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SDC/XDC create_clock and set_false_path constraints should propigate through Yosys #1291

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litghost opened this issue Jan 31, 2020 · 9 comments
Open

SDC/XDC create_clock and set_false_path constraints should propigate through Yosys #1291

litghost opened this issue Jan 31, 2020 · 9 comments
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@tmichalak

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@litghost
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litghost commented Jan 31, 2020 •
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Currently SDC constraints are passed directly to VPR, which means the net names need to be the output net names from Yosys. However, constraints should be specified on the nets in the input verilog, and then a new SDC constraints file should be output from Yosys with the new net names.

Adding this feature to the XDC plugin should work?

@tmichalak / @acomodi I believe this would allow the LiteX XDC constraints to be used throughout the flow?

@mithro FYI
@litghost litghost mentioned this issue Jan 31, 2020
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@mithro
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mithro commented Jan 31, 2020

Yes.

We can also do the clock constraint propagation through PLLs and and other boxes here.

@mithro
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mithro commented Feb 1, 2020

@daveshah1 - FYI this is the direction we are planning on going with constraints with VPR. Do you think it would be useful for nextpnr at all?

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I implemented net aliases in nextpnr, so a constraint in nextpnr can be any of the names that Yosys gives to a net. But this could be useful too

@mithro
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mithro commented Feb 1, 2020

@daveshah1 How do you figure out the net aliases?

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In the JSON, the netnames section format allows more than one name for each net. I don't know if there is a BLIF equivalent.

@acomodi acomodi mentioned this issue Feb 3, 2020
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@litghost
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Can you close this issue once this is merged?

@tmichalak
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tmichalak commented Nov 2, 2020 •
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create_clock propagates through Yosys with this SDC plugin PR and a fix.
set_false_path is not propagated, currently we simply write out what we get from the commands.

@tmichalak
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All related PRs have been merged. Closing.

@mithro
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mithro commented Dec 5, 2020

Is there a separate issue so that set_false_path is handled correctly?

@mithro mithro reopened this Dec 5, 2020
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@mithro @litghost @daveshah1 @tmichalak