Unexpected null screen in Linux framebuffer backend

[huaxinliao]

After merging #88, I encountered issues while executing fbdev. To identify the source of the error, I conducted several experiments. I observed that the error originates from the twin_screen_update function, as I was able to successfully execute fbdev using the original twin_screen_update function.

To resolve this issue, I modified the code introduced in #88 to ensure that the updated twin_screen_update function is compatible with fbdev. Below are the modifications I made:

static twin_pixmap_t *twin_active_pixmap(twin_screen_t *screen,
                                         twin_pixmap_t **active_pix)
{
    twin_pixmap_t *p = NULL, *prev_active_pix = NULL;
    /*
     * Identify the previously active pixel map and the currently active pixel
     * map, which is on the topmost layer.
     */
    for (p = screen->bottom; p; p = p->up) {
        if (p->window->active == true) {
            prev_active_pix = p;
            prev_active_pix->window->active = false;
        }
        (*active_pix) = p;
    }
+    if(*active_pix)
+        (*active_pix)->window->active = true;
-   (*active_pix)->window->active = true;
    return prev_active_pix;
}
...
        prev_active_pix = twin_active_pixmap(screen, &active_pix);
	//active_pix = prev_active_pix;
        /*
         * Mark the previously active pixel map as damaged to update its
         * changes.
         */
-       if (prev_active_pix && active_pix != prev_active_pix)
+       if ((prev_active_pix && active_pix != prev_active_pix) && prev_active_pix){
            twin_pixmap_damage(prev_active_pix, 0, 0, prev_active_pix->width, prev_active_pix->height);
            twin_window_draw(prev_active_pix->window);
        }

        /* Mark the active pixel map as damaged to update its changes. */
+       if(active_pix){
+            twin_pixmap_damage(active_pix, 0, 0, active_pix->width,active_pix->height);
+            twin_window_draw(active_pix->window);
+       }
-        twin_pixmap_damage(active_pix, 0, 0, active_pix->width,active_pix->height);
-        twin_window_draw(active_pix->window);

The condition if(*active_pix) ensures that *active_pix is valid, as I previously encountered failures in this part of the code. Similarly, if(active_pix) verifies that active_pix is not NULL. These modifications are designed to prevent unexpected situations from occurring. Finally, I obtained the output from my VM.

If I eliminate the code below, I am able to successfully run fbdev.

/* Mark the active pixel map as damaged to update its changes. */
- twin_pixmap_damage(active_pix, 0, 0, active_pix->width,
-                   active_pix->height);
- twin_window_draw(active_pix->window);

Modification.mp4