backends/drm: always prefer 10bpc buffers when supported

The environment variable is primarily meant as a workaround for displays
and drivers that misbehave when more than 8 bits per color is used. To simplify
the code, this commit makes the environment variable only control the bpc
used for displays, instead of also controlling which buffer formats get
used.

src/backends/drm/drm_egl_backend.cpp

@@ -172,13 +172,6 @@ std::pair<std::shared_ptr<KWin::GLTexture>, ColorDescription> EglGbmBackend::tex
     return std::make_pair(layer->texture(), layer->colorDescription());
 }
 
-bool EglGbmBackend::prefer10bpc() const
-{
-    static bool ok = false;
-    static const int preferred = qEnvironmentVariableIntValue("KWIN_DRM_PREFER_COLOR_DEPTH", &ok);
-    return !ok || preferred == 30;
-}
-
 std::shared_ptr<DrmPipelineLayer> EglGbmBackend::createPrimaryLayer(DrmPipeline *pipeline)
 {
     return std::make_shared<EglGbmLayer>(this, pipeline);

src/backends/drm/drm_egl_backend.h

@@ -51,7 +51,6 @@ public:
     OutputLayer *cursorLayer(Output *output) override;
 
     void init() override;
-    bool prefer10bpc() const override;
     std::shared_ptr<DrmPipelineLayer> createPrimaryLayer(DrmPipeline *pipeline) override;
     std::shared_ptr<DrmPipelineLayer> createCursorLayer(DrmPipeline *pipeline) override;
     std::shared_ptr<DrmOutputLayer> createLayer(DrmVirtualOutput *output) override;

src/backends/drm/drm_egl_layer_surface.cpp

@@ -342,11 +342,11 @@ std::unique_ptr<EglGbmLayerSurface::Surface> EglGbmLayerSurface::createSurface(c
         }
     }
 
-    const auto sort = [this](const auto &lhs, const auto &rhs) {
+    const auto sort = [](const auto &lhs, const auto &rhs) {
         if (lhs.drmFormat == rhs.drmFormat) {
             // prefer having an alpha channel
             return lhs.alphaBits > rhs.alphaBits;
-        } else if (m_eglBackend->prefer10bpc() && ((lhs.bitsPerColor == 10) != (rhs.bitsPerColor == 10))) {
+        } else if ((lhs.bitsPerColor == 10) != (rhs.bitsPerColor == 10)) {
             // prefer 10bpc / 30bpp formats
             return lhs.bitsPerColor == 10;
         } else {

src/backends/drm/drm_pipeline.cpp

@@ -313,11 +313,10 @@ bool DrmPipeline::prepareAtomicModeset(DrmAtomicCommit *commit)
         commit->addProperty(m_connector->underscanHBorder, hborder);
     }
     if (m_connector->maxBpc.isValid()) {
-        uint64_t preferred = 8;
-        if (auto backend = dynamic_cast<EglGbmBackend *>(gpu()->platform()->renderBackend()); backend && backend->prefer10bpc()) {
-            preferred = 10;
-        }
-        commit->addProperty(m_connector->maxBpc, preferred);
+        // TODO migrate this env var to a proper setting
+        static bool ok = false;
+        static const int preferred = qEnvironmentVariableIntValue("KWIN_DRM_PREFER_COLOR_DEPTH", &ok);
+        commit->addProperty(m_connector->maxBpc, ok && preferred == 24 ? 8 : 10);
     }
     if (m_connector->hdrMetadata.isValid()) {
         commit->addBlob(m_connector->hdrMetadata, createHdrMetadata(m_pending.colorDescription.transferFunction()));

src/platformsupport/scenes/opengl/abstract_egl_backend.cpp

@@ -199,13 +199,11 @@ void AbstractEglBackend::initWayland()
         return formats;
     };
 
-    if (prefer10bpc()) {
-        m_tranches.append({
-            .device = deviceId(),
-            .flags = {},
-            .formatTable = filterFormats(10, false),
-        });
-    }
+    m_tranches.append({
+        .device = deviceId(),
+        .flags = {},
+        .formatTable = filterFormats(10, false),
+    });
     m_tranches.append({
         .device = deviceId(),
         .flags = {},
@@ -284,11 +282,6 @@ dev_t AbstractEglBackend::deviceId() const
     return m_deviceId;
 }
 
-bool AbstractEglBackend::prefer10bpc() const
-{
-    return false;
-}
-
 EGLImageKHR AbstractEglBackend::importBufferAsImage(GraphicsBuffer *buffer, int plane, int format, const QSize &size)
 {
     std::pair key(buffer, plane);

src/platformsupport/scenes/opengl/abstract_egl_backend.h

@@ -42,7 +42,6 @@ public:
 
     QList<LinuxDmaBufV1Feedback::Tranche> tranches() const;
     dev_t deviceId() const;
-    virtual bool prefer10bpc() const;
 
     std::shared_ptr<GLTexture> importDmaBufAsTexture(const DmaBufAttributes &attributes) const;
     EGLImageKHR importDmaBufAsImage(const DmaBufAttributes &attributes) const;