Negative side of diff pair is exposed on ECP5

[jeanthom]

Code example:

from nmigen import *
from nmigen_boards.ecpix5 import ECPIX585Platform

class Top(Elaboratable):
    def elaborate(self, platform):
        m = Module()

        ddr3 = platform.request("ddr3", 0, dir={"dqs":"-"})
        m.submodules += [
        	Instance("BB",
				i_I=1,
				i_T=1,
				o_O=Signal(),
				io_B=ddr3.dqs.n[0])
        ]

        return m

if __name__ == "__main__":
    ECPIX585Platform().build(Top(), do_program=True)

Console output:

ERROR: IO 'ddr3_0__dqs__n[0]' is unconstrained in LPF (override this error with --lpf-allow-unconstrained)
0 warnings, 1 error
Traceback (most recent call last):
  File "diffpairissue.py", line 20, in <module>
    ECPIX585Platform().build(Top(), do_program=True)
  File "/home/jeanthomas/Documents/nmigen/nmigen/build/plat.py", line 94, in build
    products = plan.execute_local(build_dir)
  File "/home/jeanthomas/Documents/nmigen/nmigen/build/run.py", line 95, in execute_local
    subprocess.check_call(["sh", "{}.sh".format(self.script)])
  File "/usr/lib64/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['sh', 'build_top.sh']' returned non-zero exit status 255.