Using ClockSignal in platform.add_clock_constraint

[rroohhh]

Currently one cannot use a ClockSignal in platform.add_clock_constraint:

from nmigen import *
from nmigen_boards.microzed_z010 import *

plat = MicroZedZ010Platform()
plat.add_clock_constraint(ClockSignal(), 25e6)

fails with

Traceback (most recent call last):
  File "a.py", line 5, in <module>
    plat.add_clock_constraint(ClockSignal(), 25e6)
  File "/home/robin/.guix-profile/lib/python3.8/site-packages/nmigen/vendor/xilinx_7series.py", line 184, in add_clock_constraint
    super().add_clock_constraint(clock, frequency)
  File "/home/robin/.guix-profile/lib/python3.8/site-packages/nmigen/build/res.py", line 223, in add_clock_constraint
    raise TypeError("Object {!r} is not a Signal".format(clock))
TypeError: Object (clk sync) is not a Signal

It be nice to be able to use a ClockSignal directly in platform.add_clock_constraint.

[whitequark]

That's not possible--the Platform does not know the context of any particular ClockSignal, and without the context, it is ambiguous.

[awygle]

I'd like to recommend enhancing the error message for this case, or else explaining in more detail here for future reference. Even with your comment above, it's not immediately obvious to me why plat.add_clock_constraint(ClockSignal(), 25e6) doesn't work but

cs = ClockSignal()
s = Signal()
m.d.comb += s.eq(cs)
plat.add_clock_constraint(s, 25e6)

does.

[awygle]

Throwing some spaghetti at the wall for the error message:

TypeError: Object (clk sync) is not a Signal, it is a ClockSignal. You may be able to assign the value of (clk sync) to a Signal, which can then be used to add a clock constraint.

Thoughts?