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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">&amp;</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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	<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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