Using an external clock setting program

# 1 Using an external clock setting program

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XFree86 provides a facility for setting the clock frequency on a graphics card by an external program. This is provided to make it possible to deal with cards that use clock selection methods not supported by the standard drivers.

This facility is enabled by adding a `ClockProg` line to the `Device` section of the `XF86Config` file. The format of this line is:

> ``ClockProg "`commandpath`"``

where commandpath is the full pathname of the clock setting program. No flags are allowed in commandpath.

When using this option, a Clocks line is required in the Device section of the `XF86Config` file to tell the server which clock frequencies are available to it. In the case of a card that has a fixed set of preset clocks, the ordering of the clocks in the `Clocks` line should correspond to what the card/program expects. In the case of a card which has a fully programmable clock (like the SS24) the ordering is not important, and the values chosen can be anything in the range supported by the card. Up to 32 clock values may be specified.

The server calls the external program when it needs to change the clock frequency. This occurs at startup and when switching modes with the hot-key sequences. The command is passed two command-line arguments. The first is the clock frequency in MHz (as a floating point number -- currently specified to the nearest 0.1 MHz). The second argument is the index of the clock value in the Clocks list (the first clock is index 0). Cards with a fixed set of clocks would probably make use of the index, while cards with a fully programmable clock would use the frequency argument.

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