This is a (very) brief summary of some of things that need to be
done to write a Frame Lock NV-CONTROL client.

Please see samples/nv-control-framelock.c for a basic Frame Lock client.  See
the file ctkframelock.c for a much more involved example of how the
nvidia-settings utility programs the Frame Lock attributes.

The constants referenced below are defined in NVCtrl.h.  Please see the
comments in that file for an explanation of each attribute.  It may
also be instructive to grep ctkframelock.c to see how each attribute
is used.

Querying frame lock capabilities of a system:

    - Query the number of Quadro Sync devices on the system by passing
      NV_CTRL_TARGET_TYPE_FRAMELOCK to XNVCTRLQueryTargetCount()

    - Query the GPUs attached to each Quadro Sync device by querying the
      NV_CTRL_BINARY_DATA_GPUS_USING_FRAMELOCK attribute for each Quadro
      Sync device.

    - Query the enabled (available) display devices on each GPU through
      the NV_CTRL_BINARY_DATA_DISPLAYS_CONNECTED_TO_GPU and
      NV_CTRL_DISPLAY_ENABLED attributes.

    - Query whether each display may be configured as Frame Lock server or
      client through the NV_CTRL_FRAMELOCK_DISPLAY_CONFIG attribute.

Configuring the Frame Lock group (This must be done while Frame Lock Sync is
disabled on the GPU/Quadro Sync Device):

    - Set one display device as Frame Lock server through the
      NV_CTRL_FRAMELOCK_DISPLAY_CONFIG_SERVER attribute.

    - Set the rest of the display devices as Frame Lock clients through the
      NV_CTRL_FRAMELOCK_DISPLAY_CONFIG_CLIENT attribute.

    - Set NV_CTRL_USE_HOUSE_SYNC to NV_CTRL_USE_HOUSE_SYNC_TRUE or
      NV_CTRL_USE_HOUSE_SYNC_FALSE on the server Quadro Sync device (this is
      the Quadro Sync device that is attached to the GPU that contains the
      display device currently set as the Frame Lock server.)  Depending on
      whether or not you will be using a House Sync signal.

    - Set NV_CTRL_FRAMELOCK_POLARITY for each client Quadro Sync device in the
      cluster; most likely you want NV_CTRL_FRAMELOCK_POLARITY_RISING_EDGE.

    - Set NV_CTRL_FRAMELOCK_SYNC_DELAY as appropriate for each Quadro Sync
      device. Most likely you want 0.

    - Set NV_CTRL_FRAMELOCK_SYNC_INTERVAL as appropriate if you
      are using house sync.  Most likely, you just want 0.

Enabling/Disabling Frame Lock:

    - Enable Frame Lock on each GPU by setting NV_CTRL_FRAMELOCK_SYNC
      to NV_CTRL_FRAMELOCK_SYNC_ENABLE.

    - (optional) set NV_CTRL_FRAMELOCK_TEST_SIGNAL to
      NV_CTRL_FRAMELOCK_TEST_SIGNAL_ENABLE followed immediately by
      NV_CTRL_FRAMELOCK_TEST_SIGNAL_DISABLE (on the GPU that contains the
      server Frame Lock display device.)  This guarantees accuracy of the
      Universal Frame Count (as returned by glXQueryFrameCountNV()).

    - When you are done, set NV_CTRL_FRAMELOCK_SYNC to
      NV_CTRL_FRAMELOCK_SYNC_DISABLE on each GPU to disable Frame Lock.

During operation, you can also query any of these on each GPU or Frame Lock
device:

    NV_CTRL_FRAMELOCK_PORT0_STATUS
    NV_CTRL_FRAMELOCK_PORT1_STATUS
    NV_CTRL_FRAMELOCK_HOUSE_STATUS
    NV_CTRL_FRAMELOCK_SYNC_READY
    NV_CTRL_FRAMELOCK_STEREO_SYNC
    NV_CTRL_FRAMELOCK_ETHERNET_DETECTED
    NV_CTRL_FRAMELOCK_SYNC_RATE
    NV_CTRL_FRAMELOCK_TIMING

(nvidia-settings registers a gtk+ timeout function and queries the
state of these on each X screen every few seconds).