Implement Linux framebuffer backend

[jserv]

Mado should provide a Linux framebuffer/DRM backend, which is more suitable for embedded systems. In addition, the use of the Linux input subsystem is crucial for integration.

Reference:

• Back to the Linux Framebuffer!, FOSDEM 2020
• yamui
• DirectFB2
• Playground with the Linux Framebuffer

[shengwen-tw]

Pull request #15 implements the Linux framebuffer backend for the Mado.

[jserv]

With the inclusion of kconfiglib, the build system is now fully configurable like "menuconfig" when building the Linux kernel. This flexibility is also useful for developers to integrate additional backends such as Linux framebuffer and DRM device support. (#18)

[jserv]

LVGL implements both Linux framebuffer and DRM support. See:

• docs/integration/driver/display/fbdev.rst
• lv_port_linux
• src/drivers/display/drm/lv_linux_drm.h
• src/drivers/display/drm/lv_linux_drm.c
• src/drivers/display/fb/lv_linux_fbdev.h
• src/drivers/display/fb/lv_linux_fbdev.c

awtk-linux-fb is more complete.

[shengwen-tw]

Add information of Virtual Terminal (Console) as the Linux framebuffer/DRM backend should also take care of the VT switching:

• Virtual console - Wikipedia
• Linux console - Wikipedia

[shengwen-tw]

In GNU/Linux, the user can switch between different virtual terminals using Ctrl+Alt+Fn. The first virtual terminal (VT1, accessed with Ctrl+Alt+F1) is typically used by the graphical desktop environment.

We should deactivate the virtual terminal when Mado is running and restore it in opposite case.

[jserv]

drm-framebuffer is a simple application which can be used to test a Linux DRM device.

[shengwen-tw]

Frame Buffer device internals

[shengwen-tw]

pslash, a lightweight framebuffer splashscreen for embedded devices, provides good reference for virtual terminal (VT) management.

[jserv]

Let's focus on Linux framebuffer support for now. DRM support can be considered as a follow-up once the non-SDL display target and VT implementation are merged.

[shengwen-tw]

DirectFB uses the event device and udev to manage Human Interface Devices (HIDs), in which the udev is for handling device plug-in and removal.

Let's handle the mouse events (i.e., EV_REL and EV_ABS) first by reading all event devices (i.e., /dev/input/eventX) using poll() (or any suitable APIs) in a thread and later adding support for udev.

• evdev thread in DirectFB
• udev thread in DirectFB

[jserv]

Let's handle the mouse events (i.e., EV_REL and EV_ABS) first by reading all event devices (i.e., /dev/input/eventX) using poll() (or any suitable APIs) in a thread and later adding support for udev.

Related: #4

[jserv]

According to Linux 6.12 To Optionally Display A QR Code During Kernel Panics, a new feature allows the display of a QR code within the DRM panic handler infrastructure when a Linux kernel panic occurs. We could consider integrating parts of the Mado codebase into the Linux kernel (as a module) to enhance text rendering capabilities. As a graphical library and window system, Mado does not rely on floating-point operations, making it suitable for Linux kernel integration where no FPU is available.