The screen actually have two interfaces, one is DPI, another is SPI. Actual pixel data is transmitted via the DPI, but the screen also needed to be configured via the SPI before it can display normally.

I decided to connect the SPI to STM32 so I do not need to modify Raspberry Pi's kernel in order to initialize the screen. The driver was adopted from Openmoko's linux kernel source code. One thing to mention is that this screen use 9-Bit SPI with first bit used to indicate which this transfer contains data or instruction.

The full source code is available in GitHub.