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| <section id="per-thread-interrupt-controls"> |
| <h1>Per-Thread Interrupt Controls<a class="headerlink" href="#per-thread-interrupt-controls" title="Permalink to this heading"></a></h1> |
| <p>Using NuttX, you will find that the interrupts enabled/disabled state is not a |
| global property. You can not just turn interrupts off and on for all tasks. |
| Rather, enabling and disabling interrupts effects only while the single task |
| that is controlling the interrupts runs. Consider the following sequence:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="n">irqstate_t</span><span class="w"> </span><span class="n">flags</span><span class="p">;</span> |
| |
| <span class="n">flags</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">irqsave</span><span class="p">();</span><span class="w"> </span><span class="cm">/* Disable interrupts */</span> |
| <span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">);</span><span class="w"> </span><span class="cm">/* Sleep for 5 seconds */</span> |
| <span class="n">irqrestore</span><span class="p">(</span><span class="n">flags</span><span class="p">);</span><span class="w"> </span><span class="cm">/* Re-enable interrupts */</span> |
| </pre></div> |
| </div> |
| <p>What happens while the task sleeps? Does that mean that interrupts will be |
| disabled for five seconds? No, interrupts will (probably) be re-enabled while |
| the task is sleeping. How does this work?</p> |
| <p>It is really very simple. Each time a context switches occurs, a set of |
| registers are saved for the task that is being suspended. Then those registers |
| are restored from the previously saved registers for a next task that will run. |
| This is why we often describe a context switch as just setjmp/longjmp on steroids: |
| A context switch works just like setjmp (save a set of registers) and longjmp |
| (restore a set of registers), except that more registers are saved and restored.</p> |
| <p>For the the ARMv7-M, as an example, you can see the set of registers that are |
| stored in <code class="docutils literal notranslate"><span class="pre">arch/arm/include/armv7-m/irq.h</span></code></p> |
| <p>Among those registers are saved and restore are the register(s) that determine if |
| interrupts are enable or not. For the ARMv7-M family that is either the <code class="docutils literal notranslate"><span class="pre">PRIMASK</span></code> |
| register or the <code class="docutils literal notranslate"><span class="pre">BASEPRI</span></code> registers. So if a task disables interrupts then suspends, |
| the current value of <code class="docutils literal notranslate"><span class="pre">PRIMASK</span></code>/<code class="docutils literal notranslate"><span class="pre">BASEPRI</span></code> register is saved and replaced with the |
| stored value of the <code class="docutils literal notranslate"><span class="pre">PRIMASK</span></code>/<code class="docutils literal notranslate"><span class="pre">BASEPRI</span></code> register for the next task that will run, |
| thus re-enabling interrupts while the rist task is suspended.</p> |
| <p>So interrupt enabled/disable is a per-thread property, not a global property. |
| If you have been working with bare metal systems for a long time, this might seem |
| foreign to you.</p> |
| <p>By the way, locking the scheduler via <code class="docutils literal notranslate"><span class="pre">sched_lock()</span></code> behaves in this same way |
| (but the mechanism is a little different).</p> |
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