[platforms/fbdev] Properly detect a BGR image format

Summary:
If the format of the framebuffer is BGR we cannot create an RGB image
format from it - the rendering is incorrect. Unfortunately QImage does
not support a BGR image format.

To solve this problem we still use an RGB image format but on rendering
the front buffer is rgbSwapped to convert the RGB image to a BGR image.

BUG: 365243
FIXED-IN: 5.7.2

Test Plan: Tested on a neon kvm

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2134

plugins/platforms/fbdev/fb_backend.cpp

@@ -92,6 +92,7 @@ void FramebufferBackend::openFrameBuffer()
     }
     m_fd = fd;
     queryScreenInfo();
+    initImageFormat();
     setReady(true);
     emit screensQueried();
 }
@@ -153,9 +154,14 @@ void FramebufferBackend::unmap()
 }
 
 QImage::Format FramebufferBackend::imageFormat() const
+{
+    return m_imageFormat;
+}
+
+void FramebufferBackend::initImageFormat()
 {
     if (m_fd < 0) {
-        return QImage::Format_Invalid;
+        return;
     }
 
     qCDebug(KWIN_FB) << "Bits Per Pixel: " << m_bitsPerPixel;
@@ -178,7 +184,7 @@ QImage::Format FramebufferBackend::imageFormat() const
             m_green.offset == 8 &&
             m_red.offset == 16) {
         qCDebug(KWIN_FB) << "Framebuffer format is RGB32";
-        return QImage::Format_RGB32;
+        m_imageFormat = QImage::Format_RGB32;
     } else if (m_bitsPerPixel == 24 &&
             m_red.length == 8 &&
             m_green.length == 8 &&
@@ -187,7 +193,8 @@ QImage::Format FramebufferBackend::imageFormat() const
             m_green.offset == 8 &&
             m_red.offset == 16) {
         qCDebug(KWIN_FB) << "Framebuffer Format is RGB888";
-        return QImage::Format_RGB888;
+        m_bgr = true;
+        m_imageFormat = QImage::Format_RGB888;
     } else if (m_bitsPerPixel == 16 &&
             m_red.length == 5 &&
             m_green.length == 6 &&
@@ -196,10 +203,9 @@ QImage::Format FramebufferBackend::imageFormat() const
             m_green.offset == 5 &&
             m_red.offset == 11) {
         qCDebug(KWIN_FB) << "Framebuffer Format is RGB16";
-        return QImage::Format_RGB16;
+        m_imageFormat = QImage::Format_RGB16;
     }
     qCWarning(KWIN_FB) << "Framebuffer format is unknown";
-    return QImage::Format_Invalid;
 }
 
 }

plugins/platforms/fbdev/fb_backend.h

@@ -71,10 +71,17 @@ public:
         return m_bitsPerPixel;
     }
     QImage::Format imageFormat() const;
+    /**
+     * @returns whether the imageFormat is BGR instead of RGB.
+     **/
+    bool isBGR() const {
+        return m_bgr;
+    }
 
 private:
     void openFrameBuffer();
     bool queryScreenInfo();
+    void initImageFormat();
     QSize m_resolution;
     QSize m_physicalSize;
     QByteArray m_id;
@@ -91,6 +98,8 @@ private:
     quint32 m_bufferLength = 0;
     int m_bytesPerLine = 0;
     void *m_memory = nullptr;
+    QImage::Format m_imageFormat = QImage::Format_Invalid;
+    bool m_bgr = false;
 };
 
 }

plugins/platforms/fbdev/scene_qpainter_fb_backend.cpp

@@ -79,7 +79,7 @@ void FramebufferQPainterBackend::present(int mask, const QRegion &damage)
         return;
     }
     QPainter p(&m_backBuffer);
-    p.drawImage(QPoint(0, 0), m_renderBuffer);
+    p.drawImage(QPoint(0, 0), m_backend->isBGR() ? m_renderBuffer.rgbSwapped() : m_renderBuffer);
 }
 
 bool FramebufferQPainterBackend::usesOverlayWindow() const