[openib-general] ib_unmap_fmr

[Roland Dreier]

    Sean> The current API for ib_unmap_fmr takes an array.  A couple
    Sean> of questions: Is it worth it for ib_unmap_fmr to operate on
    Sean> more than a single fmr?  If so, should a linked list be used
    Sean> instead?  And, should the access layer API assume that the
    Sean> FMRs all come from the same device?  My initial thought is
    Sean> to restrict ib_unmap_fmr to a single fmr, _but_ if there's a
    Sean> reason to unmap multiples, update struct ib_fmr to allow for
    Sean> a linked list.  The access layer would assume all FMRs come
    Sean> from the same device in such a case, to avoid walking the
    Sean> list.

One of the main reasons that FMRs (in the Mellanox Tavor sense) are a
win is that the cost of flushing the HCA's memory mapping cache can be
amortized across multiple FMRs with a single unmap call.  So having
unmap_fmr operate on multiple FMRs at once is central to its
usefulness.

With that said I prefer using a linked list (and that's what I have on
my branch), and I agree with the assumption that all the FMRs come
from the same device.

 - R.