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| <section id="frame-buffer-drivers"> |
| <h1>Frame Buffer Drivers<a class="headerlink" href="#frame-buffer-drivers" title="Permalink to this heading"></a></h1> |
| <p>A framebuffer is a memory-mapped buffer that represents all the pixels necessary to drive a video display.</p> |
| <p>The Frame Buffer driver is intended to be used in the following scenarios:</p> |
| <ol class="arabic simple"> |
| <li><p>Whenever it is necessary to hold all the pixels that would be used to drive a video display. This includes:</p> |
| <ol class="arabic simple"> |
| <li><p>Graphics libraries that directly access the underlying framebuffer;</p></li> |
| <li><p>Advanced UIs (e.g. alpha blending) that need to read back the image data;</p></li> |
| </ol> |
| </li> |
| <li><p>Applications that expect the framebuffer to exist;</p></li> |
| </ol> |
| <section id="binding"> |
| <h2>Binding<a class="headerlink" href="#binding" title="Permalink to this heading"></a></h2> |
| <p>LCD and frame buffer drivers usually are not directly accessed by user code, but are usually bound to another, higher-level device driver. |
| In general, the binding sequence is:</p> |
| <ol class="arabic simple"> |
| <li><p>Get an instance of <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">fb_vtable_s</span></code> from the hardware-specific frame buffer device driver, and</p></li> |
| <li><p>Provide that instance to the initialization method of the higher-level device driver.</p></li> |
| </ol> |
| <section id="generic-lcd-frame-buffer"> |
| <span id="genericlcdfb"></span><h3>Generic LCD Frame Buffer<a class="headerlink" href="#generic-lcd-frame-buffer" title="Permalink to this heading"></a></h3> |
| <p>This example will walk through the path from userspace to hardware-specific details on how an LCD screen is bound to a framebuffer.</p> |
| <ol class="arabic"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/nuttx/video/fb.h</span></code> provides all structures and APIs needed to work with frame buffer drivers:</p> |
| <ol class="arabic simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">drivers/video/fb.c</span></code> is the higher-level device driver. An instance of <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">fb_vtable_s</span></code> will be provided to it;</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">fb_register</span></code> registers the framebuffer character device at <code class="docutils literal notranslate"><span class="pre">/dev/fbN</span></code> where N is the display number;</p></li> |
| <li><p>It also provides the prototype of <code class="docutils literal notranslate"><span class="pre">up_fbinitialize</span></code>, which may be defined by:</p> |
| <ol class="arabic simple"> |
| <li><p>An specific device into <code class="docutils literal notranslate"><span class="pre">arch/<arch>/src/<chip></span></code> directory;</p></li> |
| <li><p>By the LCD framebuffer adapter in <code class="docutils literal notranslate"><span class="pre">drivers/lcd/lcd_framebuffer.c</span></code>, which provides an intermediary interface between the Frame Buffer Driver and the LCD screen drivers;</p></li> |
| </ol> |
| </li> |
| </ol> |
| </li> |
| <li><p>Let’s consider we are using the LCD framebuffer (<code class="docutils literal notranslate"><span class="pre">CONFIG_LCD_FRAMEBUFFER</span> <span class="pre">=</span> <span class="pre">y</span></code>):</p> |
| <ol class="arabic simple"> |
| <li><p>This interface implements the <code class="docutils literal notranslate"><span class="pre">up_fbinitialize</span></code> which:</p> |
| <ol class="arabic simple"> |
| <li><p>Provides the instance of <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">fb_vtable_s</span></code> (a member of <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">lcdfb_dev_s</span></code>);</p></li> |
| <li><p>Calls <code class="docutils literal notranslate"><span class="pre">board_lcd_initialize</span></code> and <code class="docutils literal notranslate"><span class="pre">board_lcd_getdev</span></code> LCD-specific functions. These functions are defined in <code class="docutils literal notranslate"><span class="pre">boards/<arch>/<chip>/<board>/src</span></code> and prototyped in <code class="docutils literal notranslate"><span class="pre">include/nuttx/board.h</span></code>;</p></li> |
| </ol> |
| </li> |
| </ol> |
| </li> |
| <li><p>Finally, the LCD screen drivers are usually available at <code class="docutils literal notranslate"><span class="pre">drivers/lcd/</span></code> and implement the callbacks defined at <code class="docutils literal notranslate"><span class="pre">include/nuttx/lcd/lcd.h</span></code>:</p> |
