tc_clk_xilinx.sv: Add IS_FUNCTIONAL parameter on clk_mux

[CyrilKoe]

The goal is to disable the default instantiation of the BUFGMUX, as it consumes lots of clock resources when it is not actually needed. It can also create long BUF chains that can fail DRCs.
This can often fail Vivado implementations.
When buffering is needed, who's instantiating the IP has to explicitly set it.

[luca-valente]

@CyrilKoe We need to add the parameter also in the RTL target file so that it is actually possible to use it. Here as for the clock gating cell.

[niwis]

@CyrilKoe I'm not sure if IS_FUNCTIONAL makes sense here. A multiplexer is always functional. Why is the tc_clk_mux used for non-clock signals?

[niwis]

The default needs to remain 1'b1.

[luca-valente]

@niwis the goal is to disable the default instantiation of the BUFGMUX, as it consumes lots of clock resources when it is not actually needed. When needed, who's instantiating the IP has to explicitly set it.

[luca-valente]

We can find a different name for the parameter 😄

[CyrilKoe]

EN_BUF_FPGA ?

[niwis]

I understand from the comment that the BUFGMUX is not wanted if you multiplex a non-clock signal. My question is: why do you use tc_clk_mux for these non-clock signals in the first place? Maybe the place to solve this would be one hierarchy level above (by not instantiating this module). But I might be missing something; happy to discuss this offline.

[luca-valente]

Lots of "legacy" code using the tc_clk_mux... let's discuss it offline 😄

[CyrilKoe]

@niwis @luca-valente
Canged now, if you agree with using this parameter I will start a Carfield CI with the change and I will comment here if it passed.