WindowServer: Implement support for combined buffer flipping + flushing

Some devices may require DMA transfers to flush the updated buffer
areas prior to flipping. For those devices we track the areas that
require flushing prior to the next flip. For devices that do not
support flipping, but require flushing, we'll simply flush after
updating the front buffer.

This also adds a small optimization that skips these steps entirely for
a screen that doesn't have any updates that need to be rendered.

Kernel/API/FB.h

@@ -38,12 +38,13 @@ ALWAYS_INLINE int fb_set_buffer(int fd, int index)
     return ioctl(fd, FB_IOCTL_SET_BUFFER, index);
 }
 
-ALWAYS_INLINE int fb_flush_buffers(int fd, FBRect const* rects, unsigned count)
+ALWAYS_INLINE int fb_flush_buffers(int fd, int index, FBRect const* rects, unsigned count)
 {
-    FBRects fb_rects;
-    fb_rects.count = count;
-    fb_rects.rects = rects;
-    return ioctl(fd, FB_IOCTL_FLUSH_BUFFERS, &fb_rects);
+    FBFlushRects fb_flush_rects;
+    fb_flush_rects.buffer_index = index;
+    fb_flush_rects.count = count;
+    fb_flush_rects.rects = rects;
+    return ioctl(fd, FB_IOCTL_FLUSH_BUFFERS, &fb_flush_rects);
 }
 
 __END_DECLS

Kernel/Graphics/VirtIOGPU/VirtIOFrameBufferDevice.cpp

@@ -148,14 +148,16 @@ int VirtIOFrameBufferDevice::ioctl(FileDescription&, unsigned request, FlatPtr a
         return 0;
     }
     case FB_IOCTL_FLUSH_BUFFERS: {
-        FBRects user_dirty_rects;
-        if (!copy_from_user(&user_dirty_rects, (FBRects*)arg))
+        FBFlushRects user_flush_rects;
+        if (!copy_from_user(&user_flush_rects, (FBFlushRects*)arg))
             return -EFAULT;
-        if (Checked<unsigned>::multiplication_would_overflow(user_dirty_rects.count, sizeof(FBRect)))
+        if (user_flush_rects.buffer_index != 0)
+            return -EINVAL;
+        if (Checked<unsigned>::multiplication_would_overflow(user_flush_rects.count, sizeof(FBRect)))
             return -EFAULT;
-        for (unsigned i = 0; i < user_dirty_rects.count; i++) {
+        for (unsigned i = 0; i < user_flush_rects.count; i++) {
             FBRect user_dirty_rect;
-            if (!copy_from_user(&user_dirty_rect, &user_dirty_rects.rects[i]))
+            if (!copy_from_user(&user_dirty_rect, &user_flush_rects.rects[i]))
                 return -EFAULT;
             if (m_are_writes_active) {
                 VirtIOGPURect dirty_rect {

Userland/Libraries/LibC/sys/ioctl_numbers.h

@@ -30,7 +30,8 @@ struct FBRect {
     unsigned height;
 };
 
-struct FBRects {
+struct FBFlushRects {
+    int buffer_index;
     unsigned count;
     struct FBRect const* rects;
 };

Userland/Services/WindowServer/Compositor.cpp

@@ -80,13 +80,18 @@ const Gfx::Bitmap& Compositor::front_bitmap_for_screenshot(Badge<ClientConnectio
 
 void Compositor::ScreenData::init_bitmaps(Compositor& compositor, Screen& screen)
 {
+    m_has_flipped = false;
+    m_have_flush_rects = false;
+    m_buffers_are_flipped = false;
+    m_screen_can_set_buffer = screen.can_set_buffer();
+
     auto size = screen.size();
 
     m_front_bitmap = Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::BGRx8888, size, screen.scale_factor(), screen.pitch(), screen.scanline(0));
     m_front_painter = make<Gfx::Painter>(*m_front_bitmap);
     m_front_painter->translate(-screen.rect().location());
 
-    if (screen.can_set_buffer())
+    if (m_screen_can_set_buffer)
         m_back_bitmap = Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::BGRx8888, size, screen.scale_factor(), screen.pitch(), screen.scanline(screen.physical_height()));
     else
         m_back_bitmap = Gfx::Bitmap::create(Gfx::BitmapFormat::BGRx8888, size, screen.scale_factor());
@@ -97,9 +102,6 @@ void Compositor::ScreenData::init_bitmaps(Compositor& compositor, Screen& screen
     m_temp_painter = make<Gfx::Painter>(*m_temp_bitmap);
     m_temp_painter->translate(-screen.rect().location());
 