| <blockquote> |
| <div><ol class="arabic simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/nuttx/lcd/lcd.h</span></code> provides structures and APIs needed to work with LCD screens, whereas using the framebuffer adapter or the <a class="reference internal" href="lcd.html"><span class="doc">LCD Character Drivers</span></a>;</p></li> |
| </ol> |
| </div></blockquote> |
| </li> |
| </ol> |
| </section> |
| </section> |
| <section id="vsync"> |
| <h2>VSYNC<a class="headerlink" href="#vsync" title="Permalink to this heading"></a></h2> |
| <p>Vertical synchronization (VSync) synchronizes the frame rate of an application’s graphics with the refresh rate of the display, helping to establish stability. |
| If not synchronized, it may cause screen tearing, which is the effect of the image appearing to have horizontal jagged edges or ghosting across the entire screen.</p> |
| <section id="vsync-offset"> |
| <h3>VSYNC Offset<a class="headerlink" href="#vsync-offset" title="Permalink to this heading"></a></h3> |
| <p>During the VSYNC event, the screen starts displaying frame N while the renderer begins compositing window for frame N+1. |
| The application processes waiting input and generates frame N+2. |
| When the renderer has a short rendering time, it can cause a delay of almost two frames from the end of rendering to the completion of screen display. |
| To solve this problem, <code class="docutils literal notranslate"><span class="pre">FBIOSET_VSYNCOFFSET</span></code> can be used to set the VSYNC offset time (in microseconds) and reduce the delay from input device to screen using the VSYNC offset.</p> |
| </section> |
| </section> |
| <section id="examples"> |
| <h2>Examples<a class="headerlink" href="#examples" title="Permalink to this heading"></a></h2> |
| <p>Examples apply to specific cases of the <a class="reference internal" href="#genericlcdfb"><span class="std std-ref">Generic LCD Frame Buffer</span></a>:</p> |
| <section id="ttgo-t-display-esp32-board"> |
| <span id="ttgotdisplayesp32-fb"></span><h3>TTGO T-Display ESP32 board<a class="headerlink" href="#ttgo-t-display-esp32-board" title="Permalink to this heading"></a></h3> |
| <p>This board contains an ST7789 TFT Display (135x240). |
| By selecting the <code class="docutils literal notranslate"><span class="pre">ttgo_t_display_esp32:lvgl_fb</span></code> config, the <code class="docutils literal notranslate"><span class="pre">lvgldemo</span></code> example will be built with the framebuffer interface.</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">boards/xtensa/esp32/ttgo_t_display_esp32/src/esp32_bringup.c</span></code> registers the framebuffer driver:</p></li> |
| </ul> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#ifdef CONFIG_VIDEO_FB</span> |
| <span class="w"> </span><span class="n">ret</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">fb_register</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span> |
| <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">ret</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">syslog</span><span class="p">(</span><span class="n">LOG_ERR</span><span class="p">,</span><span class="w"> </span><span class="s">"ERROR: Failed to initialize Frame Buffer Driver.</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="cp">#endif</span> |
| </pre></div> |
| </div> |
| <ul> |
| <li><p><code class="docutils literal notranslate"><span class="pre">up_fbinitialize</span></code> from the frame buffer adapter will then be called as <code class="docutils literal notranslate"><span class="pre">CONFIG_LCD_FRAMEBUFFER</span> <span class="pre">=</span> <span class="pre">y</span></code>:</p> |
| <blockquote> |
| <div><ul> |
| <li><p><code class="docutils literal notranslate"><span class="pre">board_lcd_initialize</span></code> and <code class="docutils literal notranslate"><span class="pre">board_lcd_getdev</span></code> are defined at <code class="docutils literal notranslate"><span class="pre">boards/xtensa/esp32/common/src/esp32_st7789.c</span></code>:</p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">board_lcd_initialize</span></code> initializes the LCD hardware on the board by defining the SPI interface which is connected to the display controller;</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">board_lcd_getdev</span></code> calls the <code class="docutils literal notranslate"><span class="pre">st7789_lcdinitialize</span></code> and returns a reference to the LCD object for the specified LCD;</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">st7789_lcdinitialize</span></code> is part of the LCD screen driver at <code class="docutils literal notranslate"><span class="pre">drivers/lcd/st7789.c</span></code>;</p></li> |
