Support 16/32 bit color depth

Mado currently supports 32-bit color depth for desktop and laptop
display devices. To enhance its compatibility across diverse Linux
framebuffer environments, I have made the following changes to support
both 16-bit and 32-bit color depths:

- Distinguish the color format and perform color format conversion.
- Examine the correctness of the framebuffer format based on the color
  format.

These updates improve Mado's compatibility, enabling it to be used on
devices with low memory overhead, such as the Raspberry Pi, in addition
to desktop display devices.

Author: huaxinliao

backend/fbdev.c

@@ -22,6 +22,20 @@
 #define FBDEV_DEFAULT "/dev/fb0"
 #define SCREEN(x) ((twin_context_t *) x)->screen
 #define PRIV(x) ((twin_fbdev_t *) ((twin_context_t *) x)->priv)
+#define ARGB32_TO_RGB565(pixel)                                  \
+    (((pixel & 0x00f80000) >> 8) | ((pixel & 0x0000fc00) >> 5) | \
+     ((pixel & 0x000000f8) >> 3))
+
+/* Requires validation in 24-bit per pixel environments. */
+#define ARGB32_TO_RGB888(pixel) (0xff000000 | (pixel))
+#define GET_TWIN_FBDEV(left, top, closure, dest)                               \
+    do {                                                                       \
+        twin_screen_t *screen = SCREEN(closure);                               \
+        twin_fbdev_t *tx = PRIV(closure);                                      \
+        off_t off = (top) * (screen->width) + (left);                          \
+        *(dest) =                                                              \
+            (uint32_t *) ((uintptr_t) tx->fb_base + (off * sizeof(uint32_t))); \
+    } while (0)
 
 typedef struct {
     twin_screen_t *screen;
@@ -43,22 +57,43 @@ typedef struct {
     size_t fb_len;
 } twin_fbdev_t;
 
-static void _twin_fbdev_put_span(twin_coord_t left,
-                                 twin_coord_t top,
-                                 twin_coord_t right,
-                                 twin_argb32_t *pixels,
-                                 void *closure)
+static void _twin_fbdev_put_span16(twin_coord_t left,
+                                   twin_coord_t top,
+                                   twin_coord_t right,
+                                   twin_argb32_t *pixels,
+                                   void *closure)
 {
-    twin_screen_t *screen = SCREEN(closure);
-    twin_fbdev_t *tx = PRIV(closure);
+    uint32_t *dest;
+    GET_TWIN_FBDEV(left, top, closure, &dest);
+    twin_coord_t width = right - left;
+    for (int i = 0; i < width; i++) {
+        dest[i] = ARGB32_TO_RGB565(pixels[i]);
+    }
+}
 
-    if (tx->fb_base == MAP_FAILED)
-        return;
+static void _twin_fbdev_put_span24(twin_coord_t left,
+                                   twin_coord_t top,
+                                   twin_coord_t right,
+                                   twin_argb32_t *pixels,
+                                   void *closure)
+{
+    uint32_t *dest;
+    GET_TWIN_FBDEV(left, top, closure, &dest);
+    twin_coord_t width = right - left;
+    for (int i = 0; i < width; i++) {
+        dest[i] = ARGB32_TO_RGB888(pixels[i]);
+    }
+}
 
+static void _twin_fbdev_put_span32(twin_coord_t left,
+                                   twin_coord_t top,
+                                   twin_coord_t right,
+                                   twin_argb32_t *pixels,
+                                   void *closure)
+{
+    uint32_t *dest;
+    GET_TWIN_FBDEV(left, top, closure, &dest);
     twin_coord_t width = right - left;
-    off_t off = top * screen->width + left;
-    uint32_t *dest =
-        (uint32_t *) ((uintptr_t) tx->fb_base + (off * sizeof(*dest)));
     memcpy(dest, pixels, width * sizeof(*dest));
 }
 
@@ -88,6 +123,27 @@ static bool twin_fbdev_work(void *closure)
     return true;
 }
 
+static bool twin_fbdev_is_rgb565(twin_fbdev_t *tx)
+{
+    return tx->fb_var.red.offset == 11 && tx->fb_var.red.length == 5 &&
+           tx->fb_var.green.offset == 5 && tx->fb_var.green.length == 6 &&
+           tx->fb_var.blue.offset == 0 && tx->fb_var.blue.length == 5;
+}
+
+static bool twin_fbdev_is_rgb888(twin_fbdev_t *tx)
+{
+    return tx->fb_var.red.offset == 16 && tx->fb_var.red.length == 8 &&
+           tx->fb_var.green.offset == 8 && tx->fb_var.green.length == 8 &&
+           tx->fb_var.blue.offset == 0 && tx->fb_var.blue.length == 8;
+}
+
+static bool twin_fbdev_is_argb32(twin_fbdev_t *tx)
+{
+    return tx->fb_var.red.offset == 16 && tx->fb_var.red.length == 8 &&
+           tx->fb_var.green.offset == 8 && tx->fb_var.green.length == 8 &&
+           tx->fb_var.blue.offset == 0 && tx->fb_var.blue.length == 8;
+}
+
 static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
 {
     /* Read changable information of the framebuffer */
@@ -111,10 +167,29 @@ static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
         return false;
     }
 
-    /* Check bits per pixel */
-    if (tx->fb_var.bits_per_pixel != 32) {
-        log_error("Failed to set framebuffer bpp to 32");
-        return false;
+    /* Examine the framebuffer format */
+    switch (tx->fb_var.bits_per_pixel) {
+    case 16:  // RGB565
+        if (!twin_fbdev_is_rgb565(tx)) {
+            log_error("Invalid framebuffer format for 16 bpp");
+            return false;
+        }
+        break;
+    case 24:  // RGB888
+        if (!twin_fbdev_is_rgb888(tx)) {
+            log_error("Invalid framebuffer format for 24 bpp");
+            return false;
+        }
+        break;
+    case 32:  // ARGB32
+        if (!twin_fbdev_is_argb32(tx)) {
+            log_error("Invalid framebuffer format for 32 bpp");
+            return false;
+        }
+        break;
+    default:
+        log_error("Unsupported bits per pixel: %d", tx->fb_var.bits_per_pixel);
+        break;
     }
 
     /* Read unchangable information of the framebuffer */
@@ -220,9 +295,20 @@ twin_context_t *twin_fbdev_init(int width, int height)
         goto bail_vt_fd;
     }
 
+    /* Examine if framebuffer mapping is valid */
+    if (tx->fb_base == MAP_FAILED) {
+        log_error("Failed to map framebuffer memory");
+        return;
+    }
+
     /* Create TWIN screen */
-    ctx->screen =
-        twin_screen_create(width, height, NULL, _twin_fbdev_put_span, ctx);
+    ctx->screen = twin_screen_create(
+        width, height, NULL,
+        (tx->fb_var.bits_per_pixel == 16)   ? _twin_fbdev_put_span16
+        : (tx->fb_var.bits_per_pixel == 24) ? _twin_fbdev_put_span24
+        : (tx->fb_var.bits_per_pixel == 32) ? _twin_fbdev_put_span32
+                                            : NULL,
+        ctx);
 
     /* Create Linux input system object */
     tx->input = twin_linux_input_create(ctx->screen);