LCD core move buf ptr and address look up function viewport struct

I'm currently running up against the limitations of the lcd_draw functions
I want these functions to be able to be used on any size buffer not
just buffers with a stride matching the underlying device

[DONE] allow the framebuffer to be decoupled from the device framebuffer
[DONE need examples] allow for some simple blit like transformations
[DONE] remove the device framebuffer from the plugin api
[DONE}ditto remote framebuffer
[DONE] remove _viewport_get_framebuffer you can call struct *vp = lcd_set_viewport(NULL) and vp->buffer->fb_ptr

while remote lcds may compile (and work in the sim) its not been tested on targets

[FIXED] backdrops need work to be screen agnostic

[FIXED] screen statusbar is not being combined into the main viewport correctly yet

[FIXED] screen elements are displayed incorrectly  after switch to void*

[FIXED] core didn't restore proper viewport on splash etc.

[NEEDS TESTING] remote lcd garbled data

[FIXED] osd lib garbled screen on bmp_part

[FIXED] grey_set_vp needs to return old viewport like lcd_set_viewport

[FIXED] Viewport update now handles viewports with differing buffers/strides by copying to the main buffer

[FIXED] splash on top of WPS leaves old framebuffer data (doesn't redraw)
[UPDATE] refined this a bit more to have clear_viewport set the clean bit and have skin_render do its own screen clear
scrolling viewports no longer trigger wps refresh
also fixed a bug where guisyncyesno was displaying and then disappearing

[ADDED!] New LCD macros that allow you to create properly size frame buffers in you desired size without wasting bytes
(LCD_ and LCD_REMOTE_)
LCD_STRIDE(w, h) same as STRIDE_MAIN
LCD_FBSTRIDE(w, h) returns target specific stride for a buffer W x H
LCD_NBELEMS(w, h) returns the number of fb_data sized elemenst needed for a buffer W x H
LCD_NATIVE_STRIDE(s) conversion between rockbox native vertical and lcd native stride (2bitH)
test_viewports.c has an example of usage

[FIXED!!] 2bit targets don't respect non-native strides
[FIXED] Few define snags

Change-Id: I0d04c3834e464eca84a5a715743a297a0cefd0af

16 files changed, 111 insertions, 49 deletions

diff --git a/apps/gui/bitmap/list.c b/apps/gui/bitmap/list.c
index 2d3141b8cc..27121eac56 100644
--- a/apps/gui/bitmap/list.c
+++ b/apps/gui/bitmap/list.c
@@ -146,7 +146,7 @@ void list_draw(struct screen *display, struct gui_synclist *list)
     struct viewport *list_text_vp = &list_text[screen];
     int indent = 0;
 
-    display->set_viewport(parent);
+    struct viewport * last_vp = display->set_viewport(parent);
     display->clear_viewport();
     display->scroll_stop_viewport(list_text_vp);
     *list_text_vp = *parent;
@@ -332,7 +332,7 @@ void list_draw(struct screen *display, struct gui_synclist *list)
     }
     display->set_viewport(parent);
     display->update_viewport();
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
 }
 
 #if defined(HAVE_TOUCHSCREEN)
diff --git a/apps/gui/color_picker.c b/apps/gui/color_picker.c
index 03096e5589..a32f1ee179 100644
--- a/apps/gui/color_picker.c
+++ b/apps/gui/color_picker.c
@@ -164,7 +164,7 @@ static void draw_screen(struct screen *display, char *title,
     struct viewport vp;
 
     viewport_set_defaults(&vp, display->screen_type);
-    display->set_viewport(&vp);
+    struct viewport * last_vp = display->set_viewport(&vp);
 
     display->clear_viewport();
 
@@ -323,7 +323,7 @@ static void draw_screen(struct screen *display, char *title,
     }
 
     display->update_viewport();
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
 }
 
