question:using SPI as frame buffer driver

I'm using SPI LCD screen as system screen, I wanna rewrite the frame buffer driver, but one thing I could not make clear: What event triggers frame buffer data transfer to LCD? I was told it was done by hardware like LCDC, but still frame buffer needs copy_from_user before an updated data transfer to LCD, so, anybody could tell me how I could rewrite the SPI LCD driver as a standard frame buffer driver? I want to use QT as UI so standard frame buffer driver is needed. and also, small size LCD uses i8080 interface, that's the same question how to trigger screen write while refreshing the screen. Best regards, Pan