| </ul> |
| </div></blockquote> |
| </li> |
| </ul> |
| </div></blockquote> |
| </li> |
| <li><p>The LVGL demo application (<code class="docutils literal notranslate"><span class="pre">lvgldemo</span></code>) makes use of the <code class="docutils literal notranslate"><span class="pre">ioctl</span></code> system call to trigger a <code class="docutils literal notranslate"><span class="pre">FBIO_UPDATE</span></code> request to the higher-level device driver to refresh the LCD screen with framebuffer data:</p></li> |
| </ul> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">ioctl</span><span class="p">(</span><span class="n">state</span><span class="p">.</span><span class="n">fd</span><span class="p">,</span><span class="w"> </span><span class="n">FBIO_UPDATE</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">long</span><span class="p">)((</span><span class="kt">uintptr_t</span><span class="p">)</span><span class="o">&</span><span class="n">fb_area</span><span class="p">));</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="nuttx-simulator"> |
| <h3>NuttX Simulator<a class="headerlink" href="#nuttx-simulator" title="Permalink to this heading"></a></h3> |
| <p><a class="reference internal" href="../../../platforms/sim/sim/index.html"><span class="doc">NuttX Simulator</span></a> provides a X11-based framebuffer driver to simulate the framebuffer usage into a X11-compatible host.</p> |
| <p>By selecting the <code class="docutils literal notranslate"><span class="pre">sim:lvgl_fb</span></code> config, the <code class="docutils literal notranslate"><span class="pre">lvgldemo</span></code> example will be built with the framebuffer driver.</p> |
| <ul> |
| <li><p><code class="docutils literal notranslate"><span class="pre">boards/sim/sim/sim/src/sim_bringup.c</span></code> registers the framebuffer driver the same way <a class="reference internal" href="#ttgotdisplayesp32-fb"><span class="std std-ref">TTGO T-Display ESP32 board</span></a>;</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">arch/sim/src/sim/up_framebuffer.c</span></code> and <code class="docutils literal notranslate"><span class="pre">arch/sim/src/sim/up_x11framebuffer.c</span></code> will be built as <code class="docutils literal notranslate"><span class="pre">CONFIG_SIM_FRAMEBUFFER</span> <span class="pre">=</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">CONFIG_SIM_X11FB</span> <span class="pre">=</span> <span class="pre">y</span></code> are set, respectively;</p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">up_framebuffer.c</span></code> provides <code class="docutils literal notranslate"><span class="pre">up_fbinitialize</span></code> and,</p></li> |
| <li><p>calls <code class="docutils literal notranslate"><span class="pre">up_x11initialize</span></code> from <code class="docutils literal notranslate"><span class="pre">up_x11framebuffer.c</span></code> that initializes a X11-based window as a framebuffer. This is the underlying “driver”.</p></li> |
| </ul> |
| </div></blockquote> |
| </li> |
| <li><p>The LVGL demo application (<code class="docutils literal notranslate"><span class="pre">lvgldemo</span></code>) makes use of the <code class="docutils literal notranslate"><span class="pre">ioctl</span></code> system call to trigger a <code class="docutils literal notranslate"><span class="pre">FBIO_UPDATE</span></code> request to the higher-level device driver in order to refresh the LCD screen with framebuffer data as usual;</p></li> |
| </ul> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>One must consider that framebuffer requires that the entire display’s pixels to be represented. |
| Considering a 320x480 @RGB565 LCD screen, that would be 300KiB, which it’d be too much for a memory-constrained device.</p> |
| <p>However, when memory is not a constraint, framebuffer may offer applications a faster way to update display contents once writing to the RAM-mapped buffer is faster than doing multiple SPI transfers.</p> |
| <p>For memory-constrained devices, consider using <a class="reference internal" href="lcd.html"><span class="doc">LCD Character Drivers</span></a>.</p> |
| </div> |
| </section> |
| </section> |
| </section> |
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