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| <li class="toctree-l3 current"><a class="current reference internal" href="#">Notes on AVR context switch</a><ul> |
| <li class="toctree-l4"><a class="reference internal" href="#used-terms-and-context-switch-basics">Used terms and context switch basics</a><ul> |
| <li class="toctree-l5"><a class="reference internal" href="#context-creation">Context creation</a></li> |
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| <section id="notes-on-avr-context-switch"> |
| <h1>Notes on AVR context switch<a class="headerlink" href="#notes-on-avr-context-switch" title="Permalink to this heading"></a></h1> |
| <p>This document describes the ways and circumstances in which context |
| switches happen in AVR MCUs.</p> |
| <section id="used-terms-and-context-switch-basics"> |
| <h2>Used terms and context switch basics<a class="headerlink" href="#used-terms-and-context-switch-basics" title="Permalink to this heading"></a></h2> |
| <section id="context-creation"> |
| <h3>Context creation<a class="headerlink" href="#context-creation" title="Permalink to this heading"></a></h3> |
| <p>There are two ways context is created when a task is suspended. |
| Either the task is suspended in response to a hardware interrupt |
| (context created <code class="docutils literal notranslate"><span class="pre">in-interrupt</span></code> in the following text), or it is |
| suspended voluntarily, eg. by calling sleep(), read() etc. |
| (<code class="docutils literal notranslate"><span class="pre">in-task</span></code> context.)</p> |
| <p>The resulting context is identical and interchangeable with two differences:</p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">SREG</span></code> - <code class="docutils literal notranslate"><span class="pre">in-interrupt</span></code> context has global interrupt enabled (“I”-flag) set</p></li> |
| <li><p>position in the program where the task resumes running (arbitrary point for <code class="docutils literal notranslate"><span class="pre">in-interrupt</span></code> vs. inside <code class="docutils literal notranslate"><span class="pre">up_switch_context()</span></code> for <code class="docutils literal notranslate"><span class="pre">in-task</span></code></p></li> |
| </ul> |
| </div></blockquote> |
| </section> |
| <section id="task-resumption"> |
| <h3>Task resumption<a class="headerlink" href="#task-resumption" title="Permalink to this heading"></a></h3> |
| <p>Task can be resumed in two corresponding situations - context switch |
| in response to a hardware interrupt (<code class="docutils literal notranslate"><span class="pre">by-interrupt</span></code>) or in response |
| to other task relinquishing the CPU (<code class="docutils literal notranslate"><span class="pre">by-task</span></code>)</p> |
| </section> |
| </section> |
| <section id="context-switch"> |
| <h2>Context switch<a class="headerlink" href="#context-switch" title="Permalink to this heading"></a></h2> |
| <p>Two ways of context creation combined with two ways of task resumption |
| give 4 possibilities of context switch process, two of which are |
| not interesting:</p> |
| <section id="st-combination"> |
| <h3>1st combination<a class="headerlink" href="#st-combination" title="Permalink to this heading"></a></h3> |
| <p><code class="docutils literal notranslate"><span class="pre">in-task</span></code> context resumed in <code class="docutils literal notranslate"><span class="pre">by-task</span></code> context switch. Context |
| to be resumed has “I” flag cleared and SREG is restored with that flag cleared. |
| The <code class="docutils literal notranslate"><span class="pre">ret</span></code> instruction is used to resume the task,”returning” to the point |
| where it gave the CPU up, which is inside <code class="docutils literal notranslate"><span class="pre">up_switch_context()</span></code>.</p> |
| <p>This function is supposed to be executed with interrupt disabled (“This |
| function is called only from the NuttX scheduling logic. Interrupts |
| will always be disabled when this function is |
| called.” <a class="reference external" href="https://nuttx.apache.org/docs/latest/reference/os/arch.html">https://nuttx.apache.org/docs/latest/reference/os/arch.html</a> ) Caller |
| of the context switch method is therefore responsible |
| for re-enabling interrupts.</p> |
| </section> |
| <section id="nd"> |
| <h3>2nd<a class="headerlink" href="#nd" title="Permalink to this heading"></a></h3> |
| <p><code class="docutils literal notranslate"><span class="pre">in-interrupt</span></code> resumed in <code class="docutils literal notranslate"><span class="pre">by-interrupt</span></code> context switch. The task |
