46 lines
2.1 KiB
ReStructuredText
46 lines
2.1 KiB
ReStructuredText
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=============================
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Per-Thread Interrupt Controls
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=============================
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Using NuttX, you will find that the interrupts enabled/disabled state is not a
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global property. You can not just turn interrupts off and on for all tasks.
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Rather, enabling and disabling interrupts effects only while the single task
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that is controlling the interrupts runs. Consider the following sequence:
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.. code-block:: C
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irqstate_t flags;
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flags = irqsave(); /* Disable interrupts */
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sleep(5); /* Sleep for 5 seconds */
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irqrestore(flags); /* Re-enable interrupts */
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What happens while the task sleeps? Does that mean that interrupts will be
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disabled for five seconds? No, interrupts will (probably) be re-enabled while
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the task is sleeping. How does this work?
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It is really very simple. Each time a context switches occurs, a set of
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registers are saved for the task that is being suspended. Then those registers
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are restored from the previously saved registers for a next task that will run.
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This is why we often describe a context switch as just setjmp/longjmp on steroids:
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A context switch works just like setjmp (save a set of registers) and longjmp
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(restore a set of registers), except that more registers are saved and restored.
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For the the ARMv7-M, as an example, you can see the set of registers that are
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stored in ``arch/arm/include/armv7-m/irq.h``
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Among those registers are saved and restore are the register(s) that determine if
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interrupts are enable or not. For the ARMv7-M family that is either the ``PRIMASK``
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register or the ``BASEPRI`` registers. So if a task disables interrupts then suspends,
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the current value of ``PRIMASK``/``BASEPRI`` register is saved and replaced with the
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stored value of the ``PRIMASK``/``BASEPRI`` register for the next task that will run,
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thus re-enabling interrupts while the rist task is suspended.
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So interrupt enabled/disable is a per-thread property, not a global property.
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If you have been working with bare metal systems for a long time, this might seem
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foreign to you.
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By the way, locking the scheduler via ``sched_lock()`` behaves in this same way
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(but the mechanism is a little different).
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