[ofa-general] opensm support for toroidal meshes

[Robert Pearson]

We have been involved in a project to deliver a large system based on a
toroidal mesh fabric. One of the requirements for this system is to be able
to guarantee a deadlock free routing of the fabric. The lash routing engine
in opensm did not work in this case because required number of VLs for the
machine as configured was 12 which exceeded the number of VLs supported by
Mellanox switch ASICs. It turns out that if one has the freedom to reorder
the order of the port assignments used by lash optimally that lash can
successfully route the fabric but that is impractical in the hardware. The
attached note describes an algorithm for automatically recognizing when a
Cartesian mesh fabric is a torus, determining its size and optimally
reordering the ports in opensm so that lash can generate a route with the
smallest number of VLs.

We have implemented a set of changes to opensm that implement this algorithm
and will submit the changes as patches. This note will help to understand
the code.
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