 #ifdef HAVE_TOUCHSCREEN
@@ -341,7 +341,7 @@ static int touchscreen_slider(struct screen *display,
     struct viewport vp;
 
     viewport_set_defaults(&vp, display->screen_type);
-    display->set_viewport(&vp);
+    struct viewport *last_vp = display->set_viewport(&vp);
 
     button = action_get_touchscreen_press_in_vp(&x, &y, &vp);
     if (button == ACTION_UNKNOWN || button == BUTTON_NONE)
@@ -373,7 +373,7 @@ static int touchscreen_slider(struct screen *display,
             char_height*2             + /*  + margins for bottom */
             MARGIN_BOTTOM;              /* colored rectangle     */
 
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
 
     if (y < text_top)
     {
diff --git a/apps/gui/pitchscreen.c b/apps/gui/pitchscreen.c
index 0d31193fa6..b5b719ef02 100644
--- a/apps/gui/pitchscreen.c
+++ b/apps/gui/pitchscreen.c
@@ -239,6 +239,7 @@ static void pitchscreen_draw(struct screen *display, int max_lines,
     char buf[32];
     int w, h;
     bool show_lang_pitch;
+    struct viewport *last_vp = NULL;
 
      /* "Pitch up/Pitch down" - hide for a small screen,
       * the text is drawn centered automatically
@@ -249,7 +250,7 @@ static void pitchscreen_draw(struct screen *display, int max_lines,
     {
         int w, h;
         struct viewport *vp = &pitch_viewports[PITCH_TOP];
-        display->set_viewport(vp);
+        last_vp = display->set_viewport(vp);
         display->clear_viewport();
 #ifdef HAVE_TOUCHSCREEN
         /* two arrows in the top row, left and right column */
@@ -405,7 +406,7 @@ static void pitchscreen_draw(struct screen *display, int max_lines,
                         rightlabel);
     }
     display->update_viewport();
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
 }
 
 static int32_t pitch_increase(int32_t pitch, int32_t pitch_delta, bool allow_cutoff
diff --git a/apps/gui/quickscreen.c b/apps/gui/quickscreen.c
index 704133c1c4..421cc267ca 100644
--- a/apps/gui/quickscreen.c
+++ b/apps/gui/quickscreen.c
@@ -172,7 +172,7 @@ static void gui_quickscreen_draw(const struct gui_quickscreen *qs,
     char buf[MAX_PATH];
     unsigned const char *title, *value;
     int temp;
-    display->set_viewport(parent);
+    struct viewport *last_vp = display->set_viewport(parent);
     display->clear_viewport();
 
     for (i = 0; i < QUICKSCREEN_ITEM_COUNT; i++)
@@ -225,7 +225,7 @@ static void gui_quickscreen_draw(const struct gui_quickscreen *qs,
 
     display->set_viewport(parent);
     display->update_viewport();
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
 }
 
 static void talk_qs_option(const struct settings_list *opt, bool enqueue)
diff --git a/apps/gui/skin_engine/skin_backdrops.c b/apps/gui/skin_engine/skin_backdrops.c
index 243fc30a3a..caf705af54 100644
--- a/apps/gui/skin_engine/skin_backdrops.c
+++ b/apps/gui/skin_engine/skin_backdrops.c
@@ -204,11 +204,28 @@ bool skin_backdrops_preload(void)
     return retval;
 }
 
-void* skin_backdrop_get_buffer(int backdrop_id)
+void skin_backdrop_set_buffer(int backdrop_id, struct skin_viewport *svp)
 {
-    if (backdrop_id < 0)
-        return NULL;
-    return backdrops[backdrop_id].buffer;
+    if (UNLIKELY(!svp))
+        return;
+    else if (backdrop_id < 0)
+    {
+        svp->vp.buffer = NULL; /*Default*/
+        return;
+    }
+
+    enum screen_type screen = backdrops[backdrop_id].screen;
+    svp->framebuf.ch_ptr = backdrops[backdrop_id].buffer;
+#if defined(HAVE_REMOTE_LCD)
+    if (screen == SCREEN_REMOTE)
+        svp->framebuf.elems = REMOTE_LCD_BACKDROP_BYTES / sizeof(fb_remote_data);
+    else
+#endif
+    {
+        svp->framebuf.elems = LCD_BACKDROP_BYTES / sizeof(fb_data);
+    }
+    svp->framebuf.get_address_fn = NULL; /*Default iterator*/
+    screens[screen].viewport_set_buffer(&svp->vp, &svp->framebuf);
 }
 