-    m_buffers_are_flipped = false;
-    m_screen_can_set_buffer = screen.can_set_buffer();
-
     // Recreate the screen-number overlay as the Screen instances may have changed, or get rid of it if we no longer need it
     if (compositor.showing_screen_numbers()) {
         m_screen_number_overlay = compositor.create_overlay<ScreenNumberOverlay>(screen);
@@ -232,6 +234,7 @@ void Compositor::compose()
     auto& cursor_screen = ScreenInput::the().cursor_location_screen();
 
     for (auto& screen_data : m_screen_data) {
+        screen_data.m_have_flush_rects = false;
         screen_data.m_flush_rects.clear_with_capacity();
         screen_data.m_flush_transparent_rects.clear_with_capacity();
         screen_data.m_flush_special_rects.clear_with_capacity();
@@ -268,6 +271,7 @@ void Compositor::compose()
         dbgln_if(COMPOSE_DEBUG, "    -> flush opaque: {}", rect);
         VERIFY(!screen_data.m_flush_rects.intersects(rect));
         VERIFY(!screen_data.m_flush_transparent_rects.intersects(rect));
+        screen_data.m_have_flush_rects = true;
         screen_data.m_flush_rects.add(rect);
         check_restore_cursor_back(screen, rect);
     };
@@ -281,6 +285,7 @@ void Compositor::compose()
                 return;
         }
 
+        screen_data.m_have_flush_rects = true;
         screen_data.m_flush_transparent_rects.add(rect);
         check_restore_cursor_back(screen, rect);
     };
@@ -554,6 +559,8 @@ void Compositor::compose()
         Screen::for_each([&](auto& screen) {
             auto& screen_data = m_screen_data[screen.index()];
             update_animations(screen, screen_data.m_flush_special_rects);
+            if (!screen_data.m_flush_special_rects.is_empty())
+                screen_data.m_have_flush_rects = true;
             return IterationDecision::Continue;
         });
         // As long as animations are running make sure we keep rendering frames
@@ -564,9 +571,6 @@ void Compositor::compose()
     if (need_to_draw_cursor) {
         auto& screen_data = m_screen_data[cursor_screen.index()];
         screen_data.draw_cursor(cursor_screen, cursor_rect);
-        screen_data.m_flush_rects.add(cursor_rect.intersected(cursor_screen.rect()));
-        if (previous_cursor_screen && cursor_rect != previous_cursor_rect)
-            m_screen_data[previous_cursor_screen->index()].m_flush_rects.add(previous_cursor_rect);
     }
 
     Screen::for_each([&](auto& screen) {
@@ -578,16 +582,45 @@ void Compositor::compose()
 void Compositor::flush(Screen& screen)
 {
     auto& screen_data = m_screen_data[screen.index()];
+
+    bool device_can_flush_buffers = screen.can_device_flush_buffers();
+    if (!screen_data.m_have_flush_rects && (!screen_data.m_screen_can_set_buffer || screen_data.m_has_flipped)) {
+        dbgln_if(COMPOSE_DEBUG, "Nothing to flush on screen #{} {}", screen.index(), screen_data.m_have_flush_rects);
+        return;
+    }
+    screen_data.m_have_flush_rects = false;
+
     if (m_flash_flush) {
         for (auto& rect : screen_data.m_flush_rects.rects())
             screen_data.m_front_painter->fill_rect(rect, Color::Yellow);
     }
 
-    if (screen_data.m_screen_can_set_buffer)
+    auto screen_rect = screen.rect();
+    if (device_can_flush_buffers && screen_data.m_screen_can_set_buffer) {
+        if (!screen_data.m_has_flipped) {
+            // If we have not flipped any buffers before, we should be flushing
+            // the entire buffer to make sure that the device has all the bits we wrote
+            screen_data.m_flush_rects = { screen.rect() };
+        }
+
+        // If we also support buffer flipping we need to make sure we transfer all
+        // updated areas to the device before we flip. We already modified the framebuffer
+        // memory, but the device needs to know what areas we actually did update.
+        for (auto& rect : screen_data.m_flush_rects.rects())
+            screen.queue_flush_display_rect(rect.translated(-screen_rect.location()));
+        for (auto& rect : screen_data.m_flush_transparent_rects.rects())
+            screen.queue_flush_display_rect(rect.translated(-screen_rect.location()));
+        for (auto& rect : screen_data.m_flush_special_rects.rects())
+            screen.queue_flush_display_rect(rect.translated(-screen_rect.location()));
+
+        screen.flush_display((!screen_data.m_screen_can_set_buffer || screen_data.m_buffers_are_flipped) ? 0 : 1);
+    }
+
+    if (screen_data.m_screen_can_set_buffer) {
         screen_data.flip_buffers(screen);
+        screen_data.m_has_flipped = true;
+    }
 
