Below is from the document.
Yes, it’s length of per frame unit is number of line. The sensor datasheet should have detail explain.
Sensor drivers controlling sensor exposure and frame rate must implement V4L2_CID_FRAME_LENGTH and V4L2_CID_COARSE_TIME controls. These controls are used to update sensor frame length and coarse integration time registers with the values calculated from user-mode camera stack.
If the sensor driver has special requirements or direct calculation of the exposure and frame rate is required, V4L2_CID_FRAME_RATE and V4L2_CID_EXPOSURE controls can be implemented instead. When these two controls are implemented, they receive unprocessed frame rate and exposure values in fixed point format. The driver must perform any necessary determination of final register settings.
The V4L2_CID_FRAME_LENGTH and V4L2_COARSE_TIME controls take 32-bit input parameters and V4L2_CID_FRAME_RATE and V4L2_EXPOSURE take 64-bit input parameters. Use either set of controls but do not mix or use both sets at once.