What are GPIO pins?
GPIO means General Purpose I/O. Video decoder chips like the SAA7134 or the BT878 have a number of I/O pins that are not used by the video decoder itself. They have no special purpose, each manufacturer can do whatever he wants with them. Sometimes, 2 or 3 of them are used to control an audio multiplexer. On most cards, many or all GPIOs are unused. Unused Pins are usually programmed as inputs.
These GPIOs can _not_ be read/written from user space. There is no IOCTL for them. They can only be used from within the kernel driver.
GPIOs of SAA713x
The SAA713x has 27 of these pins. Each of them can be programmed to be an input or an output. On the Terratec Cinergy 400TV mobile, GPIO 18 is used to switch the tuner on (0) or off (1). Unused inputs are usually programmed as inputs. Because of weak internal pull-down resistors in the SAA713x, open inputs are read as zeroes. If a bit is read as "1" after loading the saa7134 module, this means the manufacturer soldered a pull-up resistor to that pin. As they have no resistors to waste, this usually means, they do something with that pin...
GPIOs of BT8x8
(To be written)