Changing stepgen position-scale parameter dynamically

Hi
The thing I want is being able to do some calibration on axes using hm2_7i90.0.stepgen.00.position-scale parameter dynamically.
How can I access to this parameter?
I tried to define a new halpin in my hal file by writing:
net stepScale_Axis_X hm2_7i90.0.stepgen.00.position-scale
But I get the following error:
./my_LinuxCNC_machine.hal:616: Pin 'hm2_7i90.0.stepgen.00.position-scale' does not exist

so if its pin is not available what can I do to access this parameter for example in my python code for qt designer.

Parameters can be changed with a hal setp command
 Note that dynamically changing the stepgen position scale
will result in following errors.

PCW wrote:

Parameters can be changed with a hal setp command
 Note that dynamically changing the stepgen position scale
will result in following errors.
 

You mean I need to write a hal command (setp) in my python code?
and to avoid errors I need to restart the linuxcnc after changing that parameter, right?

You don't need to restart LinuxCNC before changing the stepgen scale
you just have to make sure motion.motion-enabled is false when changing it.