 void skin_backdrop_show(int backdrop_id)
diff --git a/apps/gui/skin_engine/skin_engine.c b/apps/gui/skin_engine/skin_engine.c
index cd763def1c..049629b181 100644
--- a/apps/gui/skin_engine/skin_engine.c
+++ b/apps/gui/skin_engine/skin_engine.c
@@ -312,7 +312,11 @@ struct wps_state *skin_get_global_state(void)
 bool skin_do_full_update(enum skinnable_screens skin,
                             enum screen_type screen)
 {
-    bool ret = skins[skin][screen].needs_full_update;
+    struct viewport *vp = *(screens[screen].current_viewport);
+
+    bool vp_is_dirty = ((vp->flags & VP_FLAG_VP_SET_CLEAN) == VP_FLAG_VP_DIRTY);
+
+    bool ret = (skins[skin][screen].needs_full_update || vp_is_dirty);
     skins[skin][screen].needs_full_update = false;
     return ret;
 }
diff --git a/apps/gui/skin_engine/skin_engine.h b/apps/gui/skin_engine/skin_engine.h
index 55839608be..3b757a5f8b 100644
--- a/apps/gui/skin_engine/skin_engine.h
+++ b/apps/gui/skin_engine/skin_engine.h
@@ -70,7 +70,7 @@ void skin_backdrop_show(int backdrop_id);
 void skin_backdrop_load_setting(void);
 void skin_backdrop_unload(int backdrop_id);
 #define BACKDROP_BUFFERNAME "#backdrop_buffer#"
-void* skin_backdrop_get_buffer(int backdrop_id);
+void skin_backdrop_set_buffer(int backdrop_id, struct skin_viewport *svp);
 
 /* do the button loop as often as required for the peak meters to update
  * with a good refresh rate. 
diff --git a/apps/gui/skin_engine/skin_parser.c b/apps/gui/skin_engine/skin_parser.c
index f3a23377ef..e1a8118190 100644
--- a/apps/gui/skin_engine/skin_parser.c
+++ b/apps/gui/skin_engine/skin_parser.c
@@ -259,7 +259,7 @@ static int parse_statusbar_tags(struct skin_element* element,
     }
     else
     {
-        struct skin_viewport *default_vp = SKINOFFSETTOPTR(skin_buffer, first_viewport->data);
+        struct skin_viewport *skin_default = SKINOFFSETTOPTR(skin_buffer, first_viewport->data);
         if (first_viewport->params_count == 0)
         {
             wps_data->wps_sb_tag = true;
@@ -267,11 +267,11 @@ static int parse_statusbar_tags(struct skin_element* element,
         }
         if (wps_data->show_sb_on_wps)
         {
-            viewport_set_defaults(&default_vp->vp, curr_screen);
+            viewport_set_defaults(&skin_default->vp, curr_screen);
         }
         else
         {
-            viewport_set_fullscreen(&default_vp->vp, curr_screen);
+            viewport_set_fullscreen(&skin_default->vp, curr_screen);
         }
 #ifdef HAVE_REMOTE_LCD
         /* This parser requires viewports which will use the settings font to
@@ -279,7 +279,7 @@ static int parse_statusbar_tags(struct skin_element* element,
          * the current real font id. So force 1 here it will be set correctly
          * at the end
          */
-        default_vp->vp.font = 1;
+        skin_default->vp.font = 1;
 #endif
     }
     return 0;
diff --git a/apps/gui/skin_engine/skin_render.c b/apps/gui/skin_engine/skin_render.c
index 51c58fc196..00981f5b67 100644
--- a/apps/gui/skin_engine/skin_render.c
+++ b/apps/gui/skin_engine/skin_render.c
@@ -349,6 +349,8 @@ static void do_tags_in_hidden_conditional(struct skin_element* branch,
 {
     struct gui_wps *gwps = info->gwps;
     struct wps_data *data = gwps->data;
+    struct viewport *last_vp;
+
     /* Tags here are ones which need to be "turned off" or cleared 
      * if they are in a conditional branch which isnt being used */
     if (branch->type == LINE_ALTERNATOR)
@@ -420,22 +422,23 @@ static void do_tags_in_hidden_conditional(struct skin_element* branch,
 #if (LCD_DEPTH > 1) || (defined(HAVE_REMOTE_LCD) && (LCD_REMOTE_DEPTH > 1))
                             if (skin_viewport->output_to_backdrop_buffer)
                             {
-                                void *backdrop = skin_backdrop_get_buffer(data->backdrop_id);
-                                gwps->display->set_framebuffer(backdrop);
+                                skin_backdrop_set_buffer(data->backdrop_id, skin_viewport);
                                 skin_backdrop_show(-1);
                             }
 #endif
-                            gwps->display->set_viewport(&skin_viewport->vp);
+                            last_vp = gwps->display->set_viewport(&skin_viewport->vp);
                             gwps->display->clear_viewport();
-                            gwps->display->set_viewport(&info->skin_vp->vp);
+                            gwps->display->set_viewport_ex(&info->skin_vp->vp, 0);
                             skin_viewport->hidden_flags |= VP_DRAW_HIDDEN;
 
