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| <section id="removing-device-drivers-with-nsh"> |
| <h1>Removing Device Drivers with NSH<a class="headerlink" href="#removing-device-drivers-with-nsh" title="Permalink to this heading"></a></h1> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>Migrated from: |
| <a class="reference external" href="https://cwiki.apache.org/confluence/display/NUTTX/Removing+Device+Drivers+with+NSH">https://cwiki.apache.org/confluence/display/NUTTX/Removing+Device+Drivers+with+NSH</a></p> |
| </div> |
| <section id="nuttx-and-unix-like-operating-systems-compared"> |
| <h2>NuttX and Unix-like Operating Systems Compared<a class="headerlink" href="#nuttx-and-unix-like-operating-systems-compared" title="Permalink to this heading"></a></h2> |
| <p>There are many things that are called device drivers. In this context, the |
| discussion is limited to <strong>character device drivers</strong>. In NuttX, character |
| device drivers are represented by device driver nodes in the top-level <a class="reference internal" href="../components/filesystem/pseudofs.html"><span class="doc">pseudo |
| filesystem</span></a>.</p> |
| <p>Standard Unix-like operating systems also support device driver nodes, which |
| superficially resemble NuttX device driver nodes: Both look like files and |
| usually reside under the top-level <code class="docutils literal notranslate"><span class="pre">/dev</span></code> directory. Both can be accessed |
| with standard POSIX file system commands such as <code class="docutils literal notranslate"><span class="pre">open()</span></code>, <code class="docutils literal notranslate"><span class="pre">close()</span></code>, |
| <code class="docutils literal notranslate"><span class="pre">read()</span></code>, <code class="docutils literal notranslate"><span class="pre">write()</span></code>, and so forth. However, the similarity ends there.</p> |
| <p>The payload of a standard Unix-like operating system device driver node is a |
| device major and minor number. These major and minor device numbers are used to |
| look up the actual device driver interface using internal OS logic and data |
| structures. A NuttX device node, by contrast, directly holds the device driver |
| interface with no intervening lookup. This design is less flexible, but it is |
| more efficient and conserves limited resources in an embedded system.</p> |
| <p>In standard Unix-like operating systems, the device node can simply be deleted |
| using the shell command <code class="docutils literal notranslate"><span class="pre">rm</span></code> or the programmatic interface <code class="docutils literal notranslate"><span class="pre">unlink()</span></code>. The |
| node is removed, and nothing special happens to the underlying device driver |
| (except that it may no longer be accessible).</p> |
| <p>In NuttX, if the device node were removed in the same way, the entire device |
| interface would also be removed, effectively breaking the driver. Internally, |
| NuttX supports a function called <code class="docutils literal notranslate"><span class="pre">unregister_driver()</span></code> that can be invoked |
| to remove a device driver. Therefore, removing the device driver node must |
| behave as though <code class="docutils literal notranslate"><span class="pre">unregister_driver()</span></code> were called.</p> |
| </section> |
| <section id="the-unlink-method"> |
| <h2>The unlink() Method<a class="headerlink" href="#the-unlink-method" title="Permalink to this heading"></a></h2> |
| <p>How is this accomplished in NuttX? It is done via a special device driver |
| method called <code class="docutils literal notranslate"><span class="pre">unlink()</span></code>.</p> |
| <p>NuttX device drivers are implemented via a vtable of function pointers. That |
| vtable defines the interface between the pseudo-file system and the device |
| driver. This vtable is the structure <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">file_operations</span></code> defined in |
| <code class="docutils literal notranslate"><span class="pre">[nuttx]/include/nuttx/fs/fs.h</span></code>. It provides several interfaces that closely |
| match the standard POSIX interfaces—<code class="docutils literal notranslate"><span class="pre">open()</span></code>, <code class="docutils literal notranslate"><span class="pre">close()</span></code>, <code class="docutils literal notranslate"><span class="pre">read()</span></code>, |
| <code class="docutils literal notranslate"><span class="pre">write()</span></code>, etc.—and also includes a method called <code class="docutils literal notranslate"><span class="pre">unlink()</span></code>. This |
| <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> method is called by the NuttX VFS when a user removes a device |
| driver node.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>Removal of device driver nodes is only permitted if |
| <code class="docutils literal notranslate"><span class="pre">CONFIG_DISABLE_PSEUDOFS_OPERATIONS</span></code> is <strong>not</strong> defined. All pseudo-file |
| system operations may be suppressed to reduce the FLASH footprint in systems |
| with extremely limited resources.</p> |
| </div> |
| </section> |
| <section id="removing-a-device-node-from-nsh"> |
| <h2>Removing a Device Node from NSH<a class="headerlink" href="#removing-a-device-node-from-nsh" title="Permalink to this heading"></a></h2> |
| <p>Below is a summary of what happens when a device node is deleted using the NSH |
| <code class="docutils literal notranslate"><span class="pre">rm</span></code> command:</p> |
| <ol class="arabic simple"> |
| <li><p>The user enters the <code class="docutils literal notranslate"><span class="pre">rm</span></code> command. The NSH parser recognizes the command |
| and transfers control to the NSH function <code class="docutils literal notranslate"><span class="pre">cmd_rm()</span></code>.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">cmd_rm()</span></code> verifies the command, then calls the standard POSIX |
| <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> interface. The logic in the VFS <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> function in |
| <code class="docutils literal notranslate"><span class="pre">[nuttx]/fs/vfs/fs_unlink.c</span></code> is then executed.</p></li> |
| <li><p>The VFS <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> detects that the target to be removed is a device node |
| in the top-level pseudo-file system. It calls the device driver’s |
| <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> method. It also removes the device node from the |
| pseudo-filesystem. However, the underlying resources required to support |
| the device driver interface may remain until the device driver frees those |
| resources.</p></li> |
| <li><p>When the device driver’s <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> method is called, it determines if |
| the device resources can be freed immediately. If so, it frees those |
| resources. If, for example, there are still open references to the device |
| driver, it may defer freeing the resources until the last client has closed |
| the device driver and there are no open references. In such a case, it may |
| set a flag indicating that the device driver has been unlinked.</p></li> |
| <li><p>If freeing of device driver resources has been deferred, that flag will be |
| examined later. For instance, when the last client of the device driver |
| closes its reference to the driver, it checks whether the unlink operation |
| was deferred. If so, it frees any remaining device driver resources at that |
| time.</p></li> |
| </ol> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>Some character device driver instances do not implement the <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> |
| method. If problems arise when attempting to remove character drivers as |
| described in this Wiki page, a missing <code class="docutils literal notranslate"><span class="pre">unlink()</span></code> method is the most |
| likely cause.</p> |
| </div> |
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