable ONLY the normal interrupts. That is because we cannot disable SVCall interrupts and we cannot permit SVCall interrupts running at a higher priority than the high priority interrupts. Otherwise, they will introduce jitter in the high priority interrupt response time.

Hence, if you need to disable the high priority interrupt, you will have to disable the interrupt either at the peripheral that generates the interrupt or at the interrupt controller, the NVIC. Disabling global interrupts via the BASEPRI register must not be allowed to affect high priority interrupts.

Dependencies

  • CONFIG_ARCH_HAVE_IRQPRIO. Support for prioritized interrupt support must be enabled.

  • Floating Point Registers. If used with a Cortex-M4 that supports hardware floating point, you cannot use hardware floating point in the high priority interrupt handler UNLESS you use the common vector logic that supports saving of floating point registers on all interrupts.

Configuring High Priority Interrupts

How do you specify a high priority interrupt? You need to do two things:

First, You need to change the address in the vector table so that the high priority interrupt vectors to your special C interrupt handler. There are two ways to do this:

  • If you select CONFIG_ARCH_RAMVECTORS, then vectors will be kept in RAM and the system will support the interface: int up_ramvec_attach(int irq, up_vector_t vector). That interface can be used to attach your C interrupt handler to the vector at run time.

  • Alternatively, you could keep your vectors in FLASH but in order to this, you would have to develop your own custom vector table.

Second, you need to set the priority of your interrupt in NVIC to NVIC_SYSH_HIGH_PRIORITY using the standard interface: int up_prioritize_irq(int irq, int priority);

Example Code

You can find an example that tests the high priority, nested interrupts in the NuttX source:

  • ViewTool STM32F103/F107 Description of the configuration

  • nuttx/boards/arm/stm32/viewtool-stm32f107/highpri Test configuration

  • nuttx/boards/arm/stm32/viewtool-stm32f107/src/stm32_highpri Test driver.