[ofa-general] mthca max_sge value... ugh.

[Olaf Kirch]

On Friday 16 May 2008 00:22:12 Roland Dreier wrote:
> Strange... as I said before though something seems to have changed to
> affect this, though I have no idea what.  I'm including the test program
> I use to check if QP creation succeeds, you can run this on any suspect
> systems and see what it prints.

I ran into this a few weeks back as well, when I tried to up the SG limit
in RDS to 32 (on a arbel memfree card).

I grepped around the code a bit, got a little confused because of all the
different max_sge, max_sg and max_gs variables :-) and eventually
convinced myself that the max_sge reported simply doesn't include the
transport specific overhead that mthca_alloc_wqe_buf factors in.

Given that you have quite different WQE overheads depending on the transport,
a conservative max_sge value that works for all transports wastes one or two
entries on some others. Maybe once the QP is created, it could report
the actual max_sge value (which may actually be greater than the conservative,
transport-independent max_sge estimate of the device).

Olaf
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