 #if (LCD_DEPTH > 1) || (defined(HAVE_REMOTE_LCD) && (LCD_REMOTE_DEPTH > 1))
                             if (skin_viewport->output_to_backdrop_buffer)
                             {
-                                gwps->display->set_framebuffer(NULL);
+                                gwps->display->set_viewport_ex(last_vp, 0);
                                 skin_backdrop_show(data->backdrop_id);
                             }
+#else
+    (void)last_vp;
 #endif
                         }
                     }
@@ -792,6 +795,7 @@ void skin_render_viewport(struct skin_element* viewport, struct gui_wps *gwps,
 
 void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
 {
+    const int vp_is_appearing = (VP_DRAW_WASHIDDEN|VP_DRAW_HIDEABLE);
     struct wps_data *data = gwps->data;
     struct screen *display = gwps->display;
     
@@ -801,7 +805,20 @@ void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
     
     int old_refresh_mode = refresh_mode;
     skin_buffer = get_skin_buffer(gwps->data);
-    
+
+    struct viewport* first_vp;
+    /* should already be the default buffer */
+    first_vp = display->set_viewport(NULL);
+
+    /* Framebuffer is likely dirty */
+    if ((refresh_mode&SKIN_REFRESH_ALL) == SKIN_REFRESH_ALL)
+    {
+        if ((first_vp->flags & VP_FLAG_VP_SET_CLEAN) == VP_FLAG_VP_DIRTY &&
+            get_current_activity() == ACTIVITY_WPS) /* only clear if in WPS */
+        {
+            display->clear_viewport();
+        }
+    }
 
