drm/ast: Check video-mode requirements against VRAM size

Each video mode's primary plane requires a minimum amount of video
memory. For double buffering, this is at most half the available
VRAM. Check this constraint.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-3-tzimmermann@suse.de

drivers/gpu/drm/ast/ast_main.c

@@ -387,8 +387,31 @@ static int ast_get_dram_info(struct drm_device *dev)
 	return 0;
 }
 
+enum drm_mode_status ast_mode_config_mode_valid(struct drm_device *dev,
+						const struct drm_display_mode *mode)
+{
+	static const unsigned long max_bpp = 4; /* DRM_FORMAT_XRGBA8888 */
+
+	struct ast_private *ast = dev->dev_private;
+	unsigned long fbsize, fbpages, max_fbpages;
+
+	/* To support double buffering, a framebuffer may not
+	 * consume more than half of the available VRAM.
+	 */
+	max_fbpages = (ast->vram_size / 2) >> PAGE_SHIFT;
+
+	fbsize = mode->hdisplay * mode->vdisplay * max_bpp;
+	fbpages = DIV_ROUND_UP(fbsize, PAGE_SIZE);
+
+	if (fbpages > max_fbpages)
+		return MODE_MEM;
+
+	return MODE_OK;
+}
+
 static const struct drm_mode_config_funcs ast_mode_funcs = {
-	.fb_create = drm_gem_fb_create
+	.fb_create = drm_gem_fb_create,
+	.mode_valid = ast_mode_config_mode_valid,
 };
 
 static u32 ast_get_vram_info(struct drm_device *dev)