FPGA routing structure and other pnr questions

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tnt
Posts: 16
Joined: Fri Jun 05, 2020 5:21 am

Is there any documents / diagram that detail how the routing structure works and what kind of "faster path" exist ?

Looking at some timing reports and some parts of the toolchain I had also some questions :

* The LUT4 from yosys get broken out in LUT3 + a F_FRAG. But will VTR stil map them properly to not use the F_MUX but the TBS one ?
* I saw a comment that the dedicated path from TBS mux to the FF isn't use and it always goes from CZ to switchbox to QDI, is that still the case ?
* Is there any dedicated / faster routing to go from Fmux to/from other signals in the same LC ?

You also mentionned you have a proprietary toolchain for that fpga, do you have any comparison or rough idea how the current yosys+vtr stack compares performance/area wise to your dedicated tool ? Just to get an idea of how much potential gain there could be by improving the synthesis and pnr part of the current OSS stack.

Cheers,

Sylvain
mithro
Posts: 6
Joined: Fri Jun 05, 2020 10:20 pm

On the comparison front, I believe there is plan to add QuickLogic support (both open source tooling and the old proprietary flow) to SymbiFlow fpga-tool-perf (https://github.com/SymbiFlow/fpga-tool-perf and https://j.mp/fpga-tool-perf-spec).

Karol Gugala from Antmicro and the QuickLogic team will have to answer your more technical questions.
rakeshm
Posts: 9
Joined: Wed Jun 03, 2020 4:59 am

We are in the process of creating detailed routing architecture description. Meanwhile we will forward you document with details of the S3 FPGA routing structure.

Regarding the LUT4 implementation, currently Yosys/Symbiflow does not use TBS to implement LUT4.
It is in our list of improvements, we will be working on it.

Yes, currently Yosys/Symbiflow does not use the dedicated path from TBS mux to the FF, instead it routes out on CZ and then into the QDI.
As above, this is also in our list of improvements, we will be working on it.

In S3 FPGA, there is no dedicated/faster routing to or from Fmux from other signals in the LC.

Regarding comparison, as Mithro mentioned, we have plans to add QuickLogic support to SymbiFlow fpga-tool-perf.
Meanwhile, we are working on comparison (Utilization/Performance/Runtime) between Symbiflow and the proprietary tool which we will share soon.
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