     viewport = SKINOFFSETTOPTR(skin_buffer, data->tree);
     skin_viewport = SKINOFFSETTOPTR(skin_buffer, viewport->data);
@@ -822,12 +839,12 @@ void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
 #if (LCD_DEPTH > 1) || (defined(HAVE_REMOTE_LCD) && LCD_REMOTE_DEPTH > 1)
         if (skin_viewport->output_to_backdrop_buffer)
         {
-            display->set_framebuffer(skin_backdrop_get_buffer(data->backdrop_id));
+            skin_backdrop_set_buffer(data->backdrop_id, skin_viewport);
             skin_backdrop_show(-1);
         }
         else
         {
-            display->set_framebuffer(NULL);
+            skin_backdrop_set_buffer(-1, skin_viewport);
             skin_backdrop_show(data->backdrop_id);
         }
 #endif
@@ -842,15 +859,14 @@ void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
             skin_viewport->hidden_flags |= VP_DRAW_WASHIDDEN;
             continue;
         }
-        else if (((skin_viewport->hidden_flags&
-                   (VP_DRAW_WASHIDDEN|VP_DRAW_HIDEABLE))
-                    == (VP_DRAW_WASHIDDEN|VP_DRAW_HIDEABLE)))
+        else if ((skin_viewport->hidden_flags & vp_is_appearing) == vp_is_appearing)
         {
             vp_refresh_mode = SKIN_REFRESH_ALL;
             skin_viewport->hidden_flags = VP_DRAW_HIDEABLE;
         }
         
-        display->set_viewport(&skin_viewport->vp);
+        display->set_viewport_ex(&skin_viewport->vp, VP_FLAG_VP_SET_CLEAN);
+
         if ((vp_refresh_mode&SKIN_REFRESH_ALL) == SKIN_REFRESH_ALL)
         {
             display->clear_viewport();
@@ -862,7 +878,6 @@ void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
         refresh_mode = old_refresh_mode;
     }
 #if (LCD_DEPTH > 1) || (defined(HAVE_REMOTE_LCD) && (LCD_REMOTE_DEPTH > 1))
-    display->set_framebuffer(NULL);
     skin_backdrop_show(data->backdrop_id);
 #endif
 
@@ -872,8 +887,8 @@ void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
          * to redraw itself */
         send_event(GUI_EVENT_NEED_UI_UPDATE, NULL);
     }
-    /* Restore the default viewport */
-    display->set_viewport(NULL);
+    /* Restore the first viewport */
+    display->set_viewport_ex(first_vp, VP_FLAG_VP_SET_CLEAN);
     display->update();
 }
 
diff --git a/apps/gui/skin_engine/wps_internals.h b/apps/gui/skin_engine/wps_internals.h
index b7d7ff35d0..bf7f52bdbf 100644
--- a/apps/gui/skin_engine/wps_internals.h
+++ b/apps/gui/skin_engine/wps_internals.h
@@ -175,6 +175,7 @@ struct gradient_config {
 #define VP_DEFAULT_LABEL_STRING "|"
 struct skin_viewport {
     struct viewport vp;   /* The LCD viewport struct */
+    struct frame_buffer_t framebuf;
     char hidden_flags;
     bool is_infovp;
     OFFSETTYPE(char*) label;
diff --git a/apps/gui/splash.c b/apps/gui/splash.c
index 5bcac80169..1415d47a70 100644
--- a/apps/gui/splash.c
+++ b/apps/gui/splash.c
@@ -53,7 +53,7 @@ static void splash_internal(struct screen * screen, const char *fmt, va_list ap)
     int maxw = 0;
 
     viewport_set_defaults(&vp, screen->screen_type);
-    screen->set_viewport(&vp);
+    struct viewport *last_vp = screen->set_viewport(&vp);
 
     screen->getstringsize(" ", &space_w, &h);
     y = h;
@@ -157,7 +157,7 @@ static void splash_internal(struct screen * screen, const char *fmt, va_list ap)
     }
     screen->update_viewport();
 end:
-    screen->set_viewport(NULL);
+    screen->set_viewport(last_vp);
 }
 
 void splashf(int ticks, const char *fmt, ...)
diff --git a/apps/gui/statusbar.c b/apps/gui/statusbar.c
index cf70b7bb39..708624b23e 100644
--- a/apps/gui/statusbar.c
+++ b/apps/gui/statusbar.c
@@ -183,6 +183,7 @@ static void gui_statusbar_init(struct gui_statusbar * bar)
 void gui_statusbar_draw(struct gui_statusbar * bar, bool force_redraw, struct viewport *vp)
 {
     struct screen * display = bar->display;
+    struct viewport *last_vp = NULL;
 