-    auto screen_rect = screen.rect();
-    bool device_can_flush_buffers = screen.can_device_flush_buffers();
     auto do_flush = [&](Gfx::IntRect rect) {
         VERIFY(screen_rect.contains(rect));
         rect.translate_by(-screen_rect.location());
@@ -625,8 +658,12 @@ void Compositor::flush(Screen& screen)
             from_ptr = (const Gfx::RGBA32*)((const u8*)from_ptr + pitch);
             to_ptr = (Gfx::RGBA32*)((u8*)to_ptr + pitch);
         }
-        if (device_can_flush_buffers)
+        if (device_can_flush_buffers) {
+            // Whether or not we need to flush buffers, we need to at least track what we modified
+            // so that we can flush these areas next time before we flip buffers. Or, if we don't
+            // support buffer flipping then we will flush them shortly.
             screen.queue_flush_display_rect(rect);
+        }
     };
     for (auto& rect : screen_data.m_flush_rects.rects())
         do_flush(rect);
@@ -634,8 +671,13 @@ void Compositor::flush(Screen& screen)
         do_flush(rect);
     for (auto& rect : screen_data.m_flush_special_rects.rects())
         do_flush(rect);
-    if (device_can_flush_buffers)
-        screen.flush_display();
+    if (device_can_flush_buffers && !screen_data.m_screen_can_set_buffer) {
+        // If we also support flipping buffers we don't really need to flush these areas right now.
+        // Instead, we skip this step and just keep track of them until shortly before the next flip.
+        // If we however don't support flipping buffers then we need to flush the changed areas right
+        // now so that they can be sent to the device.
+        screen.flush_display(screen_data.m_buffers_are_flipped ? 0 : 1);
+    }
 }
 
 void Compositor::invalidate_screen()
@@ -848,7 +890,10 @@ void Compositor::ScreenData::draw_cursor(Screen& screen, const Gfx::IntRect& cur
     auto& current_cursor = compositor.m_current_cursor ? *compositor.m_current_cursor : wm.active_cursor();
     auto screen_rect = screen.rect();
     m_cursor_back_painter->blit({ 0, 0 }, *m_back_bitmap, current_cursor.rect().translated(cursor_rect.location()).intersected(screen_rect).translated(-screen_rect.location()));
-    m_back_painter->blit(cursor_rect.location(), current_cursor.bitmap(screen.scale_factor()), current_cursor.source_rect(compositor.m_current_cursor_frame));
+    auto cursor_src_rect = current_cursor.source_rect(compositor.m_current_cursor_frame);
+    m_back_painter->blit(cursor_rect.location(), current_cursor.bitmap(screen.scale_factor()), cursor_src_rect);
+    m_flush_special_rects.add(Gfx::IntRect(cursor_rect.location(), cursor_src_rect.size()).intersected(screen.rect()));
+    m_have_flush_rects = true;
     m_last_cursor_rect = cursor_rect;
     VERIFY(compositor.m_current_cursor_screen == &screen);
     m_cursor_back_is_valid = true;
@@ -861,6 +906,8 @@ bool Compositor::ScreenData::restore_cursor_back(Screen& screen, Gfx::IntRect& l
 
     last_cursor_rect = m_last_cursor_rect.intersected(screen.rect());
     m_back_painter->blit(last_cursor_rect.location(), *m_cursor_back_bitmap, { { 0, 0 }, last_cursor_rect.size() });
+    m_flush_special_rects.add(last_cursor_rect.intersected(screen.rect()));
+    m_have_flush_rects = true;
     m_cursor_back_is_valid = false;
     return true;
 }

Userland/Services/WindowServer/Compositor.h

@@ -171,7 +171,9 @@ private:
         OwnPtr<WindowStackSwitchOverlay> m_window_stack_switch_overlay;
         bool m_buffers_are_flipped { false };
         bool m_screen_can_set_buffer { false };
+        bool m_has_flipped { false };
         bool m_cursor_back_is_valid { false };
+        bool m_have_flush_rects { false };
 
         Gfx::DisjointRectSet m_flush_rects;
         Gfx::DisjointRectSet m_flush_transparent_rects;

Userland/Services/WindowServer/Screen.cpp

@@ -364,7 +364,7 @@ void Screen::queue_flush_display_rect(Gfx::IntRect const& flush_region)
     }
 }
 
-void Screen::flush_display()
+void Screen::flush_display(int buffer_index)
 {
     VERIFY(m_can_device_flush_buffers);
     auto& fb_data = *m_framebuffer_data;
@@ -380,7 +380,7 @@ void Screen::flush_display()
         flush_rect.height *= scale_factor;
     }
 
-    if (fb_flush_buffers(m_framebuffer_fd, fb_data.pending_flush_rects.data(), (unsigned)fb_data.pending_flush_rects.size()) < 0) {
+    if (fb_flush_buffers(m_framebuffer_fd, buffer_index, fb_data.pending_flush_rects.data(), (unsigned)fb_data.pending_flush_rects.size()) < 0) {
         int err = errno;
         if (err == ENOTSUP)
             m_can_device_flush_buffers = false;

Userland/Services/WindowServer/Screen.h

@@ -165,7 +165,7 @@ public:
 
     bool can_device_flush_buffers() const { return m_can_device_flush_buffers; }
     void queue_flush_display_rect(Gfx::IntRect const& rect);
-    void flush_display();
+    void flush_display(int buffer_index);
 
 private:
     Screen(ScreenLayout::Screen&);