| essentially (from its point of view) exits interrupt handler after |
| it entered it. Instruction <code class="docutils literal notranslate"><span class="pre">reti</span></code> is used to return from the handler, |
| setting “I”-flag in the process.</p> |
| </section> |
| <section id="rd"> |
| <h3>3rd<a class="headerlink" href="#rd" title="Permalink to this heading"></a></h3> |
| <p>Third and fourth combinations are more interesting:</p> |
| <p><code class="docutils literal notranslate"><span class="pre">in-task</span></code> context resumed in <code class="docutils literal notranslate"><span class="pre">by-interrupt</span></code>. The CPU enters |
| ISR and regular program flow requires returning from ISR and setting |
| “I”-flag by <code class="docutils literal notranslate"><span class="pre">reti</span></code> which does not happen. Task is resumed with |
| interrupts disabled. However, it is resumed inside <code class="docutils literal notranslate"><span class="pre">up_switch_context()</span></code> |
| and caller of that function will set the “I”-flag at some point. |
| Task then runs with interrupts enabled, all is well.</p> |
| </section> |
| <section id="th"> |
| <h3>4th<a class="headerlink" href="#th" title="Permalink to this heading"></a></h3> |
| <p><code class="docutils literal notranslate"><span class="pre">in-interrupt</span></code> resumed in <code class="docutils literal notranslate"><span class="pre">by-task</span></code>, ie. in <code class="docutils literal notranslate"><span class="pre">up_switch_context()</span></code>. |
| <code class="docutils literal notranslate"><span class="pre">reti</span></code> is used to resume the task, setting “I”-flag in the process. |
| This would be incorrect for <code class="docutils literal notranslate"><span class="pre">up_switch_context()</span></code> - it is supposed |
| to run with interrupts disabled - but the task resumes running |
| from the point where it was interrupted, which is not inside |
| of <code class="docutils literal notranslate"><span class="pre">up_switch_context()</span></code>. All is well.</p> |
| </section> |
| </section> |
| <section id="avrdx-core-considerations"> |
| <h2>AVRDx core considerations<a class="headerlink" href="#avrdx-core-considerations" title="Permalink to this heading"></a></h2> |
| <p>Now, all of the above holds true for eg. ATmega chips which control |
| interrupt execution solely by the “I”-flag, allowing the code |
| to not care about where the context switch was triggered. Regardless |
| of that, the MCU will always end up in correct state even if the |
| context switch cause doesn’t match the context being restored |
| (cases 3 and 4.)</p> |
| <p>This is not the case for AVR Dx family which behaves differently. |
| The interrupt controller does respect the “I”-flag in a sense where |
| it considers interrupts disabled when the flag is cleared. However, |
| it is possible that interrupts are not enabled when the flag is set. |
| That depends on a logical AND between “I”-flag and “interrupt handler |
| is not executing” internal state. (Refer to the documentation |
| for more precise explanation.)</p> |
| <p>What this means is that if eg. <code class="docutils literal notranslate"><span class="pre">in-task</span></code> context gets |
| resumed in <code class="docutils literal notranslate"><span class="pre">by-interrupt</span></code> condition (case 3 above), then <code class="docutils literal notranslate"><span class="pre">ret</span></code> |
| instruction is used |
| to resume the task. As discussed above, the “I”-flag is not set this |
| way but that is not a problem, it is eventually set later. However, |
| the internal state “running the interrupt handler” is not cleared. |
| This means that the task keeps running with “global interrupts are |
| enabled” but is actually unable to be interrupted. The context switch |
| code needs to handle this.</p> |
| <p>Conversely, there is a similar problem with <code class="docutils literal notranslate"><span class="pre">in-interrupt</span></code> context |
| being resumed in <code class="docutils literal notranslate"><span class="pre">by-task</span></code> (case 4). Instruction <code class="docutils literal notranslate"><span class="pre">reti</span></code> is used |
| but there is no internal state to be cleared. Unlike the previous case, |
| no problem related to this was observed but it still looks like something |
| the code wants to avoid. It could trigger all sorts of undefined behaviour |
| in the chip otherwise.</p> |
| </section> |
| </section> |
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