feat(fbdev): add docs and allow forcing refresh (#5444)

docs/integration/driver/display/fbdev.rst

@@ -0,0 +1,41 @@
+========================
+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.
+
+.. code:: c
+
+	#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``).
+
+.. code:: c
+
+	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.

docs/integration/driver/display/index.rst

@@ -5,7 +5,10 @@ Display
 .. toctree::
     :maxdepth: 2
 
+    fbdev
+    gen_mipi
     ili9341
+    lcd_stm32_hal
     st7735
     st7789
     st7796

src/display/lv_display.c

@@ -84,6 +84,7 @@ lv_display_t * lv_display_create(int32_t hor_res, int32_t ver_res)
     disp->layer_head->color_format = disp->color_format;
 
     disp->inv_en_cnt = 1;
+    disp->last_activity_time = lv_tick_get();
 
     _lv_ll_init(&disp->sync_areas, sizeof(lv_area_t));
 

src/drivers/display/fb/lv_linux_fbdev.c

@@ -59,6 +59,7 @@ typedef struct {
     char * fbp;
     long int screensize;
     int fbfd;
+    bool force_refresh;
 } lv_linux_fb_t;
 
 /**********************
@@ -238,6 +239,12 @@ void lv_linux_fbdev_set_file(lv_display_t * disp, const char * file)
                 hor_res, ver_res, lv_display_get_dpi(disp));
 }
 
+void lv_linux_fbdev_set_force_refresh(lv_display_t * disp, bool enabled)
+{
+    lv_linux_fb_t * dsc = lv_display_get_driver_data(disp);
+    dsc->force_refresh = enabled;
+}
+
 /**********************
  *   STATIC FUNCTIONS
  **********************/
@@ -275,6 +282,13 @@ static void flush_cb(lv_display_t * disp, const lv_area_t * area, uint8_t * colo
         }
     }
 
+    if(dsc->force_refresh) {
+        dsc->vinfo.activate |= FB_ACTIVATE_NOW | FB_ACTIVATE_FORCE;
+        if(ioctl(dsc->fbfd, FBIOPUT_VSCREENINFO, &(dsc->vinfo)) == -1) {
+            perror("Error setting var screen info");
+        }
+    }
+
     lv_display_flush_ready(disp);
 }
 

src/drivers/display/fb/lv_linux_fbdev.h

@@ -33,6 +33,12 @@ lv_display_t * lv_linux_fbdev_create(void);
 
 void lv_linux_fbdev_set_file(lv_display_t * disp, const char * file);
 
+/**
+ * Force the display to be refreshed on every change.
+ * Expected to be used with LV_DISPLAY_RENDER_MODE_DIRECT or LV_DISPLAY_RENDER_MODE_FULL.
+ */
+void lv_linux_fbdev_set_force_refresh(lv_display_t * disp, bool enabled);
+
 /**********************
  *      MACROS
  **********************/