buffer_ports command using delay cells instead of buffer cells

[alexroach-rl]

Hi,
With the particular third-party liberty file I am using, the buffer_ports command, called by the resize.tcl script in the ORFS. is inserting a delay cell instead of a buffer cell for input and output port buffering.

Does anyone have any idea on how it chooses the buffer cell to insert and why it might be doing this? Is there any way to prevent it from doing this apart from using the DONT_USE_CELLS environment variable? It's just that it might be problematic putting delay cells in the DONT_USE_CELLS variable as this variable is used for other stages in the flow, where I do want delay cells to be used.

[maliberty]

@precisionmoon please respond

[precisionmoon]

buffer_ports simply picks the smallest buffer cell with the largest drive resistance. Unfortunately, it is not easy to tell whether a library cell is a delay cell or a regular buffer. Instead of using DONT_USE_CELLS environment variable, you can do something like the following:

set_dont_use { DLY_CELL1 DLY_CELL2 }
buffer_ports
unset_dont_use { DLY_CELL1 DLY_CELL2 }

Also, if your library has cell_footprint attributes that cover multiple drive strengths (BUFX1, BUFX2, BUFX4, ... have the same cell_footprint attribute), you may want to enable -match_cell_footprint in repair_design and repair_timing.

[maliberty]

We could add a command option to allow the user to specify the cell if we just use a single cell everywhere.

[chrisgelinek-rl]

Added a PR to make this easy to use from within ORFS: The-OpenROAD-Project/OpenROAD-flow-scripts#3655

[alexroach-rl]

Thanks for the suggestions, guys! I will try using the set/unset dont_use command just with the delay cells either side of the buffer_ports command. Perhaps this could be added as an environment variable option to a future version, along with the other suggestion of setting the cell to use for the port buffering. According to the rsz documentation (https://openroad.readthedocs.io/en/latest/main/src/rsz/README.html), this option is already supported, but I was not able to get it to work. Perhaps it is not fully implemented yet.

From the README file:

buffer_ports 
    [-inputs] 
    [-outputs] 
    [-max_utilization util]
    [-buffer_cell buf_cell]

[maliberty]

Looks like we just need an envar in ORFS then. Let us know if that fixes your issue.

[jhkim-pii]

buffer_ports -buffer_cell <buf_cell> has been implemented in the PR #7845 . You can use it now.