       Information for using/developing external clock setting programs

                          The XFree86 Project, Inc.

                               16 December 1994

1.  Using an external clock setting program

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, and no Clocks line is specified, it is assumed that
the card has clocks which are fully programmable (like the SS24). However if
the card has a fixed set of preset clocks a Clocks line is required in the
Device section of the XF86Config file to tell the server which clock frequen-
cies are available to it. The ordering of the clocks in the Clocks line
should correspond to what the card/program expects. Up to 128 clock values
may be specified.

The server calls the external program when it needs to change the clock fre-
quency.  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 pro-
grammable clock would use the frequency argument.

2.  Developing an extern clock setting program

When such an external program is being used, the server does not change any
register fields related to clock selection, and the external program must be
careful to only modify clock selection fields.  The program is run with stdin
and stdout set to xf86Info.consoleFd -- which is the fd to use for display-
related ioctl() operations if required.  Stderr is the same as the server's
stderr -- so error or warning messages should be directed there.  The program
is run with the uid set to the real user's ID -- so if it needs to use privi-
leged system calls it should be suid-root.  The program does not inherit any
I/O access privileges, so it will need to do whatever is required to enable
them.

The program is expected to return an exit status 0 when successful, and a
status in the range 1-254 when it fails.  If the external program fails dur-
ing the server initialisation stage, the server exits.  If it fails for a
mode switch, the mode switch is aborted (the server assumes that the clock
frequency hasn't been changed) and the server keeps running.  If necessary an
exit status may be specified in the future for which the server would exit if
the program fails for a mode switch.

A sample clock switching program is provided for use with many ET4000 cards
that have a fixed set of 8 clocks.  This program is only intended as a sam-
ple, and it is not intended for general use (the internal server code handles
this type of card).  The program is xc/pro-
grams/Xserver/hw/xfree86/etc/et4000clock.c in the source tree, and
/usr/X11R6/lib/X11/etc/et4000clock.c in the binary tree.

The idea of using an external clock program was suggested by Frank Klemm
<pfk@rz.uni-jena.de>

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