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| <section id="stm32-ccm-allocator"> |
| <h1>STM32 CCM Allocator<a class="headerlink" href="#stm32-ccm-allocator" title="Permalink to this heading"></a></h1> |
| <section id="ccm-memory"> |
| <h2>CCM Memory<a class="headerlink" href="#ccm-memory" title="Permalink to this heading"></a></h2> |
| <p>The STM32 F2, F3, and F4 families have a special block of SRAM available called |
| CCM (Core Coupled Memory). This memory has the drawback that it cannot be used |
| for STM32 DMA operations.</p> |
| <p>By default, the CCM memory is lumped in with the rest of memory when the NuttX |
| heaps are created. But this can be a problem because it will be a toss of the |
| coin if non-DMA-able CCM memory or other DMA-able memory gets returned when |
| <code class="docutils literal notranslate"><span class="pre">malloc()</span></code> is called. That usually does not matter but it certainly does make |
| a difference if you are allocating memory that will be used for DMA! In that |
| case, getting CCM memory for your DMA buffer will cause a failure.</p> |
| </section> |
| <section id="config-stm32-ccmexclude"> |
| <h2>CONFIG_STM32_CCMEXCLUDE<a class="headerlink" href="#config-stm32-ccmexclude" title="Permalink to this heading"></a></h2> |
| <p>There is a configuration option called <code class="docutils literal notranslate"><span class="pre">CONFIG_STM32_CCMEXCLUDE</span></code> that can be |
| used to exclude CCM memory from the heap. That solves the problem of getting |
| CCM memory when you want to allocate a DMA buffer. But then what do you do |
| with the CCM memory? Do you let it go unused?</p> |
| </section> |
| <section id="ccm-allocator"> |
| <h2>CCM Allocator<a class="headerlink" href="#ccm-allocator" title="Permalink to this heading"></a></h2> |
| <p>In order to make use of the CCM memory, a CCM memory allocator is available. |
| This memory allocator is automatically enabled when the following options are set:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_STM32_CCMEXCLUDE</span></code> CCM memory is excluded from the normal heap, and</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_MM_MULTIHEAP</span></code> Support for multiple heaps is enabled.</p></li> |
| </ul> |
| <p>Under those conditions, the CCM memory allocator is enabled and the allocator |
| interfaces prototyped in the <code class="docutils literal notranslate"><span class="pre">arch/arm/src/stm32/stm32_ccm.h</span></code> are available.</p> |
| <p>NOTE: These interfaces are, technically, not prototyped since they are really |
| provided via C pre-processor macros.</p> |
| <p>NOTE: In order to use the CCM memory allocator functions, you must first call |
| <code class="docutils literal notranslate"><span class="pre">ccm_initialize()</span></code> somewhere in your early boot-up logic.</p> |
| <p>With these interfaces you have a (nearly) standard way to manage memory from a |
| heap that consists of the the CCM SRAM. And, since the CCM memory is no longer |
| a part of the normal heap, all allocated I/O buffers will be DMA-able (unless you |
| have included other non-DMA-able memory regions in the stack).</p> |
| </section> |
| <section id="ccm-stacks"> |
| <h2>CCM Stacks<a class="headerlink" href="#ccm-stacks" title="Permalink to this heading"></a></h2> |
| <p>One particular problem that has been reported by Petteri Aimonen requires some |
| additional work-arounds. The STM32 SPI driver supports DMA and with SPI it is |
| sometimes necessary to do some very small transfers for which there is no real |
| gain from using DMA. In this case, Petteri has devised a clever way to both 1) make |
| use of the CMM memory and 2) to force fallback to non-DMA transfers for these small |
| stack transfers.</p> |
| <p>Here is what Petteri has done:</p> |
| <ol class="arabic"> |
| <li><p>First, he has modified <code class="docutils literal notranslate"><span class="pre">arch/arm/src/common/up_createstack.c</span></code> and |
| <code class="docutils literal notranslate"><span class="pre">up_releasestack.c</span></code> so that stacks are allocated from CCM memory. That |
| allocation is something like the following:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ccm_zalloc</span><span class="p">(</span><span class="n">size</span><span class="p">);</span> |
| <span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">!</span><span class="n">result</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="cm">/* Fall back to main heap */</span> |
| <span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">zalloc</span><span class="p">(</span><span class="n">size</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>With the matching:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">if</span><span class="w"> </span><span class="p">(((</span><span class="kt">uint32_t</span><span class="p">)</span><span class="n">p</span><span class="w"> </span><span class="o">&</span><span class="w"> </span><span class="mh">0xF0000000</span><span class="p">)</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mh">0x10000000</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">ccm_free</span><span class="p">(</span><span class="n">p</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="k">else</span> |
| <span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">free</span><span class="p">(</span><span class="n">p</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| </pre></div> |
| </div> |
| </li> |
| <li><p>Then Petteri added special DMA support enabled with <code class="docutils literal notranslate"><span class="pre">CONFIG_STM32_DMACAPABLE</span></code>. |
| That option enables an option in all of the DMA logic called:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">bool</span><span class="w"> </span><span class="nf">stm32_dmacapable</span><span class="p">(</span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">maddr</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>That will return true if it is possible to do DMA from the address and false |
| if not.</p> |
| </li> |
| <li><p>Finally, Petteri added logic to the STM32 SPI driver that use <code class="docutils literal notranslate"><span class="pre">stm32_dmacapable()</span></code>: |
| If the address is not DMA capable, then the SPI driver will fall back to |
| non-DMA operation.</p> |
| <p>With Petteri’s changes all of the large I/O buffers will be allocated from |
| DMA-able memory. All stacks will be allocated from non-DMA-able CCM memory |
| (provided that there is space). Small SPI DMA buffers on the non-DMA-able stack |
| will be detected by <code class="docutils literal notranslate"><span class="pre">stm32_dmacapable()</span></code> and in that case, the STM32 SPI driver |
| will fall back and use non-DMA-transfers.</p> |
| <p>From all reports this works quite well.</p> |
| </li> |
| </ol> |
| </section> |
| </section> |
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