     if (!display)
         return;
@@ -267,7 +268,7 @@ void gui_statusbar_draw(struct gui_statusbar * bar, bool force_redraw, struct vi
 #endif
         memcmp(&(bar->info), &(bar->lastinfo), sizeof(struct status_info)))
     {
-        display->set_viewport(vp);
+        last_vp = display->set_viewport(vp);
         display->set_drawmode(DRMODE_SOLID|DRMODE_INVERSEVID);
         display->fill_viewport();
         display->set_drawmode(DRMODE_SOLID);
@@ -343,7 +344,7 @@ void gui_statusbar_draw(struct gui_statusbar * bar, bool force_redraw, struct vi
             gui_statusbar_led(display);
 #endif
         display->update_viewport();
-        display->set_viewport(NULL);
+        display->set_viewport(last_vp);
         bar->lastinfo = bar->info;
     }
 }
diff --git a/apps/gui/usb_screen.c b/apps/gui/usb_screen.c
index 3169831322..31321ec005 100644
--- a/apps/gui/usb_screen.c
+++ b/apps/gui/usb_screen.c
@@ -179,6 +179,7 @@ static void usb_screen_fix_viewports(struct screen *screen,
 
 static void usb_screens_draw(struct usb_screen_vps_t *usb_screen_vps_ar)
 {
+    struct viewport *last_vp;
     static const struct bitmap* logos[NB_SCREENS] = {
         &bm_usblogo,
 #ifdef HAVE_REMOTE_LCD
@@ -194,7 +195,7 @@ static void usb_screens_draw(struct usb_screen_vps_t *usb_screen_vps_ar)
         struct viewport *parent = &usb_screen_vps->parent;
         struct viewport *logo = &usb_screen_vps->logo;
 
-        screen->set_viewport(parent);
+        last_vp = screen->set_viewport(parent);
         screen->clear_viewport();
         screen->backlight_on();
 
@@ -217,7 +218,7 @@ static void usb_screens_draw(struct usb_screen_vps_t *usb_screen_vps_ar)
         }
         screen->set_viewport(parent);
 
-        screen->set_viewport(NULL);
+        screen->set_viewport(last_vp);
         screen->update_viewport();
     }
 }
diff --git a/apps/gui/viewport.c b/apps/gui/viewport.c
index db58b5d72d..9fdf88e8f0 100644
--- a/apps/gui/viewport.c
+++ b/apps/gui/viewport.c
@@ -31,6 +31,7 @@
 #include "settings.h"
 #include "misc.h"
 #include "list.h"
+
 /*some short cuts for fg/bg/line selector handling */
 #ifdef HAVE_LCD_COLOR
 #define FG_FALLBACK global_settings.fg_color
@@ -44,7 +45,6 @@
 #define REMOTE_BG_FALLBACK LCD_REMOTE_DEFAULT_BG
 #endif
 
-
 /* all below isn't needed for pc tools (i.e. checkwps/wps editor)
  * only viewport_parse_viewport() is */
 #ifndef __PCTOOL__
@@ -101,6 +101,7 @@ static void toggle_theme(enum screen_type screen, bool force)
     bool enable_event = false;
     static bool was_enabled[NB_SCREENS] = {false};
     static bool after_boot[NB_SCREENS] = {false};
+    struct viewport *last_vp;
 
     FOR_NB_SCREENS(i)
     {
@@ -111,6 +112,7 @@ static void toggle_theme(enum screen_type screen, bool force)
 
     if (is_theme_enabled(screen))
     {
+        last_vp = screens[screen].set_viewport(NULL);
         bool first_boot = theme_stack_top[screen] == 0;
         /* remove the left overs from the previous screen.
          * could cause a tiny flicker. Redo your screen code if that happens */
@@ -162,7 +164,7 @@ static void toggle_theme(enum screen_type screen, bool force)
                 screens[screen].clear_viewport();
                 screens[screen].update_viewport();
             }
-            screens[screen].set_viewport(NULL);
+            screens[screen].set_viewport(last_vp);
         }
         intptr_t force = first_boot?0:1;
 
