Mention sample program execution with Linux framebuffer and input system

Author: shengwen-tw

README.md

@@ -60,30 +60,54 @@ benefiting the entire application stack.
 
 ## Build and Verify
 
+### Prerequisites
+
 `Mado` is built with a minimalist design in mind. However, its verification
 relies on certain third-party packages for full functionality and access to all
-its features. To ensure proper operation, the development environment should
-have the [SDL2 library](https://www.libsdl.org/), [libjpeg](https://www.ijg.org/), and [libpng](https://github.com/pnggroup/libpng) installed.
+its features. We encourage the development environment to be installed with all optional
+packages, including [libjpeg](https://www.ijg.org/), [libpng](https://github.com/pnggroup/libpng),
+and the [SDL2 library](https://www.libsdl.org/).
 * macOS: `brew install sdl2 jpeg libpng`
 * Ubuntu Linux / Debian: `sudo apt install libsdl2-dev libjpeg-dev libpng-dev`
 
-Configure via [Kconfiglib](https://pypi.org/project/kconfiglib/)
+### Configure the demo program
+
+Configure via [Kconfiglib](https://pypi.org/project/kconfiglib/), you should select either SDL
+video output or the Linux framebuffer.
 ```shell
 $ make config
 ```
 
-Build the library and demo program.
+### Build the library and demo program
+
 ```shell
 $ make
 ```
 
-Run sample `Mado` program:
+### Run demo program with SDL backend
+
 ```shell
 $ ./demo-sdl
 ```
 
 Once the window appears, you should be able to move the windows and interact with the widgets.
 
+### Run demo program with the Linux framebuffer backend
+
+Normal users don't have access to `/dev/fb0` so use `sudo`:
+
+```shell
+$ sudo ./demo-fbdev
+```
+
+Alternatively, you can add the user to the video group to avoid typing `sudo` every time:
+
+```shell
+$ sudo usermod -a -G video $USERNAME
+```
+
+In addition, the framebuffer and mouse device files can be assigned via the environment variable `FRAMEBUFFER` and `MOUSE`.
+
 ## License
 
 `Mado` is available under a MIT-style license, permitting liberal commercial use.

backend/fbdev.c

@@ -142,6 +142,8 @@ static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
     return true;
 }
 
+/* FIXME: Let's remove this later, in case we actually need it */
+#if 0
 static bool twin_find_free_vt(int *vt_num)
 {
     char vt_path[30];
@@ -163,6 +165,7 @@ static bool twin_find_free_vt(int *vt_num)
     /* No free VT found */
     return false;
 }
+#endif
 
 static int twin_vt_open(int vt_num)
 {
@@ -187,7 +190,7 @@ static bool twin_vt_setup(twin_fbdev_t *tx)
     }
 
     /* Find next free virtual terminal */
-    if (!twin_find_free_vt(&tx->vt_num)) {
+    if ((ioctl(tx->vt_fd, VT_OPENQRY, &tx->vt_num) < 0)) {
         printf("Failed to find a free virtual terminal.\n");
         return false;
     }