The QSGAbstractRenderer::setClearMode will save a ClearMode member, this is a enum value:

enum ClearModeBit
    {
        ClearColorBuffer    = 0x0001,
        ClearDepthBuffer    = 0x0002,
        ClearStencilBuffer  = 0x0004
    };

ClearMode control the renderer how to clear buffers when render, but the QSGAbstractRenderer::clearMode is not using in any place. If you call QSGAbstractRenderer::setClearMode, you can see any effect.

I need use QSGAbstractRenderer::setClearMode in my project, I want preserve color buffer before render, because I can control the render tareget, I know clear color buffer is unnecessary.