openXC7 - no Bels remaining of type 'BUFMR'

[kerimbavcic]

When using openXC7 (place and route step) we get an error in BUFMR module, 'u_csi_rx_top.u_phy_clk.u_bufmr' to be specific. We know @hansfbaier that you mentioned this could happen in the previous issue (your message: "I already fixed that in my branch, but now the problem is that nextpnr-xilinx does not support BUFM").

Anyways, here is the exact message we've gotten:
ERROR: Unable to place cell 'u_csi_rx_top.u_phy_clk.u_bufmr', no Bels remaining of type 'BUFMR' 2 warnings, 1 error make: *** [Makefile:53: top.fasm] Error 255

These changes are included - fix inverted check condition and error message typos for OSERDESE2 and it fixed previous issue.

Our question now is, what are the next steps?

[lehaifeng000]

why do you need to use BUFMR?

[chili-chips-ba]

Why not?!

One of the goals of this project is to help openXC7 get as close as it can to proprietary tools. And, while we perhaps could brush this bug under the carpet, such as by restructuring the design sources, that would simply defer this issue until someone else runs into it.

Moreover, due to physical limitations of Artix7 internal architecture and clock routing resources within this particular device, it may not even be possible to cheat that way and mask out the problem.

Note that BUFMR is driving two BUFRs:

• https://github.com/chili-chips-ba/openeye-CamSI/blob/main/1.hw/csi_rx/csi_rx_phy_clk.sv

• What is the problem at looking at this error as a problem that needs fixing?

[lehaifeng000]

It is difficult to support BUFMR for openxc7
they lack segbits in cmt_top, cmt_lower and hclk_cmt

[hansfbaier]

@lehaifeng000 Should probably be quite easy to write a fuzzer for it in prjxray

[lehaifeng000]

@hansfbaier Yes, that's right!

[chili-chips-ba]

Is the plan then for openXC7 team to pursue this bug to extinction, or for us to try brushing it under the carpet?!

[chili-chips-ba]

@Juninho99 as it looks like openXC7 team cannot attend to this bug before late May 2025, we should try to work out a design recipe that avoids the use of BUFMR.