@@ -282,11 +284,11 @@ static void set_default_align_flags(struct viewport *vp)
 void viewport_set_fullscreen(struct viewport *vp,
                               const enum screen_type screen)
 {
+    screens[screen].init_viewport(vp);
     vp->x = 0;
     vp->y = 0;
     vp->width = screens[screen].lcdwidth;
     vp->height = screens[screen].lcdheight;
-
 #ifndef __PCTOOL__
     set_default_align_flags(vp);
 #endif
@@ -312,9 +314,25 @@ void viewport_set_fullscreen(struct viewport *vp,
 #endif
 }
 
+void viewport_set_buffer(struct viewport *vp, struct frame_buffer_t *buffer,
+                                                const enum screen_type screen)
+{
+    if (!vp) /* NULL vp grabs current framebuffer */
+        vp = *(screens[screen].current_viewport);
+
+    /* NULL sets default buffer */
+    if (buffer && buffer->elems == 0)
+        vp->buffer = NULL;
+    else
+        vp->buffer = buffer;
+    screens[screen].init_viewport(vp);
+}
+
 void viewport_set_defaults(struct viewport *vp,
                             const enum screen_type screen)
 {
+    vp->buffer = NULL; /* use default frame_buffer */
+
 #if !defined(__PCTOOL__)
     struct viewport *sbs_area = NULL;
     if (!is_theme_enabled(screen))
@@ -323,7 +341,7 @@ void viewport_set_defaults(struct viewport *vp,
         return;
     }
     sbs_area = sb_skin_get_info_vp(screen);
-    
+
     if (sbs_area)
         *vp = *sbs_area;
     else
diff --git a/apps/gui/viewport.h b/apps/gui/viewport.h
index 683c36fe76..be80e44721 100644
--- a/apps/gui/viewport.h
+++ b/apps/gui/viewport.h
@@ -63,6 +63,9 @@ void viewportmanager_theme_undo(enum screen_type screen, bool force_redraw);
 /* call this when a theme changed */
 void viewportmanager_theme_changed(const int);
 
+void viewport_set_buffer(struct viewport *vp, struct frame_buffer_t *buffer,
+                                                const enum screen_type screen);
+
 #ifdef HAVE_TOUCHSCREEN
 bool viewport_point_within_vp(const struct viewport *vp,
                                const int x, const int y);
diff --git a/apps/gui/yesno.c b/apps/gui/yesno.c
index 1a1645047a..d70b66f230 100644
--- a/apps/gui/yesno.c
+++ b/apps/gui/yesno.c
@@ -78,8 +78,9 @@ static void gui_yesno_draw(struct gui_yesno * yn)
     struct screen * display=yn->display;
     struct viewport *vp = yn->vp;
     int nb_lines, vp_lines, line_shift=0;
+    struct viewport *last_vp;
 
-    display->set_viewport(vp);
+    last_vp = display->set_viewport(vp);
     display->clear_viewport();
     nb_lines = yn->main_message->nb_lines;
     vp_lines = viewport_get_nb_lines(vp);
@@ -116,7 +117,7 @@ static void gui_yesno_draw(struct gui_yesno * yn)
     }
 #endif
     display->update_viewport();
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
 }
 
 /*
@@ -133,11 +134,11 @@ static bool gui_yesno_draw_result(struct gui_yesno * yn, enum yesno_res result)
     struct screen * display=yn->display;
     if(message==NULL)
         return false;
-    display->set_viewport(vp);
+    struct viewport *last_vp = display->set_viewport(vp);
     display->clear_viewport();
     put_message(yn->display, message, 0, viewport_get_nb_lines(vp));
     display->update_viewport();
-    display->set_viewport(NULL);
+    display->set_viewport(last_vp);
     return(true);
 }