What does 'positive' / 'negative' polarity mean for HVF and VCLK?

Positive polarity means that the active edge is a low-to-high transition. Negative polarity means that the active edge is a high-to-low transition.

Both the firmware encode parameter PICFG and the PMODE2 register allow you to change the polarity of HSYNC, VSYNC, FIELD, and VCLK.

Polarity is set with the PMODE2 register if the register is programmed for custom-specific mode or raw pixel mode. Otherwise, polarity is set with the ADV2x2 firmware encode parameter PICFG.