How to use fixed pin location file in ORFS flow?

[vijayank88]

This question came from gitter channel:
Is it possible to manually specify the pins locations instead of having a complete random placement.
I do see the following directive used in https://github.com/The-OpenROAD-Project/OpenROAD-flow-scripts/blob/master/flow/designs/gf12/swerv_wrapper/config.mk
export PLACE_PINS_ARGS = -exclude left: -exclude right: -exclude top:* -exclude bottom:0-10 -exclude bottom:400-700
Can I use PLACE_PINS_ARGS to specify the location of the pins? If yes what is the syntax to be used?

Given that I essentially have pins with fix locations, through which variable in the config.mk file should I pass the name of the file containing the place_pin set of directives? Would something like IO_CONSTRAINTS=my_place_pin_directives.tcl be a good choice? If not which other variable should I use in this case?

[vvbandeira]

I don't think there's a straightforward way to do this by just modifying existing flow arguments. Using IO_CONSTRAINTS would probably not get the desired result because we call place_pins after sourcing the file, see here. The user can replace the default ORFS io_placement.tcl script with their own version. The user can also file an Issue requesting the feature or submit a PR.

[maliberty]

You can use place_pin (see https://github.com/The-OpenROAD-Project/OpenROAD/tree/master/src/ppl) for manual placement.

[gkamendje]

@maliberty
Assuming I have all my 20 pins that I need to place on a specific location. In which file should I put the 20 place_pin directives? I don't think that modifying the default ORFS io_placement.tcl is the most elegant way to achieve this. Maybe having an option for the user to supply a file with pin placement directives would be more appropriate.

[maliberty]

In io_placement.tcl you can use

if {[info exists ::env(IO_CONSTRAINTS)]} {
source $::env(IO_CONSTRAINTS)
}

to give your placement.

[gkamendje]

@maliberty
vvbandeira was suggesting that using IO_CONSTRAINTS this way might not be the best solution given that place_pins is called right after sourcing the content of IO_CONSTRAINTS in io_placement.tcl. Do you suggest to comment out the call to place_pins in this case?

[maliberty]

I believe it will be a no-op if you have manually placed all the pins (or at least it should be) so there should be no harm in running it.

[laurentc2]

Is there somewhere an example using place_pin that set/force the all the pins name, location and layer ?
Such an example for that function would be nice .

Laurent

[maliberty]

I don't know that there is an example in ORFS but you can see the unit test https://github.com/The-OpenROAD-Project/OpenROAD/blob/master/src/ppl/test/place_pin1.tcl

[vijayank88]

With set_io_pin_constraint you can restrict IO placement on specific side.
Examples found here: https://github.com/The-OpenROAD-Project/OpenROAD/blob/master/src/ppl/test/add_constraint10.tcl