Linux Framebuffer Driver

Overview

The Linux framebuffer (fbdev) is a linux subsystem used to display graphics. It is a hardware-independent API that gives user space software access to the framebuffer (the part of a computer's video memory containing a current video frame) using only the Linux kernel's own basic facilities and its device file system interface, avoiding the need for libraries that implement video drivers in user space.

Prerequisites

Your system has a framebuffer device configured (usually under /dev/fb0).

Configuring the driver

Enable the framebuffer driver support in lv_conf.h, by cmake compiler define or by KConfig. Additionally you may configure the rendering mode.

#define LV_USE_LINUX_FBDEV           1
#define LV_LINUX_FBDEV_RENDER_MODE   LV_DISPLAY_RENDER_MODE_PARTIAL

Usage

To set up a framebuffer-based display, first create a display with lv_linux_fbdev_create. Afterwards set the framebuffer device node on the display (usually this is /dev/fb0).

lv_display_t *disp = lv_linux_fbdev_create();
lv_linux_fbdev_set_file(disp, "/dev/fb0");

If your screen stays black or only draws partially, you can try enabling direct rendering via LV_DISPLAY_RENDER_MODE_DIRECT. Additionally, you can activate a force refresh mode with lv_linux_fbdev_set_force_refresh(true). This usually has a performance impact though and shouldn't be enabled unless really needed.

Hide the cursor

You may encounter a blinking cursor on the screen. The method to hide it varies depending on the platform. For instance, here is how it can be done on a Raspberry Pi:

  1. Edit /boot/cmdline.txt file.

  2. Add vt.global_cursor_default=0.

Common mistakes

Default resolution issue

When the Linux kernel initializes, it sets up subsystems like the framebuffer (fbdev) to manage display output. If an HDMI display is connected, the kernel detects it and allocates a portion of RAM as the framebuffer, which holds the pixel data for rendering images.

However, a common issue arises when the display is not powered on during the boot process. If the board is powered on before the screen, the kernel may incorrectly configure the framebuffer resolution. As a result, both the visible and virtual resolutions can be incorrect, leading to display problems.

This issue often occurs with HDMI connections where the display is powered up after the system has already booted.

The following command outputs the current settings of the specified framebuffer device, such as resolution, pixel depth, and timings.

fbset -fb /dev/fb0

To prevent display-related issues, it is recommended to ensure all devices, including the HDMI display, are connected and powered on before powering up the board.