Using pre-built packages with portmanager

[cpghost]

Building packages for multiple machines on a fast CPU,
with portmanager's -bu option populates a /usr/ports/packages tree.
So far, so good.

What I'd like though, is to be able to reuse that tree (mounted via
NFS or rsynced over) on other machines with much slower CPUs.

The fast build machine and the other slow machines are not synchronized
w.r.t. the set of installed ports. What is needed is that portmanager
uses packages from /usr/ports/packages if available, and compiles from
source the remaining ports.

How do I get portmanager to upgrade ports, using
  1. pre-built packages from /usr/ports/packages (ONLY),
and only if there's no binary package there,
  2. build from source as usual?

Additional limit (preventing use of portupgrade -P) is that only
local (/usr/ports/packages), and no remote packages should be
used.

Any idea how to do this with portmanager?

Sorry if it's obvious, but I'm running low on coffeine... ;)

Thanks
-cpghost.

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