I'm using the st-link on stm32f4 discovery. I had upgrated the firmware to V2.J21.S0, and there's something wrong when using the printf via SWO. The printf data number inceases fast but there is no data shown in the window. After reconnecting the USB port, and restart the SWO viewer, it works correctly. The problem occors again when the program of target board is reprogramed, or reconnecting the ST-Link in ST-Link utility. I tried downgrade the firmware from V2.J21.S0 to V2.J17.S0 and it just works fine. So I think there's a bug in the firmware.